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  • 7/30/2019 Boat Electrics


    Selecting the correct size and type of cable for any particular job.

    This isn't anywhere near as difficult as some people believe. It is also more complicated than other people


    Selecting the wrong size or type of cable can result in, at best, a system that does not operate correctly or as

    desired, a system that is unreliable and subject to continual failures, an instalaltion that fails surveys or safety

    inspections etc. It can lead to a system that costs far more than it needs to or, as a worst possible scenario, it can

    result in fires or electrocution.

    There are 4 rules that must be followed. Each one is relatively simple but important in it's own right.

    Rule 1

    The first rule is that the cable must be of the correct type for the voltage. This is related to the insulation

    breakdown voltage. Cable is specified by the manufacturer (following testing) as being suitable for use up to a

    certain voltage. This specification will be written on the cable drum. As long as this specification is higher than

    the system voltage everything is fine. That is to say it is perfectly acceptable to use 1000 volt cable on a 24 volt

    system. Obviously a cable rated for 24 volts would be totally unsuitable for use on a 1000 volt system.

    Rule 2

    The 2nd rule is even simpler. This relates to the physical strength and durability of the cable. This one really is

    down to nothing more than common sense. For instance, cables inside mobile telephones and calculators aretiny. Suppose there is a piece of equipment at the bow of a boat that draws extremely low current but needs

    powering from the stern. Say a 0.001 amp load. A tiny cable of around 0.1mm2 as found in a mobile 'phone will

    handle the current. But physically this cable is not going to survive very long on a boat due to vibration and

    chaffing etc. The size of the cable and the physical strength of the insulation must be up to the job.

    Further, the cable insulation must be tolerant of any other chemicals it may come into contact with. For instance

    cables in engine bays should be oil and fuel resistant. Those of us on the UK Inland Waterways are more than

    aware of the problems with PVC insulation when in contact with expanded polystyrene.

    The last 2 rules are slighly more complicated and are related to the actual size of the conductor. This defines

    how much current the cable can safely carry.Rule 3

    The third rule is that the cable must be able to safely handle the current without overheating the cable and/or it's

    insulation. This specification can be calculated from the current through the cable and the resistance of the

    cable (which will show how much heat will be generated). This can then be used with further figures relating to

    the type and make up of the cable, the ambient air temperature etc to calculate the cable temperature rise.

    Fortunately this has been made much simpler for us as international standards bodies have drawn up tables

    which show this in a simple tabulated format. One simply looks up the cable size, the table then shows the

    maximum safe current for a cable in free air or a cable in a conduit etc. Even more fortunate for us is that the

    cable suppliers take the worst examples from these tables and specify that as being the current capability for

    each particular wire.For instance 2.5mm2 cable is usually specified by the standards bodies as being suitable for 30 amps in free air

    or 20 amps when in a conduit. The manufacturers therefore specify this size cable as being safe for use up to 20


    Any cable you buy should have the current carrying capacity specified on it's packaging. This is sometimes

    referred to as the cable's "ampacity".

    Staying within this specification ensures that the cable will not be overheated.

    Rule 4

    For volt drop purposes the required cable size in mm2=

    18/((volt drop [volts]*1000/current [amps])/length [metres])

    = 18/((volts*1000/amps)/metres)

    The final rule is one that usually means a much larger cable than that specified by rule 3 must be used. This is

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    almost always the case for low voltage (i.e. 12 or 24 volt) systems.

    The reason for this is that rule 3 takes in to account the possibility of overheating the cable only. Rule 4 is

    regarding the acceptable volt drop down the cable at a certain current. This is usually more of a problem in low

    voltage systems than it is in higher voltage systems.

    An explanation is in order here.

    As stated above, 2.5mm2 cable is specified as being safe up to 20 amps.

    Now suppose we have a 230 volt load drawing 20 amps on the end of 20 metres of this 2.5mm2

    cable. Theresistance of this 40 metres (20 metres each way) of cable is approximately 0.288 ohms. This sounds like

    nothing. Using Ohm's law (V=I*R : V=volts, I=amps, R=resistance) we can calculate the total volt drop to be

    20amps*0.288Ohms = 5.8 volts. So our 230 volt load at the end of the cable will see 224.8 volts instead of 230

    volts. This is well within the specification for a 230 volt supply (the acceptable voltage for a 230 volt supply

    ranges from 216 volts to 253 volts).

    So this cable is perfectly acceptable for a 20 amp 230 volt load.

    However, irrespective of the voltage the system is running at, a 20 amp load will drop 5.8 volts down a total of

    40 metres of this cable. So if our load at the end is a 12 volt load drawing 20 amps then by the time the power

    gets there, it will now be at 12 - 5.8 = 6.2 volts. Obviously this is no use to us whatsoever! The cable, although

    being run within it's rating is dropping too much voltge. It was OK at 230 volts, but no use at all at 12 volts.

    So this leads to the question of "what size cable should I use?"

    The answer is surprisingly simple, and we are somewhat amazed at how often we see the wrong size cable

    being used.

    In fact, the answer is so easy, I'm going to say this again. It's easy!

    There are large, complicated tables available that one can carry round showing the resistance of various sized

    cables, the volt drop per km (or per metre or furlong or whatever) at various current draws etc etc etc. One

    never seems to have one at hand when it is needed.

    Fortunately, since metrication, things have become very simple. This is because the resistance of cable (and

    therefore the volt drop) is directly, inversely, proportional to the cross sectional area of the cable. And the cross

    sectional area of cable is now how they are specified and sold.

    So. Decide on the acceptable volt drop for the job in hand. For instance a 12 volt light really needs to run on a

    minimum of 11.0 volts to operate correctly. So in this case the maximum allowable volt drop is 1.0 volt. In a

    split charge system the cables between various batteries ideally need to drop no more than 0.05 volts in order to

    allow the system to operate at it's best.

    So anyway, decide on the acceptable volt drop in volts.

    Multiply this by 1000.

    Divide this by the current in amps.

    Now divide the result by the actual total cable run length (both positive and negative) in metres.

    Now divide 18 by the result of the above. Hey presto, that is the required cable size in mm2. Obviously most of

    the time this will come out to a silly required wire size so you just choose the next up, standard, available, wire


    Finally choose whichever wire is the largest from rule 3 and rule 4. On low voltage systems rule 4 will almost

    always dictate the wire size. On high voltage systems rule 3 will usually dictate the wire size.

    I told you it was simple. The trick is that the resistance of copper wire is roughly 18/size=Ohms/Km

    cable size [mm2]=18/((volt drop*1000/amps)/metres)

    This rather messy looking formula was kept in this format because it is easy to work with as the above example

    shows. However it is much nicer and easier to remember when rearranged thus:-

    cable size[mm2]=18*metres*amps/(V*1000)

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    NOTE - Always build a safety margin in to the cable sizes. i.e. increase the cable size from the calculated

    size by around 30%. Never try to run cables at their maximum specified current limit.

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    No one seems to agree. Should the electrical system be bonded to the hull or not?

    Earthing in a marine installation

    How should earthing arrangements be made on a boat?

    This is one of the most common questions asked and one of the most misunderstood subjects in the field. Even

    so called "experts" very often get it completely wrong.

    The subject is also, unfortunately, one of the most complicated to answer. Consequently this document is rather

    long.The complexity problems arise because there are so many separate, unrelated, aspects to consider. In order of

    priority they are........

    1. The matter of safety for those on board and for those not on board. The matter of safety for those not on

    board is often overlooked or completely disregarded by the uninitiated.

    2. The matter of galvanic corrosion.

    3. The reliability of the equipment on board.

    Now there are two ways we can answer this question. Some people will not believe a technical argument,

    perhaps because they don't understand it, perhaps because their interpretation of technical matters is incorrect.

    And for those people we will answer the question like this:-

    The AC electrical system earth should be bonded to the hull because:-

    1. The European Recreational Craft Directive says so.

    2. The British Marine Electronics Association "Code of Practice" says so.

    3. The book "The Boat Owners Electrical and Mechanical Manual" by Nigel Calder (a world renowned expert)

    says so.

    4. The ABYC (American Boat and Yacht Council) recommends so.

    How many more references do you need?

    The above people and organisation didn't come to the conclusion that the ground should be bonded to the hull

    on a whim. They came to this conclusion because they spent a lot of time and effort studying every possible

    fault and condition and drawing the conclusion that to bond the ground to the hull is, on balance, far safer than

    to leave the system floating.

    The other way to answer the question is like this:-

    Let's tackle the matter of safety for those on board and those not on board first.

    The first safety matter is obviously that of electric shock. Clearly we have to reduce the risk of this as much as


    Electrical equipment is manufactured to 2 distinct standards. "Single insulated" (Class I) and "double insulated"

    (Class II).

    "Single insulated" equipment is manufactured in such a way that the AC mains electricity is insulated from the

    casing so that the two do not touch. In the event of an internal fault, this may no longer be the case. For instance

    a live cable, inside the equipment, may become loose and contact the case of the equipment. This obviously

    presents a shock hazard if the equipment casing is metal. So the casing is earthed to the green/yellow conductor

    in the power cord.

    In the event of the fault described above, the live cable connects to the case of the equipment, which is

    connected to the earth conductor, which causes an enormous current to flow, which blows the incoming fuse

    thus interrupting the supply.

    That is the safety mechanism of single insulated equipment.

    You will instantly see that the integrity of the earth conductor is vital to the safety of the equipment.

    "Double insulated" equipment is manufactured in such a way that even if the same internal fault arises, the

    cable simply cannot contact anything metal that is accessible from the outside of the equipment. So even if the

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    live conductor becomes detatched inside the equipment, it does not present a shock hazard to the user. "Double

    insulated" equipment, by definition, has two insulation barriers between the electrical parts and the user. This

    usually consists of the insulation on the cables and connections inside the equipment as the first barrier, then an

    insulating case as the second barrier. This equipment usually consists of an entirely non conductive outer

    casing (i.e. plastic).

    "Double insulated" equipment does not require an earth conductor in the power cord.

    These are the two accepted standards throughout Europe (and most of the rest of the world) for safety in AC

    powered equipment."Double insulated" still suffers from the problem that the equipment can still cause an electric shock if it gets

    wet and the water gets to the insides of the equipment. This danger cannot arise with "single insulated"

    equipment as the casing is "held" at the same voltage as the ground upon which we stand by the earth conductor

    in the power cord. Therefore no voltage difference can appear between the case of the equipment and the

    ground. Therefore no current can flow through someone holding the equipment and stood on the ground.

    A vessel with AC and shorepower facilities

    If we now take a look at a boat (let's take a metal hulled boat for this purpose), plugged into shorepower, we can

    instantly see that, from the point of view of someone not on the boat it is a piece of equipment on the end of a

    power cord. We can further see that it is a "single insulated" piece of equipment and therefore must have it's

    casing (the hull) connected to the ground conductor in order to ensure the safety of those outside the equipment(the vessel).

    If the hull is not connected to the ground conductor, and we get the fault of a loose live cable touching the hull,

    we get the situation where the hull is at 230 volts (in Europe) and the ground around it is at 0 volts. It obviously

    depends upon the conductivity of the water (whether fresh or salt water, pollution levels etc) but it is far from

    certain (particularly in fresh water) that this fault will cause sufficient current to blow the fuse on the

    shorepower point.

    Anyone touching the boat and the ground (perhaps climbing aboard) will instantly receive an electric shock.

    Anyone (or anything) swimming in the water will have an enormous voltage differential presented across their

    body which may be sufficient to electrocute them, and even if not, will almost certainly paralyse them causingthem to drown.

    This fact cannot be argued against. The hull must be bonded to the incoming earth conductor in order to ensure

    the safety of those not on board the vessel.

    And I really think this is the crux of the matter. When the safety of those not on the vessel is considered there is

    no argument whatsoever for not bonding the ground to the hull. It is that simple.

    Now it could be argued that the presence of a Residial Current Device will sense this fault and trip thereby

    ensuring the safety of everyone in the vicinity of the vessel however there are a couple of problems here that

    means this cannot be relied upon.

    Firstly, RCDs are not manadatory in many parts of the world (including the UK), so there may not actually beone fitted.

    Secondly, even if one is fitted, whilst it is almost certain that it will trip in salt water, this is not the case in fresh

    water (because freshwater is a much poorer conductor than salt water). Especially on a small boat, or one with

    most of the underwater steelwork painted, or a GRP or wooden boat with some underwater metalwork.

    Many people argue that the AC system should not be bonded to the earth conductor (for the reason that it can

    cause galvanic corrosion problems which we will come to later), but when presented with the above scenario,

    hold their head in shame and admit that it is something they had never even considered. They had only

    considered the safety of those on board the vessel.

    That one argument alone confirms that, in the case of a vessel that has the facility to use shorepower, the AC

    ground must be bonded to the hull. To not bond them is leaving oneself open to electrocution of people(perhaps oneself or one's own family), sleepless nights in the event it should happen, litigation and possibly

    even criminal proceedings for manslaughter for ignoring all the codes of practice, guidlines and actual laws

    which state, quite categorically, that they should be bonded.

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    A vessel with an AC system but no facilities to use shorepower

    The power in this case would come from either a generator or an inverter. It is still 230 volt (in Europe) power,

    it is just as dangerous as shorepower.

    However in this case the risk to those not on board does not exist.

    There are many possible causes of electric shock, the most common being someone touching live and earth at

    the same time. Now in the case of a generator or inverter, if the system is totally isolated from the hull (which

    we can do in this case as the risk to those not on board no longer exists) this danger no longer exists.

    However, if the system is totally isolated from the hull, and a fault arises that connects live to the hull

    somewhere in the installation, no resultant problem will ensue. The system will continue to operate perfectly......

    until someone touches the hull (which is at 230 volts) and the case of a "single insulated" piece of equipment

    (which is at 0 volts) at the same time. They will receive an electric shock.

    Had the AC system had it's earth conductor bonded to the hull this situation could not arise because as soon as

    the live cable touched the hull either the fuse would blow or the inverter or generator would cut-out having

    detected an overload.

    Now in order to ensure full safety, we need to also bond neutral and earth at the output of the inverter and/or

    generator and install RCCDs on the outputs of the inverter and/or generator.

    Either way, we still need to bond the AC system to the hull.

    AC in general

    So whether the system can use shorepower or not, if we have AC on board, the earth conductormust be bonded

    to the hull.

    DC system with no AC on board

    From the point of view of safety it makes no difference whether or not the DC system is bonded to the hull.

    From the point of view of galvanic corrosion it makes no difference whether or not the DC system is bonded to

    the hull (but the hull must never be used as a return path in the manner of vehicle wiring).

    However, if the DC positive side is bonded to the hull this can have huge implications for galvanic corrosoion.

    Remember, the most positive (voltage wise) point will be the point that erodes. The negative point will receive

    a plating from the most positive point.

    This diagram shows a single battery, with

    the negative side bonded to the hull. It

    also shows a load switch, a load and two

    areas of dampness represented by the

    lines with arrowheads at each end.

    No current will flow through the damp

    area at A as both ends are held at the

    same voltage by the bonding to the hull.

    Current will flow through the damp area

    at B and, due to electrolysis, the wire will

    erode (as it is more positive) and plate

    the section of the hull at B (which is

    more negative) with a small amount of


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    This diagram shows exactly the same

    installation but this time the positive side

    has been bonded to the hull.

    This time no current will flow through

    the damp area at B because both ends are

    held at the same voltage by the bonding

    to the hull.

    Current flows through the damp area at

    A, but this time the hull will be eroded

    (as it is more positive) and plate the wire

    (as it is more negative) with a small

    amount of steel from area A.

    This is stray current erosion. The hull is being eaten away. Purely and simply by bonding the positive to the hull

    instead of bonding the negative.

    If the DC system is not bonded to the hull at all then obviously this cannot happen.

    However, if the positive becomes bonded as a result of a fault (a frayed wire perhaps), the system will continue

    to operate, the fault will not show itself with any symptoms. So the vessel now has a positive bonded system

    without the owner knowing anything about it. And with it, the vessel also has all the problems of a positive

    bonded DC system - i.e. greatly accelerated stray current erosion.

    There are two options to protect against this....

    1. Bond the negative to the hull. If a fault occurs that attempts to bond the positive to the hull, the main fuse

    will blow, alerting the owner to the problem.

    2. Keep the system isolated from the hull and fit a device that detects if the positive somehow becomes bonded

    to the hull. I am not aware of any such device and do not see a market big enough to warrant designing one.

    So even with a DC system and no AC system, bonding the DC system to the hull is still required. Without doing

    so, you may find you have inadvertently created a positive grounded system.

    AC system and DC system

    If both electrical systems are installed then all of the above applies. i.e. it is imperative that both systems be

    bonded to the hull.

    There is also another scenario in this case to further convince the doubters.

    Assume the AC system is bonded to the hull but the DC system is not.

    Some equipment is connected to both systems. This sounds rare - in actual fact it isn't, dual voltage fridges,battery chargers and inverters are all connected to both systems.

    A fault in one of these items could cause AC mains to be presented to the DC side. If both systems are bonded

    to the hull, this will instantly cause the incoming fuses or circuit breakers to blow.

    If one of the systems is isolated from the hull this will not happen. The result will be that the DC system (which

    we all assume is safe to touch, and which usually has components with insulation rated for about 50 volts) will

    be sat at 230 volts with respect to the hull or the other electrical system. Clearly this is highly dangerous.

    In summary, whatever electrical system is fitted, it is imperative that the system is bonded to the hull.

    Galvanic corrosion problems as a result of bonding the AC system to the hull

    When plugged into shorepower, and the AC system ground is bonded to the hull, the quayside, other boats and

    your boat creates a battery which, 9 times out of ten, causes a current to flow in such a direction that your hull

    erodes. This is galvanic erosion.

    There are 2 simple remedies to this problem. One is to fit an isolation transformer, the other is to fit a galvanic

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    isolator. Both will cure the problem.

    Bonding the DC system cannot affect this.

    All bonding should be done at one central point. It is not acceptable to bond various parts of the system in

    various separate places. This can cause voltage differentials between various parts of the hull which can lead to

    stray current erosion.

    Finally, a few people with steel hulled narrowboats have mentioned that the resistance between the hull (which

    is usually almost completely bare steel in a narrowboat) and the actual ground (of the world) is so low (even in

    fresh water) that an RCD will always trip in the event of a live-earth fault and in fact some have gone on to saythat the resistance is so low that a main circuit breaker or fuse will blow.

    We accept that this is true most of the time however there are narrowboats out there which have completely or

    almost completely painted hulls and in this case is it far from certain that an RCD or circuit breaker will blow.

    Further, narrowboat owners seem to forget that not all boats have steel hulls. Many are fibreglass or wooden.

    Some are carbon composite etc. One has to think further afield than "my boat". These regulations and

    guidelines have to coverall boats not just one or two!

    There is a further discusion of galvanic corrosionhere.
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    How to correctly interconnect multiple batteries to form one larger bank.

    Two things I have noticed in my (more than) 20 years in this business are that:-

    A. Many "specialists" simply tell you..... "do it this way, this is the correct way" without ever showing why they

    consider it to be the correct way, and often it isn't, which is perhaps why they couldn't show you why it is(!)

    B. Some things have been done for so long, in a certain manner, that it seems they must be the best way of

    doing it. Otherwise why hasn't another method appeared?

    Here at SmartGauge Electronics we always show you why one method is better. We don't expect you to takeour word for it. We will happily use practical examples, theory, maths or whatever else it takes to show the

    results of various ways of doing things.

    Interconnecting multiple batteries to form one larger bank is one case in point. Though in this case, newer

    methods have emerged over the years. Unfortunatley they still aren't perfect.

    Here is a diagram showing the old way of interconnecting 4 batteries to form one larger bank. This is a method

    that we still see in many installations.

    Method 1

    Notice that the connections to the main installation are all taken

    from one end, i.e. from the end battery.

    The interconnecting leads will have some resistance. It will be

    low, but it still exists, and at the level of charge and discharge

    currents we see in these installations, the resistance will be

    significant in that it will have a measurable effect.

    Typically the batteries are linked together with 35mm cable in a

    good installation (often much smaller in a poor installation).

    35mm copper cable has a resistance of around 0.0006 Ohms

    per metre so the 20cm length between each battery will have a

    resistance of 0.00012 Ohms. This, admittedly, is close to

    nothing. But add onto this the 0.0002 Ohms for each

    connection interface (i.e. cable to crimp, crimp to battery post

    etc) and we find that the resistance between each battery post is

    around 0.0015 Ohms.

    If we draw 100 amps from this battery bank we will effectively be drawing 25 amps from each battery. Or so

    we think.

    In actual fact what we find is that more current is drawn from the bottom battery, with the current draw getting

    progressively less as we get towards the top of the diagram.

    The effect is greater than would be expected.

    Whilst this diagram looks simple, the calculation is incredibly difficult to do completely because the internal

    resistance of the batteries affects the outcome so much.

    However look at where the load would be connected. It is clear that the power coming from the bottom battery

    only has to travel through the main connection leads. The power from the next battery up has to travel throughthe same main connection leads but in addition also has to travel through the 2 interconnecting leads to the next

    battery. The next battery up has to go through 4 sets of interconnecting leads. The top one has to go through 6

    sets of interconnecting leads. So the top battery will be providing much less current than the bottom battery.

    During charging exactly the same thing happens, the bottom battery gets charged with a higher current than the

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    top battery.

    The result is that the bottom battery is worked harder, discharged harder, charged harder. It fails earlier. The

    batteries are not being treated equally.

    Now in all fairness, many people say "but the difference is negligible, the resistances are so small, so the effect

    will also be small".

    The problem is that in very low resistance circuits (as we have here) huge differences in current can be

    produced by tiny variations in battery voltage. I'm not going to produce the calculations here because they

    really are quite horrific. I actually used a PC based simulator to produce these results because it is simply tootime consuming to do them by hand.

    Battery internal resistance = 0.02 Ohms

    Interconnecting lead resistance = 0.0015 Ohms per link

    Total load on batteries = 100 amps

    The bottom battery provides 35.9 amps of this.

    The next battery up provides 26.2 amps.

    The next battery up provides 20.4 amps.

    The top battery provides 17.8 amps.

    So the bottom battery provides over twice the current of the top battery.This is an enormous imbalance between the batteries. The bottom battery is being worked over twice as hard as

    the top battery. The effects of this are rather complex and do not mean that the life of the bottom battery will be

    half that of the top battery, because as the bottom battery loses capacity quicker (due to it being worked harder)

    the other three batteries will start to take more of the load. But the nett effect is that the battery bank, as a

    whole, ages much quicker than with proper balancing.

    I have to be honest now and say that when I first did this calculation in about 1990 I completely refused to

    believe the results. The results seemed so exaggerated. So much so that I wired up a battery bank and did the

    experiment for real, taking real measurements. The calculations were indeed correct.

    Method 2

    All that has changed in this diagram is that the main feeds to

    the rest of the installation are now taken from diagonally

    opposite posts.

    It is simple to achieve but the difference in the results are truly

    astounding for such a simple modification.

    The connecting leads, in fact, everything else in the installation

    remains identical.

    Also, it doesn't matter which lead (positive or negative) is

    moved, Whichever is easiest is the correct one to move.

    The results of this modification, when compared to the original

    diagram are shown below. Only that one single connection has

    been moved.

    After this simple modification, with the same 100 amp load....

    The bottom battery provides 26.7 amps of this.The next battery up provides 23.2 amps.

    The next battery up provides 23.2 amps.

    The top battery provides 26.7 amps.

    This is quite clearly a massive improvement over the first method. The batteries are much closer to being

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    correctly balanced. However they are still not perfectly balanced.

    How far is it necessary to go to get the matching equal?

    Well, the better the quality of the batteries, the more important it becomes. The lower the internal resistance of

    the batteries, the more important it is to get them properly balanced.

    So that now leaves the question of whether or not there is a wiring method to perfectly balance the batteries.

    Before getting to that, it should be pointed out that doing the calculation is not actually required in order to

    arrive at the ultimate interconnection method. I simply did them to show the magnitude of the problem.In order to get a better balancing it is simply necessary to get the number of interconnecting links as close as

    equal between each battery and the final loads.

    In the first example the power from the bottom battery passed through no interconnecting links. The top battery

    passed through 6 links.

    In the 2nd example (the much improved one), the power from the top and bottom battery both passed through a

    total of 3 links. That from the middle 2 batteries also both passed through 3 links which begs the question "why

    were they not therefore perfectly balanced?". The answer is that some of the links have to pass more total

    current and this therefore increases the voltage drop along their length.

    And now we get to the correctly wired version where all the batteries are perfectly balanced.

    Method 3

    This looks more complicated.

    It is actually quite simple to achieve but

    requires two extra interconnectng links and

    two terminal posts.

    Note that it is important that all 4 links on

    each side are the same length otherwise oneof the main benefits (that of equal resistance

    between each battery and the loads) is lost.

    The difference in results between this and

    the 2nd example are much smaller than the

    differences between the 1st and 2nd (which

    are enormous) but with expensive batteries

    it might be worth the additional work. Most

    people (myself included) don't consider the

    expense and time to be worthwhile unless

    expensive batteries are being fitted or if thenumber of batteries exceeds 8.

    This method isn't always so easy to install because of the required terminal posts. In some installations there is

    simply no room to fit these. So, thanks to a colleague, we can also present another wiring method that achieves

    perfect battery balancing...............

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    Method 4

    And here it is.

    This looks odd but it's actually quite simple.

    What has been done here is to start with 2

    pairs of batteries. Each wired in the proper

    "cross diagonal" method. Then each pair is

    wired together, again in the cross diagonal


    Notice that for each individual battery, the

    current always goes through a total of one

    long link and one short link before reaching

    the loads.

    This method also achieves perfect balance

    between all 4 batteries and may be easier to

    wire up in some installations. Many thanks

    to "smileypete" from for this idea.

    There really is no excuse whatsoever (apart from, perhaps, incompetence or laziness) for using the first example

    given at the top of this page.

    The other three methods achieve much better balancing with the final two achieving perfect balancing between

    all four batteries.I think I am right in saying that this is the only example I have ever come across where doing something the

    correct way actually looks less elegant than doing it incorrectly.

    Finally, if you only have 2 batteries, then simply linking them together and taking the main feeds from

    diagonally opposite corners cannot be improved upon.

    Once the number of batteries gets to 3 or more then these other methods have to be looked at.

    With a large number of batteries it may be necessary to go to the 3rd method shown above.

    Even with 8 batteries it is possible to get reasonable balancing by placing the main "take off" feeds from

    somewhere down the chain instead of from the end batteries. Remember, count the number of links each battery

    needs to run through to reach the final loads and get these as equal as possible.

    Finally, if your battery bank has various take off points on different batteries, change it now! It is extremely bad

    practice. Not only does it mess up the battery balancing, it also makes trouble shooting very much more

    complicated and looks awful.

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    And finally, finally, we keep getting asked where the chargers should be connected to. We didn't address this

    question because it seemed so blatantly obvious where they should be connected that it never occurred to us

    that anyone might be unsure. The chargers should always be connected to the same points as the loads. Without


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    Narrowboat AC Electrical systems.

    We used to get called in to cure all sorts of installation problems on narrowboats. Often the installation started

    as one system, then over the years had been the recipient of so many changes and modifications that the final

    system was often very complicated to operate, unreliable and in many cases downright dangerous.

    There really is no need for this if a logical approach is taken to the installation.

    This page is not intended as a concise education in AC electrical installations. That would require an entire

    book. It is intended as a starting point in order to allow the various options to be considered. Until further notice

    it should be considered as a work in progress and will be added to periodically.

    If you have a particularly unique system please tell us about it. If we consider it of wider appeal we would love

    to include it within these pages.

    This first system usually consists of nothing more than a shorepower facility. Even this can be installed

    incorrectly. There are 4 main points that must be adhered to.

    1. Despite what some people think (they are wrong), the AC ground must be bonded to the hull. See here for an

    explanation of why.

    2. The installation must include one or more RCDs that protect all circuits.

    3. The cabling must be protected by the correct size fuses or circuit breakers.4. All components used must be of the correct type, i.e. correct ratings and manufactured specifically for use

    with 230 volt systems.

    So, that having been said let's look at the most basic system. A shorepower plug (yes it is indeed a plug not a

    socket) feeding the boat AC system.

    Note the electrical position of the galvanic isolator and the earth/hull bonding. Anywhere else is wrong.

    Here is exactly the same system but using an isolation transformer instead of a galvanic isolator.
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    Another possible installation uses just an inverter with no facility for shorepower. Let's now take a look at this

    installation. This same installation is also applicable to a generator instead of an inverter.

    Note that no galvanic isolator is required in this case.

    Often it is desirable to have an inverter for use when away from the mooring and also shorepower for use when

    in the marina. This then complicates matters in that some means to switch between the two is required.

    There are probably more ways to achieve this than there ways to skin a cat. The most basic method is to have a

    plug feeding the boat electrical system which is simply plugged into shorepower or the inverter. This is far from

    ideal. Usually a more permanent solution is sought.

    One possible problem that has to be guarded against is any possibility of shorepower finding it's way to the

    output of the inverter. This would, in nearly all cases, totally destroy the inverter, instantly, whether or not it is

    switched on. Partly for this reason both the Live and the Neutral have to be switched. Also "break before make"

    switches must be used.

    Here is one way of switching between shore power and inverter.

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    And here is exactly the same system but using an isolation transformer instead of a galvanic isolator.

    This method uses a manual change-over switch to select between shorepower or inverter. It is simple and

    reliable. The only downside is that it requires user intervention to select between the two. This does not bother

    many people.

    Note that in this diagram there is also a separate feed for running equipment that would, if run from the inverter,

    quickly flatten the batteries. This feed only receives power from the shore-line. It cannot be powered by the

    inverter. Obviously a battery charger would be connected to this feed.

    Note that as a result of having 2 separate circuits (one for loads that may run from the inverter and a separate

    one that only runs from shoreline), it is necessary to install 2 RCDs.

    RCDs are quite expensive, so another way of achieving the same result (i.e. having a separate circuit fed by

    shoreline only) is to use a 4 pole switch, which only connects certain loads when in the position for shoreline.

    Here is the diagram for such a system. Thanks to John Gwalter for this idea. Whilst this idea is simple, we were

    very impressed with it. It shows great ingenuity.

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    And, again, here is the same system using an isolation transformer instead of a galvanic isolator.

    In the two systems shown directly above the shoreline and inverter both feed into the manual changeover

    switch in the usual manner. The output from the changeover switch then feeds into a single RCD and on to the

    rest of the boat. The output from the RCD also feeds back to the other two sets of contacts in the switch so that

    the final power feed is only "live" when the switch is in the "shorepower" position.

    This has the advantage that only one RCD is required. The downside is that a more expensive switch is needed

    and the wiring is slightly more complicated.

    For many people, the manual changeover switch is an inconvenience. The job can be automated with the use of

    a relay having a 230 volt coil. Here is the diagram for such a system. It is essentially the same as the first

    manual system above. It also uses 2 RCDs for full safety protection.

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    And, again, with an isolation transformer instead of a galvanic isloator.

    Note that, as is usual practice, the relay is shown with the contacts in the resting position.

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    And in the same way that the manual system can be modified to use a single RCD by using a 4 pole switch, the

    automatic system can also be modified thus.

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    And the same again, but with an isolation transformer instead of a galvanic isolator.

    This is effectively the same as the single RCD manual changeover system but automated with the use of a relay.

    The diagrams above are in no way exhaustive. There are many other ways of achieving the same result. But

    often they are overcomplicated and achieve nothing more than those above.

    The diagrams shown here can be expanded. For instance an installation may also have a generator. The usual

    practice would then be to use another changeover switch (either manual or automatic) to select between

    shorepower or generator, then the second switch selects between this or the inverter. Many other options are

    possible. But the systems shown above are a good starting point.

    Finally here is the system as installed on Lionheart No 2

    This looks complicated. In actual fact it's quite simple but a few things might be worth noting.

    We often have our kids on the boat. This means things like computer games, X-Boxes, Playstations, DVD

    players etc. We often have 2 or more TVs on at the same time. When the kids are with us, the idea of no 230

    volt electricity doesn't bear thinking about. For this reason there are 2 inverters. If one breaks, we can plug into

    the other one and keep the kids quiet.

    As we needed 2 inverters it made sense to split the loads between them. So we have one feeding the usual

    sockets round the boat. The other one feeds the tumble dryer and washing machine.

    As the loads are split we have quite small inverters. They are both 1500 watt, pure sinewave.

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    One word of warning. Do not be tempted to overcomplicate things. Try to keep it as simple as possible.

    Simplicity in this case is usually elegant. And an elegant design usually works better than anything else.

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    Which is the best choice? A Galvanic Isolator or an Isolation Transformer?

    Until relatively recently this was quite a simple question. With a simple set of answers.

    It came down to this.

    Galvanic isolator.



    Very simple to install


    Unit can fail without symptoms thus requiring regular checking. The failure could either endanger life by

    failing open circuit or remove the galvanic protection by failing short circuit. In fairness none of these failures

    are likely unless there is a large fault which blows the shorepower main circuit breaker. But the possibility does

    exist. The solution is to test the galvanic isolator after any problem with the AC system has been rectified, plus

    periodic checking.

    Isolation Transformer


    The possibility of symptomless failure is almost zero. A failure will usually result in failure of the AC supply.


    More Expensive


    More complicated to install

    And that was it. The choice was made on those points alone.

    The goalposts have now been moved and the decision is no longer this straightforward.

    In order to understand why, it is necessary to look at how each device breaks the connection between the

    shorepower earth and the hull of the boat.

    Here is the internal schematic of a standard galvanic isolator.

    It goes in-line with the earth lead between the shorepower plug on the quayside and the AC electrical system on

    the boat. Each diode in the isolator will drop around 0.6 volts before it starts to conduct. So 2 in series will

    require 1.2 volts before any conduction takes place. There are two diodes facing one way, and two facing the

    other way to enable AC fault currents to flow in both directions (thus tripping the circuit breaker or RCD).

    This means that the galvanic currents (which are usually between 0.4 and 0.8 volts) which cause galvanic

    corrosion are blocked from flowing. They would have to exceed 1.2 volts in order for a current to flow. They

    never exceed this level due to the metals involved and the water. This is the protection method to prevent

    galvanic corrosion. This is how they work.

    Now let's look at an isolation transformer. This is the correct way of wiring up such a device. You may see

    diagrams of them wired up in a different way. They are wrong.

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    Note that there is simply no connection between the shorepower earth and the earth on the boat. This is what

    protects the vessel from galvanic corrosion.

    Note also that the Neutral and Earth are bonded on the output side of the transformer. This recreates exactly the

    same sitution as we normally receive from the national grid and is what allows fuses and RCDs to operate in the

    correct manner.Finally note that there is a "safety screen" between the primary winding and the secondary winding which is

    connected to shorepower earth. This is so that any fault current in the primary (for instance from insulation

    breakdown) is returned to shorepower earth to trip the circuit breaker or RCD rather than electrifying the boat


    In effect, the transformer recreates a totally isolated AC supply just like having your own mini AC power

    station. Totally isolated from anythng else.

    So that explains how each device protects against galvanic currents which cause galvanic corrosion. Now we

    need to move on to explain what has changed that now makes isolation transformers a better choice.

    Let's take a look at the galvanaic isolator again when installed in a typical installation from some years ago and

    see what happens.

    This diagram shows shorepower operating some form of load. The load is the bit within the green box. It

    doesn't matter what, it could be an old battery charger, a table lamp, more or less anything. Nothng is connected

    to earth. Nothing changes. The galvanic isolator continues to do it's job of protecting against galvanic currents

    just like it was designed to do.

    The problem these days is that most equipment isn't built like this any more. A lot of equipment uses switch

    mode power supplies which generate lots of radio frequency interference (RFI). Under CE regulations this RFI

    is not permitted, it has to be got rid of. One of the easiest ways to get rid of it is to divert it to earth. And this is

    what much equipment these days does.

    So now we have to modify the diagram to this:-

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    The load remains within the green box.

    Note the red box. This now contains two capacitors. These are components that conduct AC but not DC. The

    higher the frequency of the AC, the more they conduct (this is a very basic summary of what a capacitor does)

    Their purpose in this case is to conduct any unwanted RFI down to ground where it is safely got rid of instead

    of it radiating everywhere and interfering with TV, Radio, mobile telephones etc.

    The problem is that in order for this to happen, they can cause the galvanic isolator to conduct the RFI to

    ground, so the galvanic isolator is almost permanently conducting.

    This almost certainly won't happen in saltwater as the parallel path of the water will conduct most of this AC to

    earth. However in freshwater it is a possibility.

    The capacitors are thus trying to place an AC voltage on the earth conductor. Normally they cannot do this as it

    is earthed. But in the case where a galvanic isolator is fitted they can put a small AC voltage across the galvanic

    isolator. Just sufficient to make it conduct. This is a mixture of line frequency AC (50Hz in Europe) and the RFI

    from the switch mode power supplies.

    These AC voltages are not so much of a problem for galvanic corrosion. But it means that the galvanic isolator

    is almost permanently conducting. And therefore it is not blocking the DC galvanic currents.

    This has become more of a problem as more and more equipment has started to be built with these components

    in place. And today this includes just about every piece of equipment you can buy. Please note that for many

    years certain items have always had these capacitors in place. However in recent years just about everythinguses them and they are also of a much higher value than they used to be which causes greater currents to flow,

    the equipment attempts to put higher voltages across the galvanic isolator and thus force it harder into


    And the more equipment of this type is installed, the greater the problem becomes.

    The capacitors used are of a very low value and the currents introduced are very small. In fact there are limits

    defined in CE, UL and CSA Standards for each item of equipment. But this doesn't prevent 10 pieces of

    equipment causing 10 times more current!

    (As an aside it is interesting to note that the problems caused by these capacitors [i.e. AC currents flowing in the

    earth conductor] has become so great that in many instances, with several items of equipment, they are

    sufficient to cause RCDs to trip in the absence of any fault!)

    For this reason, some galvanic isolator manufacturers have put a capacitor inside the galvanic isolator so that

    the capacitor conducts the AC currents thus preventing the diodes from conducting. This idea would work, if

    the capacitor was the correct size.

    However, whilst this could be done to cope with the currents produced by one piece of equipment, it cannot be

    done for those produced by, say, 15 separate items installed round the boat. For technical reasons, there is a

    limit to the size that this capacitor can be. And it would be necessary to exceed this limit in order to keep the

    AC voltage below the "conduction" voltage of the galvanic isolator. Unless the AC voltage is held below this

    level, there is no point in fitting the capacitor to the galvanic isolator. It will not achieve anything.

    The only conclusion is that if an installation has enough of this "modern" equipment to cause AC voltagesacross the galvanic isolator that are sufficient to cause it to conduct then the galvanic isolator is not doing it's

    job properly.

    An important note is in order here. Some equipment is manufactured to a standard known as "Double Insulated"

    or "Class II". This equipment has no ground conductor in the power cord. They use a 2 core cable. This

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    equipment cearly cannot put any RFI voltage on the earth conductor. Or can it?

    Tests show that it can actually do so due to the capacitance in the AC wiring. The amount of RFI introduced by

    such equipment is very much less, but with enough of them connected could still be sufficient to cause a

    problem. Admittedly there would have to be a lot of equipment.

    Further, if this equipment is also connected to the DC system (dual voltage equipment, battery chargers etc)

    then it is highly likely that a similar set of capacitors are fitted to divert the RFI down to the DC ground which

    will (or rather should) also be bonded to the AC ground. Again the effect will not be as great but it can still

    cause a problem and perhaps more than class II equipment that is not connected to the DC system.Now someone is going to ask "is there a way to measure whether this is happening? How do I know if my

    galvanic isolator is doing it's job?"

    And that's a perfectly fair question. Yes there is a test, of course there is. It isn't a difficult test however you do

    need to know what you are doing, you would need an oscilloscope and due to the fact that it involves testing on

    the AC system whilst is it live I cannot go into it here. Anyone with the knowledge and equipment to do such a

    test will know from this paragraph what needs to be done.

    Finally there is one more issue that can cause problems for galvanic isolators. That of the incoming earth lead

    conductor having a voltage on it. Tests show that this is actually far more common than many people would

    believe. Not a dangerous (to life) voltage but a voltage sufficient to force a galvanic isolator into conduction.

    There are serveral causes for this voltage being present on the earth conductor but those reasons are not relevanthere. The test is simply to measure the voltage between the earth conductor and the actual ground under your

    feet using a suitable earth rod. Any voltage present here is reducing the headroom (and thus effectiveness) of

    the galvanic isolator.

    The only alternative is to fit an isolation transformer.

    And just for those who think this all sounds very biased: We do not make, sell or install either galvanic isolators

    or isolation transfomers.

    And finally, finally, we have been asked to point out one possible safety issue with isolation transformers.......

    The ideal physical (from a safety point of view) position for an isolation transformer is on the quayside with the

    (now) isolated AC power feed running from the quayside to the boat. However in many cases this is simply notat all practical. This means the transformer has to be installed in the boat which leaves the problem that the

    incoming shorelead runs from the quayside to the boat and is not grounded to the hull.

    Some argue that this defeats the whole argument that the incoming AC must be bonded to the hull. To a certain

    extent this is correct however the only possible point of fault is where the shorelead passes onto the boat and

    down to the transformer. Whereas normally the entire electrical system has to be contended with.

    Without the isolation transformer there are possibly hundreds of different places around the vessel that could

    result in a live/hull fault. Behind plug sockets, in distribution panels etc. With an isolation transformer this is

    reduced to the feed from the shorepower socket to the transformer itself.

    Remember that AC mains on board can never be made 100% safe. That simply isn't possible. We have toaccept this and make the system as safe as is practically possible.

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    Wiring and installing an isolation transformer - Page 1.

    Isolation transformers are installed into boats to isolate the vessel "ground" from the actual "ground" (i.e. the

    earth) in order to prevent galvanic corrosion. There are further explanations of the reasons hereand here.

    Assuming you have made the decision that an isolation transformer is what you intend to fit, then it is necessary

    to install it correctly so that it, in order of priority, A) Is safe, B) Does the required job of isolating the hull from

    the shore earth and C) Is reliable.

    Well surely everyone agrees with these points, so why should there be any confusion about the best way to wire

    one up?

    Well, it seems to come down to risk assessment. AC mains electricity can never be made 100% safe. That is an

    impossibility. So we have to make it "as safe as possible". And it seems this is where the confusion lies.

    Everyone agrees about how to wire up the primary (to the shoreline live and neutral), every one agrees about

    how to wire up the secondary (to the boat live and neutral). The different viewpoints come about with regard to

    what to do with the earth connections.

    Firstly there is the question of whether the secondary side should recreate a neutral-earth bond (thus imitating

    what we get from the national grid).

    This is quite simple to answer. If no RCDs (Residual Current Device - known as GFCIs in USA) are fitted, then

    it is safer to not bond the neutral and earth on the secondary (output) side. If RCDs are fitted then in order forthem to operate, the neutral-earth bond must be made.

    As many installations will be subject to regulatory guidelines/legislation it is often a requirement that RCDs

    are fitted. That being the case the neutral-earth bond has to be made. They simply do not operate as designed

    without the neutral-earth bond.

    From the safety point of view, it is debatable which system is safest (and again with 2 schools of thought),

    RCDs and neutral earth bond, or a floating system i.e. one without the neutral-earth bond. This is a subject for a

    different webpage! For the record we believe a neutral-earth bond with RCDs offers the best protection.

    This leaves the question of what to do with the earth connection and safety screen on the transformer.

    Let's have a look at the isolation transformer with the primary and secondary already wired. Note that we havemade the neutral-earth bond in this case as most installations will be using RCDs and thus require the bond.

    The question is: What do we do with the safety screen/chassis connection in order to ensure the best safety?

    Note that this will only ever affect the outcome in the event of a fault. In the absence of a fault, it makes no

    difference whether the safety screen is connected to the shorepower earth, the boat earth or indeed left


    It seems that in order to ensure we connect the safety screen to the best place, we will have to look at possible

    faults that may ocurr, and connect the screen to whichever point ensures the best safety, in the majority of cases.

    This really is the best that we can do.
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    So let's list some possible faults, in general, then later we can look at them in further detail and try to ascertain

    which are most dangerous, which are most likely etc.

    1. Actual transformer faults. i.e. short or open circuits between various parts of the transformer.

    2. Wiring faults. i.e. loose wires in the installation, short circuits in the wiring, open circuits etc

    3. Water in the transformer or it's enclosure.

    These cover more or less all possible faults that can ocurr with an isolation transformer.

    Now, as a practicing engineer, I can state quite categorically that an actual transformer fault is far less likely

    than either of the other two. There is absolutely no question in my mind about this. I doubt, very much, manypeople would debate this point.

    So this leaves the other two possible types of fault. The relative probability of these depend upon the type

    vessel. For instance water inside the transformer enclosure is probably more likely in a small ocean going

    vessel than a large one or a vessel on the canal system. Faulty wiring is probably more likely on a vessel owned

    and maintained by a private individual than by professionals (some people would debate this!).

    Rather than argue this point any further, let's give them an equal probability. Water in the enclosure or faulty

    wiring have roughly the same probability of ocurring and both carry a much greater probability than a fault in

    the actual transformer itself.

    So the first faults (i.e. the most likely) that we need to guard against are faulty wiring and water ingress.Let's look at the transformer again, note that this time there are 4 points labelled A, B, C and D. We have also

    connected the transformer safety screen/chassis to the shoreline earth.

    Now imagine a fault as a result of a wire coming loose and each one in turn shorting to the transformer chassis

    at each of the four points A, B, C and D. Let's take them one at a time.

    1. The wire at point A comes loose from it's terminal connection on the transformer and touches the transformerchassis.

    You can instantly see that this will effectively short circuit the incoming live to earth. This will cause an

    enormous current to flow and blow the fuse on the shorepower socket.

    2. The wire at point B comes loose from it's terminal connection on the transformer and touches the transformer


    This will do nothing to blow the incoming fuse. If an RCD is fitted to the shorepower socket it will trip it

    becasue the primary current will now be returned down the earth connection instead of neutral thus causing an

    imbalance between the live and neutral currents. This is what trips RCDs. Either way, there is no immediate

    danger presented to those on or off the vessel.3. The wire at point C comes loose from it's terminal connection on the transformer and touches the transformer


    This will do nothing other than the boat will no longer have AC electricity. The live output of the transformer is

    now isolated, the actual wire that has touched the case is dead.

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    4. The wire at point D comes loose from it's terminal connection on the transformer and touches the transformer


    This will do nothing immediate. Electrically it is the same as the diagram above shows. The only problem is

    that the arcing may present a fire hazard.

    Now let's contrast the above faults with what happens in exactly the same situtations but with the transformer

    chassis and screen returned to the boat earth. Here is the relevant diagram. Note that the only thing that has

    changed is where the transformer chassis is earthed.

    1. The wire at point A comes loose from it's terminal connection on the transformer and touches the transformer


    The hull of the boat now has 230 volts on it with respect to the actual ground outside. As this current has to

    travel through the water to return to earth, it is far from certain (particularly in fresh water) that sufficient

    current will flow to blow the incoming shorepower fuse. Obviously this situation is highly dangerous. If an

    RCD is fitted to the shorepower then this may well trip, but again it is far from certain. It is however highlylikely.

    2. The wire at point B comes loose from it's terminal connection on the transformer and touches the transformer


    Apart from the boat losing AC power this will do nothing. If an RCD is fitted to shorepower it may trip. It may


    3. The wire at point C comes loose from it's terminal connection on the transformer and touches the transformer


    The boat will lose 230 volt power. No fuses will blow, no RCDs will trip. No danger exists.

    4. The wire at point D comes loose from it's terminal connection on the transformer and touches the transformerchassis.

    Loads on the boat will draw their current through this new (faulty) connection which may present a fire hazard.

    This is the only danger.

    From the above it seems clear to me that to return the earth to the shorepower presents less danger than to

    return it to the vessel earth. Add up the possible dangers. There are more, and they are greater with the chassis

    earthed to the boat earth than when earthed to the shorepower earth.

    With the transformer chassis earthed to the boat, there exists one scenario above where the hull of the vessel

    can become live. Obviously this is highly dangerous. No such danger exists with the chassis earthed to the

    shorepower earth.To analyse the problem of water ingress, exactly the same procedure can be used but in this case instead of

    assuming there exists a short circuit, treat the fault as being a resistor. And also remember that the connection to

    the transformer remains intact.

    Let's run through these.............

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    Wiring and installing an isolation transformer - Page 2.

    Here is the diagram again with the chassis earthed to the shoreline earth.

    Now let's consider water between each of the points A, B, C and D and the transformer chassis and enclosure.

    Water between point A and chassis.

    This may or may not blow the incoming fuse (almost certainly not in the case of fresh water). It may or may not

    trip the incoming RCD (if fitted). No immediate danger exists as the case is safely held at earth potential i.e. 0


    Water between point B and chassis.

    This will either do nothing or possibly trip the incoming RCD (if fitted).

    Water between point C and chassis.

    This is a difficult one! The water will attempt to ground the live side of the secondary to the shorepower earth

    (albeit through a resistor - the water in the transformer enclosure). This will therefore try to put a voltage on the

    hull of the vessel. The ratio of the voltages between the hull and real ground and that between the secondary

    live and real ground depends entirely upon the ratio of the resistance of the two water paths. It is a fair

    assumption that the resistance between the hull and real ground will be substantially lower than that between

    the secondary live and the shorepower earth. The voltage between the hull and real earth will be lower by the

    same ratio. No circuit fuses will blow and no RCDs will trip.

    Water between point D and chassis.

    Nothing, no symptoms, no danger.

    Now let's run through the same faults with the chassis returned to the boat earth.

    Here is the diagram again.

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    Water between point A and chassis.

    This produces the same final effect as the the last fault described above but with one major difference. The

    water will attempt to make the hull of the boat live (albeit through a resistor - the water in the transformer

    enclosure). The hull will try to prevent this due to the conductivity of the water that the boat is floating in. Theratio of the voltages between the hull and real ground and that between the primary live and the transformer

    chassis depends entirely upon the ratio of the resistance of the two water paths. It is a fair assumption that the

    resistance between the hull and real ground will be substantially lower than that between the primary live and

    the chassis. The voltage between the hull and real earth will be lower by the same ratio. However in this case

    there is a chance that an RCD (if fitted) on the shorepower may trip. This is far from certain (particularly in

    fresh water).

    Water between point B and chassis.

    Nothing will happen. An RCD on the shorepowermay trip but this is doubtful. No danger exists.

    Water between point C and chassis.No danger exists, No fuses will blow, no RCDs will trip. The water will probably be blown away by the current.

    Water between point D and chassis.

    No effect.

    So, in the case of water ingress to the transformer enclosure, both methods of earthing each produce one

    scenario that attempts to put voltage on the hull. Due to the massive difference in ratios between the resistance

    of the water the boat floats in, and the resistance of the water inside the transformer (say 1 litre of water

    compared to a cubic kilometre!), the possible voltage presented across the hull and real earth is most likely very

    low. However in the case of the chassis being earthed to the boat hull there is a chance (it is far from certain)

    that an RCD on the shorepower (if fitted) may trip. This isn't the case when earthed to the shorepower earth.Note that it is, of course, possible that water ingress contacts more than one point, perhaps 2 of them or even all

    of them. However this then becomes very complicated to analyse with any level of meaning.

    So the argument of which way to earth the transformer, when considering water ingress comes down to the fact

    that both methods produce one scenario that attempts to make the hull live. The chances of any level of voltage

    that is dangerous are much more remote than the chances of a dangrous voltage when we consider faulty wiring

    in the transformer enclosure.

    And when we consider faulty wiring, it is clear that to bond the tranformer chassis to the shorepower earth

    produces less danger.

    I therefore believe, on balance, mainly due to the extreme danger of the boat hull becoming live due to faultywiring that it is safer to bond the transformer safety screen and chassis to the incoming shorepower earth.

    We are not alone in this conclusion. Of all the references I found, books, websites, manufacturers manuals etc,

    all but three recommends bonding the transformer chassis to the shorepower earth. Of the remaining three, one

    recommends bonding it to the boat hull, the other two recommend simply not connecting it. Not connecting it at

  • 7/30/2019 Boat Electrics


    all leaves the installation open to a whole collection of other possible problems including, in the main, greater

    risk of electric shock.

    Note that bonding the chassis to the incoming earth means the transformer and casing have to be isolated and

    insulated from the hull. Otherwise the whole purpose of installing the transformer in the first place (i.e. for

    galvanic isolation) becomes negated.

    I have not considered the possibilities of transformer faults here because, in comparison to wiring faults and

    water ingress, they are extremely rare.