The maintenance manager's guide to circuit protection

8/14/2019 The maintenance manager's guide to circuit protection

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Maintenance Managers Guide

Power Quality

The Maintenance Managers Guide

to circuit protection

Hydraulic-magnetic CB operation; a, b, c - overcurrent; d - fault current


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www.leonardo-energy.org

1. Why is circuit protection installed?

The primary objective of Electrical Installation Practice is to provide an

installation that is safe and functional. Since it is inevitable that faults will

sometimes occur in electrical systems and the appliances that they

supply, steps need to be taken to ensure that the safety of people and

property is maintained. For the protection of people, exposure to

dangerous voltages must be prevented by, for example, good insulation of

live parts, proper earthing and earth fault detection. For property

protection it is necessary to prevent over-currents that could causeoverheating and fire and fault current, i.e. the uncontrolled flow of energy

that might lead to ignition or explosion. This document is concerned

primarily with protection against the effects of over-currents and fault

currents, but, of course, the rapid disconnection of faults enhances safety

by reducing the risk of exposure of people to dangerous voltages.

Typically, electrical installations follow a tree architecture, the root of

which is the point of common coupling (PCC) where there is a protective

device provided by the energy supplier. At this point, the supply is definedin terms of capacity the maximum power that can be normally drawn

and the prospective short circuit current the maximum short circuit

current that could flow through a solid short circuit applied at the PCC.

Next in line are the installations main switch and distribution board where

the supply splits into a number of sub-circuits, which may be final circuits

or feeders to other sub-distribution boards, each with a protective device,

as shown in Figure 1.

Figure 1 Generic installation topology


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2. How are protective devices selected?

At every point where the current carrying capacity of the conductor

changes (e.g. points a to d), there is a circuit breaker, for which there are

three specific requirements:

it must be capable of breaking the maximum prospective fault current

at that point

it must respond to over-current in such a way as to disconnect the

circuit before the excess heat generated in the load circuit cable is

sufficient to damage the cable or materials in contact with it

it must limit potential damage to the load circuit by limiting the

magnitude, duration or energy of a fault current to a safe level while

disconnecting the circuit from the supply.

In an ideal situation, only the breaker for the faulted circuit will open,

disconnecting the fault and leaving the rest of the installation unaffected.

This is essential for critical loads, but it is often difficult to achieve

completely at an affordable cost, so alternative strategies are also used.

3. Prospective fault current

The prospective fault current is the maximum current that could flow at a

particular point of the installation if a solid short circuit were to be applied

there. The prospective fault current depends on the supply impedance at

that point, including the source impedance and all cables and accessories

in the circuit, so, assuming that there are no transformers, it decreases as

electrical distance from the source increases.

The prospective fault current is very important in the selection of the

protection strategy and of the individual protection devices. Every

protective device must be either capable of breaking this current (at its

position in the installation) without excessive arcing and without being

damaged in the process or must be assisted to do so by an upstreamcircuit breaker. This is discussed further under Selectivity or

discrimination.


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4. Circuit breakers

There are three main types of circuit breaker used within installations:

Miniature circuit breakers, MCBs, are used in residential, commercial

and industrial applications for final sub-circuit protection. They are

relatively cheap, compact and available in a wide range of ratings (5

to 100 A), but have limited breaking capacity, typically 6, 10 or 15 kA.

Moulded case circuit breakers, MCCBs, are designed to high

breaking current capacity with low let-through energy. They are

available in frame sizes from 100 A up to about 3000 A forinstallations where the prospective short circuit fault currents could be

as high as 100 kA. The characteristics are not standardised and the

trip levels and trip times are often adjustable to provide the desired

type and level of discrimination.

Air circuit breakers, ACBs, are more correctly defined as Power

Circuit Breakers, with the fundamental difference being that the short

time withstand current of the ACBs is equal to the interrupting rating.

5. Characteristics

A typical CB characteristic curve is shown in Figure 2.

Figure 2 Characteristic curve of a circuit breaker


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The so-called inverse time part of the characteristic is designed to protect

against over-current. It allows for substantial short overloads without

tripping, because the rate at which the cable conductor temperature rises

due to the extra heat generated is relatively slow due to the high specific

heat of the copper conductors. As the over-current level increases, the

time to respond reduces rapidly to restrict the rise in temperature and

reduce the risk of damage. The characteristic takes advantage of the

inherent short time over-current tolerance of the cable and allows short

duration inrush currents to flow without tripping the breaker.

The instantaneous characteristic is intended to respond very rapidly to

fault current. Fast action is needed because fault currents are high

enough to pose a high risk of damage to load circuits.

The most common to achieving these characteristics is the thermal-

magnetic breaker. The thermal characteristic is provided passing the

load current through an element including a bi-metal strip which deflects

according to its temperature. Once a set deflection has been reached, the

mechanism is tripped, disconnecting the load circuit. Since the trip issensitive to temperature, a relatively small over-current will build up heat

and eventually cause tripping over an extended time, while a larger over-

current will heat up and cause tripping in a shorter time. In this area of

operation, the device will not respond to very short duration over-currents.

The magnetic characteristic is intended to protect against fault currents. It

is provided by a small solenoid which exerts a force on the release

mechanism. At a predetermined multiple of the rated current, the force is

sufficient to operate the trip mechanism, and the load circuit is

disconnected.

Breaking fault current rapidly is not easy. Without special measures, an

arc will form as the contacts separate which will be sustained until the next

current zero crossing point. It is possible that a half cycle of fault current

will flow, feeding a relatively large amount of energy into the fault.

Reducing the energy supplied to the fault requires rapid opening of the

contacts and fast quenching of the arc.


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To ensure rapid opening, the contacts are designed (Figure 3) so that the

magnetic force generated by the current flowing up one contact and down

the other tends to push the contacts apart.

Figure 3 MCB construction

As the contacts part, an arc is formed and current continues to flow. The

arc suppressor, a stack of U-shaped steel plates forming a channel

around the contacts, extinguishes the arc. The magnetic field produced

by the arc forces the ionised gasses into the plates, rapidly cooling and

dividing the gasses and so breaking the arc.

Figure 4 Arc quenching during MCB operation


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An alternative method to achieving the inverse-time operating

characteristic in circuit breakers, is to use a hydraulic-magnetic principle.

The device consists of a solenoid with a moving core which is normally

displaced by a spring so that it is outside the magnetic circuit. The core is

sealed within a cylinder filled with viscous silicone oil so that its movement

is rate limited or damped. The moving armature is coupled to the tripping

mechanism such that when the armature is attracted to the solenoid pole

piece, the breaker will trip.

Under normal load current the magnetic force generated by the air cored

coil is insufficient to overcome that exerted by the spring, so no movement

takes place (a).

Upon the occurrence of an over-current, the magnetic force induced in the

coil exceeds that of the opposing spring. The magnetic core moves

towards the pole piece at a rate determined by magnetic force, the

viscosity of the silicone oil and the mechanical clearance between the

magnetic core and the enclosing tube (b). In due course, the magneticcircuit is completed and the armature is attracted, tripping the circuit

breaker (c). The actual time-current characteristic in such devices is

easily controllable through a combination of the opposing spring force and

the viscosity of the silicone oil that is sealed inside the tube assembly.

At very high currents, such as a fault current, the magnetic field generated

by the air-cored coil is sufficiently strong to attract the moving armature

without the core being in the energised position, so the breaker trips

instantaneously (d).

Figure 5 Hydraulic-magnetic CB operation; a, b, c - overcurrent; d - fault

current


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Electronic sensing for over-current and fault circuit protection in circuitbreakers is generally restricted to higher current devices for the present,

mainly due to cost considerations. On the other hand, electronic sensing

has resulted in a step-function improvement in both the reliability and

performance of sensitive earth leakage protection, whilst costs have been

held within affordable limits.

The use of electronic sensing allows much more accurate protection and

enables developments such as annunciators, true RMS protection and

communication between circuit breakers. Once cost restrictions have

been overcome, technologies such as load and fault signature recognition

could become a reality. Intelligent circuit protection is likely to have

applications in future Smart homes.

6. Practical Characteristics

In practice, as with any other manufactured device, the performance

characteristics of circuit breakers are subject to variation. For MCBs, the

thermal characteristic which provides protection against over-current

the following test points are given in Standards EN 60898:

This relates to practice as follows. The circuit is designed to provide

power to a load or group of loads and so has an expected maximum

current, Ib. The nominal MCB rating, In, must be greater than Ib. The

conductor size for the circuit is selected to have a current carrying

capacity, Iz, that is greater than In. In addition, the current that causeseffective operation of the device within the required time, I2, must not be

greater than 1.45 times the current carrying capacity of any part of the

Current I (A) Nominal trip

current

In (A)

Result

I = 1.13 x In All Must not trip within 1 hour

I = 1.45 x In < 63A Must trip within 1 hour

I = 1.45 x In > 63 A Must trip within 2 hours

I = 2.55 x In < 32A Must trip between 1 and 60 seconds

I = 2.55 x In > 32A Must trip between 1 and 120 seconds


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circuit. As can be seen from the table, for an MCB meeting EN 60898, a

current of 1.45 x In will trip in less than one or two hours, so this condition

is deemed to be satisfied.

The magnetic trip provides protection against the effects of fault current.

Since it operates within one cycle, MCBs are sensitive to inrush, starting

and surge currents. To avoid a high level of nuisance tripping, MCBs are

available with nominal magnetic trip ratings of 5, 10 and 20 times the trip

rating as shown in Figure 2. Fault current protection must operate within a

prescribed time, which, for 230 V circuits, is 0.4 seconds. This places

another requirement on conductor sizing a short circuit at the far end of

the circuit must cause a fault current large enough to operate the

magnetic trip. Consequently, there is a maximum limit on the circuit loop

impedance according to the class of the breaker. So, if Class D MCBs are

used (to avoid nuisance tripping on inrush currents, for example), the loop

impedance must be lower than if a Class B device is used. Where the

load is concentrated, the loop impedance required by a Class D device

should be met if the circuit voltage drop has been correctly taken into

account when sizing the cable. However, this may not be the case fordistributed loads so it should always be checked.

Figure 6 shows the overall responses of Class B, C and D MCBs.

Figure 6 MCB Characteristics, showing minimum and maximum instantaneous

tripping currents


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Since circuit breakers are thermally operated devices, they are sensitiveto temperature with the nominal trip current decreasing as the ambient

temperature rises. Devices are designed and tested to operate singly in a

vertical orientation at 30 C. At higher temperatures or where the circuit

breakers are mounted in groups (as is typical in a distribution board),

derating factors must be applied. It must be remembered that circuit

breakers also generate heat due to their internal resistance.

Manufacturers publish power dissipation values and derating tables for

ambient temperatures above 30 C and for grouping factors.

As can be seen from Figure 6, the tolerances on circuit breaker

characteristics are rather wide. In overload conditions, for example, the

time to trip at 1.5 x In is a minimum of 40 seconds and a maximum of 400

seconds at 30 C. In fault conditions, a Class C device may trip at a

minimum current of 5 x In or may not trip until the current exceeds 10 x In.

These tolerances must be taken into account in design by using the worst

case. For example, when considering the required value of loop

impedance, the highest value of tripping current must be used, but whenconsidering resilience to inrush current, the lowest value is the relevant

one.

7. Selectivity or Discrimination

Ideally, when a fault occurs, only the circuit breaker immediately upstream

of the fault should open, thereby isolating the fault without disconnecting

any other circuit. In Figure 7, the fault should result in only the breaker C3opening. In order to achieve that, the discrimination between the breakers

at levels A, B and C must be total and breaker C3 must be capable of

breaking the prospective fault current at C3.

Where resilience is important, total discrimination is essential. In other

circumstances it is often necessary to compromise and adopt a less

stringent strategy such as backup protection.


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Figure 7 Typical system topology

Discrimination between breakers in a system can be achieved either on

the basis of current difference or time difference. A co-ordination study

must be undertaken to ensure predictable behaviour.

Current discrimination is achieved if the downstream device has a lower

current trip level, under all circumstances, than the upstream device.

Figure 8 shows an example of this. Since there is no overlap, the

downstream breaker will always disconnect the fault and perfect

discrimination is achieved. The difficulty is that the downstream breaker

must be capable of breaking the full prospective short circuit current,

which will often mean that a more expensive device must be used.


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Figure 8 Breaker combination giving complete current discrimination

Figure 9 Breaker combination giving limited current discrimination

8. Cascade or Backup protection

Cascade protection is intended to allow the use of lower cost circuit

breakers in positions where their current breaking capacity might be lessthan the prospective fault current. Figure 9 illustrates a case where the

breaker characteristics are allowed to overlap at a high fault current level.


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Here, the upstream breaker will operate first if the fault current exceedsthe current at which the curves cross about 2700 A in this example

otherwise the downstream breaker will operate. In practice this means

that fault currents less than 2700 A will be disconnected by the

downstream device while larger fault currents, which might exceed the

breaking capacity of the downstream device, are disconnected by the

upstream device. Many faults will result in a current considerably less

than the full prospective fault current, perhaps because the fault has some

resistance, or because it occurs (and operates the breaker) at less than

full voltage, and will be cleared solely by the downstream breaker. Note

that the characteristics in the overload area do not overlap so, for overload

conditions, the downstream breaker will always be responsible for clearing

overload currents.

The disadvantage of this approach is that a high fault current will cause

the upstream breaker to operate, removing power from healthy circuits

and increasing business disruption.

Breaker manufacturers publish comprehensive performance data for

cascade protection systems. However, it must be remembered that

cascade protection always introduces the probability that power will be

removes from a much greater part of the installation than is strictly

necessary.

Time discrimination is achieved by delaying the action of the upstream

breaker until the downstream breaker has had time to open, as shown inFigure 10. This scheme requires the use of suitable breakers; the

upstream breaker must be designed for this purpose and the downstream

breaker must be capable of breaking the full prospective fault current.


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prescribed adjustments have not been correctly carried out often

installed breakers are found in the as shipped state. It is important that

these problems are rectified by reference to the design documentation

rather than trial and error the erroneous operation may be delayed, but it

may also be catastrophic.

9.2. Post commissioning issues and modifications

The most common problems are, as for new installations, nuisance

tripping and a perceived lack of discrimination, but the cause is different

the use of the installation has changed.

Nuisance tripping usually becomes apparent following a change in the

nature, number or use of the loads connected to the network. In one

case, a tutorial room in an academic institution had been converted into a

small computer suite housing 42 personal computers. For software

management purposes, these computers were audited and updated

overnight, using Wake on Lan or magic packet instructions to bring them

out of standby and into full power mode. These instructions were sent inrapid succession, resulting in a virtually simultaneous power-up of all the

computers. The result was a large inrush current, retrospectively

measured at >500 A in the worst case, resulting in the MCBs operating.

The corrective action taken was to replace the Class B MCBs which trip

at 5 times their nominal rating with Class D devices that would trip at 20

times their nominal rating. This would have been a reasonable response,

if the loop impedance had been checked to ensure that a true fault current

would have been high enough to trip a Class D breaker. No such check

had been made, so the nuisance tripping issue was resolved, but the

circuit may not be protected in the event of a real fault! The moral is that

any maintenance action that could alter the behaviour of the protection

system should be carefully checked.

Where there is a perceived lack of discrimination it is usually brought

about by a change of function in that area of the installation, such as the

installation of more mission critical equipment, for example, or simply a

change in the perception of the criticality of the existing equipment.Complete discrimination schemes can be more costly and may have been

implemented only in areas where they were considered essential for


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critical operations. When changes of use occur, it is possible that theprotection scheme will need to be readjusted or upgraded.

10. Conclusions

The design of the protection scheme requires a systems approach. It

should not be assumed that additional circuits can be added to a

distribution board without some consideration of the possible effects on

upstream protection devices and their settings.

Any change of wiring that could affect the loop impedance such as

conductor upsizing or re-routing via a longer or shorter path must be

considered in conjunction with the effect it could have on prospective fault

currents and on clearing times for up and downstream protection devices.

Even the apparently innocuous change of substituting, say, a Class D

MCB in place of a Class B MCB requires reconsideration of loopimpedance.

The maintenance manager's guide to circuit protection

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