Welcome to the PROBLEM PAGE.

We find many people request assistance in the practical aspects of the industry. In response we will investigate the problem and endeavour to present the best possible solution.Thank you for all your questions sent in. Send your problems (and sometimes your creative solutions) to acra@netactive.co.za with problem page in the subject line. You may include pictures.

William Dlamini asks: I have seen many of your articles and see that you have given very helpful advice for the electrical part of the trade. I ask could you possibly tell me about the different types of cables as I see all different types been used but what is correct?

William, as I stated in the previous issue I will continue with the explanation by going through a step by step procedure for the fitment of the armoured cable gland.

METHOD

1. Determine and mark the required length of conductors (in this case 200 mm).

2. Fit shroud over cable end by turning it inside out and then pull it over the cable end, large end first. Pull it past the length mark, then pull it back to obtain the correct shape.

3. By using a hacksaw, ring cut the strands of the armouring halfway through at the position previously marked. Care must be taken not to cut through the armouring as the core insulation underneath the inner sheath might easily be damaged.

4.

Use a cable knife to cut along the outer PVC sheath, starting at the cable end up to the hacksaw cut.

5. Remove the outer PVC covering.

6. The armouring which has been ring cut can now easily be broken off, simply by bending it slightly to and fro at the hacksaw cut.

WARNING: Care must be taken to prevent injury!

7. Strip outer PVC to a length equal to the length of the cone nut, plus 5mm.

8. Fit gland outer unit over cable and spread armour wires.

9. The armouring can be spread by holding the cable tight with one hand just behind the gland and with the other hand grip the inner sheath and move it in a circular movement.

10. Pass cable end through the gland inner unit and lay armour wires over the cone.

11. Pass the differential nut over the cable end and screw onto the gland inner. For easier fitting of

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Problem PageBy Grant Laidlaw

the gland inner onto the outer, the nipple can be screwed out slightly. By adjusting the nipple in or out a small clearance approximately 2 mm can be obtained between the inner and outer gland faces.

12. Lock the inner and outer nuts of the gland by using a spanner on each nut.

13. Fit the shroud over the gland assembly.

14. Cut around the inner sheath approximately the length equal to its diameter from the gland, making sure that you do not damage the insulation of the inner conductors.

15. With a cable knife, starting at the top, cut along the inner sheath in the direction of the twist of the conductors and remove it. Be careful not to damage the conductor’s insulation.

EARTH TAGSEarth tags are used to obtain earth continuity when the gland is connected to a non-conductive box of any non-conductive piece of equipment. The large opening slides over or may screw onto the gland nipple. The other connection is connected to an earth conductor.

Selection of cable sizeThe copper conductors using PVC insulation are typically used in most installations. The following sizes and rated currents at 20º Celsius and an assumed ambient air temperature of 30 ºC. This is a specification taken from a manufacturer. (SANS 1507-3)

Cable size (Copper)

Current Rating Volt Drop(mV/A/m)

1mm ² 12 amps 30,1801,5mm ² 17 amps 25,8202,5mm ² 27 amps 15.3684mm ² 38 amps 8.5816mm ² 47 amps 6.391

10mm ² 64 amps 3.79316mm ² 82 amps 2.390

It is important not to exceed the rated amperage of conductors. Exceeding the rated current may result in under voltage, equipment damage, overheating of the conductor, possible fire hazard and possible failure of the conductor.

Removing the insulation from conductors

Using a knifeHolding the knife at 30º cut through the insulation down to the conductor. The angle reduces the risk of damaging the conductor. Once the insulation has been cut through around the conductor pull it off, leaving the required length of conductor exposed.

Using a wire stripperA wire stripper has notches in the jaws used for stripping different conductor sizes. The correct selection for the conductor to be stripped is important. Failure to do so will result in the damaging of the conductor or the stripper not being able to remove the insulation.

Conductor jointingJoining the conductor ends by means of a ferrule.A ferrule consists of a tinned copper tube which is crimped onto two conductor ends. Ferrules are available in various sizes to suit the conductors to be joined. Most ferrules have an internal stop situated in the centre to prevent the conductor ends from passing that point.

Insulation tapeWhen using insulation tape start at one end laying the end of the tape over the tapered end of the original insulation. Wind the tape diagonally toward the opposite end, keeping the tape stretched and letting the successive turns overlap by 50%. On reaching the opposite end work back in the same way, the individual turns of the tape will be almost 90º to the previous turns. Proceed in this manner until the applied tape is as thick as the original insulation.

Joints for conductors

Married joint This type of joint is only used on stranded conductors and constitutes a very strong joint that can withstand some mechanical stress.

For example: When used in a vertical application.

• Remove the insulation approximately 120 mm from the end of the conductors to be joined.

• Twist each conductor tightly in the direction of the lay.

• Untwist the conductors approximately 95 mm from the end and evenly spread them at approximately 45º.

• If the conductor has a centre strand, remove. Straighten strands and lightly clean using fine sandpaper. Interleave the strands of the two conductors and butt the twisted potions together.

• Wrap the left hand conductor around the right hand conductor in the direction of its lay. Proceed in the same manner with the right conductor.

• Cut off ends approximately 5 mm from the insulation in staggered position.

• Smooth over using a pair of pliers.• Solder joint.

Barrel and twist joint

This joint is used to join a solid strand conductor.

• Strip approximately 100 mm of the insulation.• Clean the strands using fine sandpaper. • Twist conductors together for approximately 50 mm. • Wind the two ends three times around the opposite

conductors.• Cut off excess length on the ends and smooth using

pliers.• Solder joint.

Screw Connectors Screw connectors can be used to join multi-strand connectors provided that the correct size connector is used and that the conductor is fitted into the connector. The screw can then be tightened to secure the conductor.

VOLT DROPVoltage drop caused by conductor resistance should be kept to within tolerances. Damage to the unit due to under-voltage occurs in the form of magnetic switches chattering and possible compressor motor burn out.In order to determine the voltage drop over a length of conductor and subsequent selection of conductor sizes refer to the example of the manufacturers table used in the description of selection of cable sizes.

Well that’s it for 2007. I wish to thank all who participated and look forward to your questions for 2008.Have a wonderful festive season and keep safe.Grant Laidlaw.

REFERENCES: Aberdare cables

Thanks to: Duncan Ferreira:Ferreia’s Electrical wholesalers