Experts Q&A

July 31st, 2012, Published in Uncategorised articles

Question:

When installing an all-steel cable management structure (tray, baskets and supports) in an all-steel container for telecoms cabling (48 V DC and signal cables) with AC power and lighting – all contained in separate PVC trunking and conduit, is cross bonding required between all component parts of the DC cable management?

Answer:

In the circumstances described I believe that the installation method should provide sufficient bonding for the trays etc. However, it still needs to be confirmed in each case, as follows:

All exposed conductive parts must be bonded, and with specific reference to cable trays and such:

6.5.2.2 Electrical continuity

All parts of rigid metallic wireways shall be bonded to earth.

Such bonding must be confirmed by testing, see the second note under 6.13, Bonding, in the code:

Note 2:

No external conductor is required if compliance with the requirements for continuity can be proved by the test in 8.7.2.


Question:

I saw some extension cables supplying power to outside equipment. The cables were braided, but the braids were cut off and taped at the 16 A plugs and sockets. Surely this cannot be right? In any case, how are you supposed to terminate the braids if you are going into a plastic
plug/enclosure?

Answer:

I assume you refer to copper braided cable, which is dealt with under 6.7.4 in the code. All braiding must be earthed and bonded, and connected to any metal enclosures. It can be used as ECC. Termination can be done by twisting the strands together firmly, and by terminating them in the normal manner. However, it remains advisable to ferrule or solder the ends to prevent such thin strands from damage.


Question:

Are cables that supply 230 V DC allowed to bunch with 230 V AC cables inside the same cable conduit or trunking? If not, what is the correct way to install both 230 V DC and AC cables? Also, what are the necessary precautions needed to avoid electric shock for the 230 V DC system? The 230 V DC source is from an LED lighting controller/power supply unit with a 230 V AC input. The LED luminaires’ voltage input is 230 V DC.

Answer:

The Wiring Code SANS 10142-1 does not permit this:

6.1.10 Conductors that form part of a DC installation shall not be run in the same wireway as conductors that form part of an A.C. installation.

It would be advisable to install the DC component as a totally separate installation with its own wireways etc. The same precautions against shock as prescribed for an AC installation would apply.

 


Question:

Where a 240/110 V transformer (either centre or line tapped) supplies a distribution board, should there be secondary protection against indirect contact or does the TX primary protection suffice?

Answer:

Protection will be required for the secondary side of the transformer as well as for the circuit connected to it.


 

Question:

What is the advantage or purpose of breaking the neutral on a switch?

Answer:

It is a safety precaution. The neutral is considered a live conductor which can become dangerous in some instances. In electrical installations, the neutral may never be switched (6.9.2), unless it is switched with the corresponding phase conductor.

The supply to all fixed appliances must be controlled by a switch-disconnector that switches both live conductors (6.16.1.3 a). The same applies to DBs (6.6.1.1). Under fault conditions, the neutral may be subject to significant voltage, which can be hazardous. Consider crossed live and neutral as can happen with meter replacement, for instance.

 


 

Question:

I seem to read conflicting requirements about the Ze external loop impedance test. Please clarify whether this test is to be carried out with the supply cable live or dead. I know that the end-user side would be shut down and isolated when doing this test.

Answer:

Considering that the circuit is included in the loop impedance test, such circuit will be interrupted by switching off the supply, which means the test cannot be done.

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