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Wiring Matters 93 - Nov 22 - High Protective Conductor Currents

GlasgowMatt

In the latest Wiring Matters issue 93, there is an article on high protective conductor currents and section 543.7.1. 

It quotes the methods of complying with reg 543.7.1.203 including "A single protective conductor, meeting regulations 543.2 (relating to conductor type) and 543.3 (relating to preserving electrical continuity), having csa of not less than 10 mm2 (to give a mechanically robust connection)."

The author then goes on to describe typical circuits and then says this about ring final circuits.

"Where a standard ring final circuit is employed, complying with Appendix 15 of BS 7671 (using typically 2.5 mm2 live conductors and 1.5 mm2 cpcs) it can be appreciated that the total csa of all of the live and protective conductors will easily exceed 10 mm2 – hence satisfying regulation 543.7.1.203."

Do others agree that the authors above interpretation is imcorrect? I dont know where they have got the sum of all live and protective conductors from?

Parents

  • It appears to me that 543.7.1.202 and .203 contradict one another. .203 requires a 10mm conductor although installed whereas flexible cables to a plug may be 4mm2. Surely the 4mm cable is at much greater risk of failure, particularly loose connections than the fixed cables? Surely too the discussion of ring final circuits (that are normally used to supply sockets) is that they must be protected by 30mA RCDs, and are therefore not really suitable to supply more than 1 load with above 10mA of leakage? It is quite obvious to me that a 32A ring final circuit is non-compliant with any of this regulation group. I suppose it might be if:

    1. It has no sockets, only fixed loads

    2.It has no RCD of a sensitive rating, ie no additional protection (of course with these Earth precautions such would not be needed with a suitable method of cable installation.

    The article needs a bit of a rethink, because it simply magnifies the confusion of the regulations themselves.

  • in reply to David Stone

    With regard to .202 and .203 contradicting each other, they kind of do, however the key difference is .202 is for a single item of equipment, .203 for multiple. 

    The example being a radial circuit supplying two 16A Commando BS EN 60309-2 sockets, to serve to Data Racks. The supply to the sockets would need to comply with 543.7.1.203 while, after each socket, it only needs to comply with 543.7.1.202. 

    .202 also says "shall be connected to the supply by". It is trying to impose conditions outside the fixed wiring after the BS EB 60309-2 socket, which are normally "outside the scope" of BS7671.

    With regards to Ring final circuits, it is only where "the total protective conductor current is likely to exceed 10mA" for fixed equipment, or "known or reasonably to be expected that the total protective conductor current in normal service will exceed 10mA" (543.7.2.201) for socket circuits, so it could supply many loads where there are a number of possible devices that together can add up to more than 10mA

    The typical one for this is a socket circuit in an office. Lots of switch mode power supplies for monitors, PCs, printers etc. These can go up to 3.5mA per device as per IEC-60950-1 limits. 

Reply
  • in reply to David Stone

    With regard to .202 and .203 contradicting each other, they kind of do, however the key difference is .202 is for a single item of equipment, .203 for multiple. 

    The example being a radial circuit supplying two 16A Commando BS EN 60309-2 sockets, to serve to Data Racks. The supply to the sockets would need to comply with 543.7.1.203 while, after each socket, it only needs to comply with 543.7.1.202. 

    .202 also says "shall be connected to the supply by". It is trying to impose conditions outside the fixed wiring after the BS EB 60309-2 socket, which are normally "outside the scope" of BS7671.

    With regards to Ring final circuits, it is only where "the total protective conductor current is likely to exceed 10mA" for fixed equipment, or "known or reasonably to be expected that the total protective conductor current in normal service will exceed 10mA" (543.7.2.201) for socket circuits, so it could supply many loads where there are a number of possible devices that together can add up to more than 10mA

    The typical one for this is a socket circuit in an office. Lots of switch mode power supplies for monitors, PCs, printers etc. These can go up to 3.5mA per device as per IEC-60950-1 limits. 

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