18th question.

AJJewsbury:

It should be noted as a final technical note, that SRCDs are not really an answer for fault protection (i.e. can't meet disconnection times) because they can't respond to an upstream fault. They can therefore only be used for additional protection

Could you clarify that Graham? It sounds like you're saying that a SRCD/FCURCD might be too slow to provide the 0.4s or 0.2s disconnection required for ADS - which sounds rather odd to me (given they'd typically open in something like 40ms for most credible L-PE faults of negligible impedance). Naturally they can't provide protection from upstream faults - but then neither can any other protective device - faults on a distribution circuit imposing hazardous voltages for hazardous durations on downstream final circuits is a well known general problem - there's nothing peculiarly lacking in a BS 7288 device on that count.  A SRCD may rely on an upstream overcurrent protective device for backup protection in the case of large fault currents - but the same is true of DIN rail mounted RCCBs - they often have a breaking capacity not more than 3kA.


Then there's the situation where Zs at the point where thee BS 7288 device is installed is fine for the upstream MCB, but the additional wiring downstream increases Zs beyond that limit (whether fixed wiring from a FCURCD or an extension lead from a SRCD) - are you suggesting that the BS 7288 device shouldn't be relied upon to provide ADS downstream - just additional protection - and so some other means of ensuring ADS downstream needs to be provided in addition?


   - Andy.

 

Sure, I won't begin to answer your points directly, but provide the clarification which I believe is simple.


BS 7671, in terms of fault protection for ADS, is concerned with fault protection up to the extent of the fixed installation - and of course the RCD in an SRCD cannot provide protection upstream of it. Protection of any fault by ADS downstream of a socket-outlet or connection point is either fortuitous, or determined by an appliance manufacturer in accordance with their product standard, along with suitable installation instructions. This is clear from Tables 41.2 to 41.4 in BS 7671 which rely only on loop impedances in the fixed wiring.


So, the clarification is, if the disconnection times in Table 41.1 cannot be met, say because of circuit length supplying the SRCD, then an SRCD is unsuitable for selection as a device for fault protection to achieve those disconnection times, because it cannot protect against faults upstream. In that case, only an RCD in the fixed installation could be used for fault protection. The SRCD could continue to provide additional protection downstream.


Likewise, an upstream RCD would be required to provide fault protection in a TT system, according to Table 41.5.



In terms of an SRCD as part of an FCU, where fixed wiring is installed downstream, again BS 7288 tells us in Note 4 to Clause 1 that where the SRCD is to provide fault protection, it must comply with BS EN 61008-1 or BS EN 61009-1:

NOTE 4 For SRCDs intended to provide isolation or fault protection, or to be used in IT systems, BS EN 61008-1 or BS EN 61009-1 should be used, as applicable, in conjunction with the requirements of BS 1363-2 for socket-outlets.

So again, BS 7288 tells us the SRCD can't provide fault protection.

AJJewsbury:

It should be noted as a final technical note, that SRCDs are not really an answer for fault protection (i.e. can't meet disconnection times) because they can't respond to an upstream fault. They can therefore only be used for additional protection

Could you clarify that Graham? It sounds like you're saying that a SRCD/FCURCD might be too slow to provide the 0.4s or 0.2s disconnection required for ADS - which sounds rather odd to me (given they'd typically open in something like 40ms for most credible L-PE faults of negligible impedance). Naturally they can't provide protection from upstream faults - but then neither can any other protective device - faults on a distribution circuit imposing hazardous voltages for hazardous durations on downstream final circuits is a well known general problem - there's nothing peculiarly lacking in a BS 7288 device on that count.  A SRCD may rely on an upstream overcurrent protective device for backup protection in the case of large fault currents - but the same is true of DIN rail mounted RCCBs - they often have a breaking capacity not more than 3kA.


Then there's the situation where Zs at the point where thee BS 7288 device is installed is fine for the upstream MCB, but the additional wiring downstream increases Zs beyond that limit (whether fixed wiring from a FCURCD or an extension lead from a SRCD) - are you suggesting that the BS 7288 device shouldn't be relied upon to provide ADS downstream - just additional protection - and so some other means of ensuring ADS downstream needs to be provided in addition?


   - Andy.

 

Sure, I won't begin to answer your points directly, but provide the clarification which I believe is simple.


BS 7671, in terms of fault protection for ADS, is concerned with fault protection up to the extent of the fixed installation - and of course the RCD in an SRCD cannot provide protection upstream of it. Protection of any fault by ADS downstream of a socket-outlet or connection point is either fortuitous, or determined by an appliance manufacturer in accordance with their product standard, along with suitable installation instructions. This is clear from Tables 41.2 to 41.4 in BS 7671 which rely only on loop impedances in the fixed wiring.


So, the clarification is, if the disconnection times in Table 41.1 cannot be met, say because of circuit length supplying the SRCD, then an SRCD is unsuitable for selection as a device for fault protection to achieve those disconnection times, because it cannot protect against faults upstream. In that case, only an RCD in the fixed installation could be used for fault protection. The SRCD could continue to provide additional protection downstream.


Likewise, an upstream RCD would be required to provide fault protection in a TT system, according to Table 41.5.



In terms of an SRCD as part of an FCU, where fixed wiring is installed downstream, again BS 7288 tells us in Note 4 to Clause 1 that where the SRCD is to provide fault protection, it must comply with BS EN 61008-1 or BS EN 61009-1:

NOTE 4 For SRCDs intended to provide isolation or fault protection, or to be used in IT systems, BS EN 61008-1 or BS EN 61009-1 should be used, as applicable, in conjunction with the requirements of BS 1363-2 for socket-outlets.

So again, BS 7288 tells us the SRCD can't provide fault protection.