In this thread....
In most senses, BS7671 appears to be saying that it is acceptable to rely upon an RCD for ADS in TN systems:
1...A footnote to Table 41.1 in 411.3.2.2 (disconnection times) says (without qualification about supply type) “Where compliance with this regulation is provided by an RCD ...”.
2...411.4.4 (specifically about TN systems) says that fault protection may be provided by an OPD or an RCD, adding (reasonably!) only that if an RCD is used for fault protection, the circuit must also have an OPD.
3...531.2.8 indicates (without qualification about supply type) that if an RCD is used for fault protection, it should be capable of coping with the possible fault currents.
4...As you have said, 531.3.1 indicates that an RCD may be used for fault protection in a TN system if 411.4.5 (essentially low enough Zs) cannot be achieved. However, it does qualify this as applying to “certain equipment in a certain part of the installation”. It is therefore questionable as to whether this could be invoked in relation to, say, a sockets circuit (since “certain equipment” would then not be defined).
415.1.1 and 411.3.3 speak (without qualification about supply type) of use of RCDs as ‘additional protection’. 411.3.3 speaks (without qualification about supply type) of an RCD as ‘additional protection’, and requires them on certain circuits (e.g. most domestic socket circuits), but does not specifically say as to whether the RCD can be ‘relied upon’ for ADS in TN systems (although some people may feel that ‘additional’ means that it cannot be the sole protection). .
The main reg cited by those who believe that, despite (1)-(4) above, an RCD cannot be be ‘relied upon’ for fault protection (i.e. ADS) is 415.1.2. This says that “the use of an RCD is not recognised as a sole means of protection and does not obviate the need to apply one of the protective measures specified in Sections 411 to 414”. However, particularly given that one of these measures (413) is ‘electrical separation’, I cannot help but wonder whether those people who cite this reg as prohibiting an RCD as the primary provider of ADS in a TN system are not perhaps ‘over-interpreting’ the reg.
Although the regs don’t always make this clear, it is obvious that with a TT system one nearly always has to rely on an RCD to provide ADS (i.e. ‘disconnection times’).
As for TN systems, I don’t know. I have to say that, taking the totality of the above into account, I personally find it fairly hard to believe that the regs are prohibiting reliance on an RCD for ADS. The question is obviously moot unless Zs is too high for the OPD to provide satisfactory disconnection times and, in that situation, 531.3.1 allows reliance on an RCD to provide fault protection for “certain equipment in a certain part of the installation”. Whether that includes things like domestic sockets circuits is seemingly open to debate.
Given the fallibility of devices, in some senses it would seem better to ‘rely upon’ an RCD for fault protection, since it can be tested, rather than to rely upon an OPD, which effectively can’t be tested.
What do others think?
Kind Regards, John
We have often discussed this, but I would hesitate to go as far as saying that we ever arrived at ‘the answer’.I think we discussed this one recently, didn't we lads? ... The answer was that BS 7671 permits the use of an RCD on circuits within TN installations where the Zs cannot be achieved, I believe. It must be the designer to decide what that means and what are the risks associated with relying on an RCD to trip within the required time period are, if any.
In most senses, BS7671 appears to be saying that it is acceptable to rely upon an RCD for ADS in TN systems:
1...A footnote to Table 41.1 in 411.3.2.2 (disconnection times) says (without qualification about supply type) “Where compliance with this regulation is provided by an RCD ...”.
2...411.4.4 (specifically about TN systems) says that fault protection may be provided by an OPD or an RCD, adding (reasonably!) only that if an RCD is used for fault protection, the circuit must also have an OPD.
3...531.2.8 indicates (without qualification about supply type) that if an RCD is used for fault protection, it should be capable of coping with the possible fault currents.
4...As you have said, 531.3.1 indicates that an RCD may be used for fault protection in a TN system if 411.4.5 (essentially low enough Zs) cannot be achieved. However, it does qualify this as applying to “certain equipment in a certain part of the installation”. It is therefore questionable as to whether this could be invoked in relation to, say, a sockets circuit (since “certain equipment” would then not be defined).
415.1.1 and 411.3.3 speak (without qualification about supply type) of use of RCDs as ‘additional protection’. 411.3.3 speaks (without qualification about supply type) of an RCD as ‘additional protection’, and requires them on certain circuits (e.g. most domestic socket circuits), but does not specifically say as to whether the RCD can be ‘relied upon’ for ADS in TN systems (although some people may feel that ‘additional’ means that it cannot be the sole protection). .
The main reg cited by those who believe that, despite (1)-(4) above, an RCD cannot be be ‘relied upon’ for fault protection (i.e. ADS) is 415.1.2. This says that “the use of an RCD is not recognised as a sole means of protection and does not obviate the need to apply one of the protective measures specified in Sections 411 to 414”. However, particularly given that one of these measures (413) is ‘electrical separation’, I cannot help but wonder whether those people who cite this reg as prohibiting an RCD as the primary provider of ADS in a TN system are not perhaps ‘over-interpreting’ the reg.
Although the regs don’t always make this clear, it is obvious that with a TT system one nearly always has to rely on an RCD to provide ADS (i.e. ‘disconnection times’).
As for TN systems, I don’t know. I have to say that, taking the totality of the above into account, I personally find it fairly hard to believe that the regs are prohibiting reliance on an RCD for ADS. The question is obviously moot unless Zs is too high for the OPD to provide satisfactory disconnection times and, in that situation, 531.3.1 allows reliance on an RCD to provide fault protection for “certain equipment in a certain part of the installation”. Whether that includes things like domestic sockets circuits is seemingly open to debate.
Given the fallibility of devices, in some senses it would seem better to ‘rely upon’ an RCD for fault protection, since it can be tested, rather than to rely upon an OPD, which effectively can’t be tested.
What do others think?
Kind Regards, John