IET announces new amendment to BS 7671 (IET Wiring Regulations)

AJJewsbury:

The product standard ( BS 7288) says they are not suitable for providing Additional protection.

It doesn't quite say that. That would put it in the the same category as a BS for chocolate tea pots. I think what's written could equally be read as "the the device doesn't protect the upstream part of the circuit and that additional protection [for that - if any is required] will have to be provided elsewhere".

They do not provide isolation as they do not meet the minimum contact clearance.

Oh yes they do - on a SRCD you just pull the plug out (just like for unswitched BS 1363 sockets). Besides is there actually a requirement for devices providing additional protection only to provide isolation? 531.1.1 only talks about devices providing ADS - additional protection is technically a different beast.


  - Andy.

Well, this is interesting. I don't want to disagree with you Andy, simply provide some points for discussion.


First, a couple of points on whether an RCD is always a device providing ADS:

• In a fault where ADS is required, if the RCD in the SRCD operates first, it is the device providing ADS whether we intended it to be or not. Is "additional protection" a form of ADS anyway

• Regulation 411.3.3 is a requirement for ADS, therefore it could be argued that an RCD provided for 411.3.3 is a protective device providing ADS (or part of the ADS solution)

• In TT systems, it often does, almost by definition.

Now on to "isolation". To be honest, we don't know what the purpose of the "isolation" required for a device providing ADS actually is, because BS 7671 (and the international standards HD 60364 / IEC 60364) doesn't tell us in simple terms ... So, I guess, possible options include something along the lines of:

• Enable isolation of the fault downstream after operation of the protective device to effectively remove the fault and enable the electrical installation to continue to be used safely, or to facilitate safe maintenance and replacement whilst maximizing the availability of the rest of the electrical installation (the plug and socket-outlet combination can do that of course - BUT a single-pole device such as a fuse in an SFCU-RCD doesn't isolate an N-E fault, so ???)

• Provide isolation immediately when the protective device because this is required for safety (a plug and socket-outlet combination cannot achieve this aim, removal of a fuse if the SRCD is in fact part of an SFCU-RCD assembly cannot either - therefore BS 7288 device not suitable ... BUT then again it could be argued that a fuse "blowing" does not, strictly, provide isolation ....)

AJJewsbury:

The product standard ( BS 7288) says they are not suitable for providing Additional protection.

It doesn't quite say that. That would put it in the the same category as a BS for chocolate tea pots. I think what's written could equally be read as "the the device doesn't protect the upstream part of the circuit and that additional protection [for that - if any is required] will have to be provided elsewhere".

They do not provide isolation as they do not meet the minimum contact clearance.

Oh yes they do - on a SRCD you just pull the plug out (just like for unswitched BS 1363 sockets). Besides is there actually a requirement for devices providing additional protection only to provide isolation? 531.1.1 only talks about devices providing ADS - additional protection is technically a different beast.


  - Andy.

Well, this is interesting. I don't want to disagree with you Andy, simply provide some points for discussion.


First, a couple of points on whether an RCD is always a device providing ADS:

• In a fault where ADS is required, if the RCD in the SRCD operates first, it is the device providing ADS whether we intended it to be or not. Is "additional protection" a form of ADS anyway

• Regulation 411.3.3 is a requirement for ADS, therefore it could be argued that an RCD provided for 411.3.3 is a protective device providing ADS (or part of the ADS solution)

• In TT systems, it often does, almost by definition.

Now on to "isolation". To be honest, we don't know what the purpose of the "isolation" required for a device providing ADS actually is, because BS 7671 (and the international standards HD 60364 / IEC 60364) doesn't tell us in simple terms ... So, I guess, possible options include something along the lines of:

• Enable isolation of the fault downstream after operation of the protective device to effectively remove the fault and enable the electrical installation to continue to be used safely, or to facilitate safe maintenance and replacement whilst maximizing the availability of the rest of the electrical installation (the plug and socket-outlet combination can do that of course - BUT a single-pole device such as a fuse in an SFCU-RCD doesn't isolate an N-E fault, so ???)

• Provide isolation immediately when the protective device because this is required for safety (a plug and socket-outlet combination cannot achieve this aim, removal of a fuse if the SRCD is in fact part of an SFCU-RCD assembly cannot either - therefore BS 7288 device not suitable ... BUT then again it could be argued that a fuse "blowing" does not, strictly, provide isolation ....)