The BS says these devices only provide "supplementary" protection they require upstream "Additional Protection" if Additional Protection is required. RCDs do not provide protection for upstream only downstream of the device. The unswitchef varaties do not in themselves provide satisfactory isolation. Yes you could isolate by removing the plug from the socket variety or remove the fuse from the SFCU variety which does put them in the catagory of a chocolate tea point.
Do you agree they are not suitable when Additional Protection is required?
If Additional Protection is required and you can't fit an RCBO in the consumer unit just fit an RCD in a 2 module one in an enclosure next to the consumer unit. I did this in my workshop for the sockets as the old metal MEM has re-wirable fuses, not exactly major engineering works! I will replace this when with a BG unit which are on sale at the moment when I have some time, a well engineered die cast aluminium powder coated finish which is IP rated, what is there not to like?
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".
I think it's simply not clear.
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 ....)
The BS says these devices only provide "supplementary" protection they require upstream "Additional Protection"
I'd assumed that the references to "supplementary protection" and "additional protection" meant the same thing - after all BS 7671 itself used to use the term "supplementary protection" to mean what we now call "additional protection" (e.g. 16th Ed BS 7671:2001+AMD2:2004 reg 412-06 "Supplementary protection by residual current device") - i.e. I suspect there's no more significance in the use of the two different terms than that the writers of BS 7288 haven't been entirely consistent in adopting the new vocabulary.
RCDs do not provide protection for upstream only downstream of the device.
Yes of course - agreed.
Do you agree they are not suitable when Additional Protection is required?
Well I can't really see the point in having Socket RCDs (or FCU RCDs) (or a BS for the same) if we necessarily have to provide 30mA RCD protection upstream. I can see the logic in them making (labouring even) the point that they don't provide upstream protection, hence if additional protection is required for the upstream wiring they you're going to have to make some other arrangements - but in the cases where BS 7671 doesn't demand any additional protection (by RCD or supplementary bonding) for the upstream wiring, I can't see the logic in BS 7288 demanding it.
The first mention of Additional Protection [AP] in BS7671 is 410.1 [ scope of Chpt 41].....I think..
The first mention of AP with reference to RCD is 411.1... I think...
415.1.1 is very specific of what is expected of an RCD for AP.
That is all straightforward.
Then 531.1.1 : Devices for protection against electric shock by ADS shall be suitable for isolation in accordance with chpt 46 and 537.
There is the table for 537 as a "guide" for suitability for switching/ isolating etc. In the RCD bit it lists BS60947-2/61008/61009 as okay for isolation. Ah you think! But a removable BS1361 fuse , a switched fused connection unit, a plug and socket are also all suitable for isolation as listed in same table.
Again you have to ask "when is an RCD not an RCD?" Or even , "when is a plug and socket not a plug and socket?"
Perhaps they can be both RCD and plug and socket or RCD and fused connection unit.
Something is only isolated [properly] when there is some means to prevent it being energised again inadvertently, such as locking off , removing the fuse or unplugging [ though strictly the last two may need further steps if the mischievous are about ].
If it is not an RCD do not call it an RCD, as it is generally expected an RCD is a protective device and BS7671 says they are for additional protection, if the correct rating.
I'd suggest not. They're two different concepts. ADS (as described by 411) requires c.p.c., earthing and protective devices co-ordinated with Zs - additional protection doesn't demand any of that and can be applied to situations where there's no ADS at all - e.g. circuits protected completely by double or reinforced insulation (non-domestic of course) - e.g. called up the cables concealed in a wall regulations.
BUT then again it could be argued that a fuse "blowing" does not, strictly, provide isolation ...
Absolutely. I've seen some BS 3036 fuse carriers where the ceramic "shield" around the fusewire as been so "spattered" with condensed(?) copper after the fusewire blew I'd wondered if it wasn't conductive itself - likewise some ruptures are so gentle that you only get a tiny gap in the fusewire. Eitherway the achievement of a 3mm+ gap and associated creepage clearances don't seem to be reliably achievable. I'm not sure of the effect of sand (or similar) in cartridge fuses, but would suspect again that we don't necessarily end up with a clear 3mm gap equivalent. There's been no suggestion of outlawing fuses for ADS, so I have a fairly strong suspicion that only manual isolation is being asked for.
In my time working as an electrician I have had to buy seven editions of the Wiring Regulations, this is the first one I bought and have on my shelf.
So when I started working as an electrician there clearly was not any issues about using BS7288 RCD devices, bearing in mind that then and now they are made to the British Standard BS7288:1990, which is still current for a few more weeks. Exactly how and why was it decided that there could be an issue with these RCD devices? I suggest that there is no issues with the BS8277:1990 versions, only the new BS8277:2016 versions that are just now being released for sale.
Also I am still making alterations and additions to electrical circuits protected by BS4293 RCDs, but they are no longer listed in the current Wiring Regulations, so is that wrong, is there an issue about using these to provide RCD protection? If there is an issue millions of consumer units with working and safe RCDs that need replacing to undertake minor alterations and additions to existing electrical installations.
Also I am still making alterations and additions to electrical circuits protected by BS4293 RCDs, but they are no longer listed in the current Wiring Regulations, so is that wrong, is there an issue about using these to provide RCD protection? If there is an issue millions of consumer units with working and safe RCDs that need replacing to undertake minor alterations and additions to existing electrical installations.
I'd say that there's no more of an issue than re-using red & black cables (which won't conform to the current version of BS 6004), BS 3871 MCBs or BS 1361 fuses. If anything a properly functioning BS 4293 RCD might even give better protection than a modern BS EN one - being that they'll disconnect within 200ms instead of 300ms (at 1x).
Further to what Alcomax said above, I pointed out in another discussion that ended up drifting onto this subject that FCURCD devices are manufactured to two British Standards, BS8277:1990 and BS1363-4:1995, so are perfectly capable of providing isolation, the IET says so.
