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18th question.

Former Community Member over 6 years ago

Hi all, any comments welcome on this one.
Is it still acceptable to have a socket outlet for a specific use not rcd protected under the 18th?
I fitted a single 13a socket outlet recently in a loft space for one of my regular customers, it is to supply a security camera system and the suppliers asked for a socket to be provided. It is supplied from the first floor lighting circuit which doesn't have rcd protection. (16th. ed. board) There is not the slightest chance of the socket being used to supply anything else and I would like to issue a mwc stating that the socket is only to be used for this specific purpose. If it's a major issue I could get back to the customer and arrange to fit an rcbo but I don't really think that is necessary? Thinking now about going back to change socket for an rcd protected one?

0 gkenyon over 5 years ago

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.
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0 AJJewsbury over 5 years ago

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

Would it be fair to say that a RCDFCU meeting BS 7288 (and possibly BS 1363) only wouldn't be suitable (possibly due to small contact gaps) - but one complying with BS61008/9 in addition to BS 7288 would be suitable?

- Andy.
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0 ebee over 5 years ago

A sort of "Get out of Jail free" card
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0 Chris Pearson over 5 years ago

This debate seems set to rumble on and on.

BS 7288 paragraph 1 Scope: "SRCDs are only intended to provide supplementary protection downstream of the SRCD." The problem, as I have mentioned in this forum previously, is that "supplementary protection" is not defined.

So what are the differences between a BS EN 61008/61009 device and a BS 7288 one? They are different. BS 7288 paragraph 0 Introduction: "Although the BS EN 61008 and BS EN 61009 series may be applicable to “residual current devices integrated in socket-outlets” it is acknowledged that due to the specific use and location of a socket-outlet, at the boundary of the fixed installation and immediately upstream of electrical equipment powered through a plug inserted into the socket-outlet, these devices require different features."

(1) The SRCD may be operated (by ordinary persons, although it doesn't say that) several times a day.

(2) Isolation is not required because that is achieved by pulling the plug. So if you just switch off at the RCD and don't pull the plug, the load is not isolated. By contrast, the operation of a BS EN 61008/9 device does provide (at least single pole) isolation and may be locked off.

(3) "The absence of permanently connected long conductors downstream of the RCD, together with a limited number of powered appliances, justifies reduced EMC levels." So the BS 7288 SRC is not as robust as a BS EN 61008/61009 device with respect to electromagnetic disturbances.

So there we have it. The devices are different and they are not approved for the same purpose. That said, I am perfectly happy that when I use equipment outdoors, I am adequately protected by my BS 7288 SRCD.
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18th question.

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