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The application of BS 7288 RCDs

Chris Pearson

The application of BS 7288 SRCDs has cropped up from time to time, so what is their application since BS 7671:2018?

531.3.6 specifies that RCDs for additional protection shall comply with BS 61008, BS 61009, or BS 62423 (type F and type B devices). BS 7288 SRCDs appear to have been acceptable up to and including BS 7671:2008+3 because there is no corresponding paragraph. This is not entirely surprising because BS 7671:2008+3 was published in 2015 whereas the current version of BS 7288 was published during the following year and in essence, it is BS 7288 which rules out SRCDs for additional protection.

Section 1 of BS 7288, Scope, says: SRCDs are only intended to provide supplementary protection downstream of the SRCD. SRCDs are intended for use in circuits where the fault protection and additional protection are already assured upstream of the SRCD.

That begs the question, supplementary to what? It can only be additional protection (provided by a BS 61008, etc. device). So why fit one?

It seems to me that there are two reasons. First, where it is desired to have protection with a sensitivity of 10 mA, perhaps at a workstation where the risk of direct contact is increased; and second, where a second level of protection is required to guard against failure of an upstream RCD.

If anybody can think of another reason for fitting a BS 7288 SRCD, do please speak up.

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    Buy NEW WIZZO BANG BREAKFAST CEREAL TODAY. SUPPLEMENTED WITH VITAMINS FOR A SHINIER NOSE, IMPROVED SEX LIFE,  AND FRESH SMELLING FEET.

    “Supplementary” = Provided in addition to something, so as to complete or improve it." e.g. Supplementary information.

    I can not find a B.S. 7671 definition though.

    Although B.S. 7671 does have a definition for “SUPPLEMENTARY INSULATION,” which is “Independent insulation applied in addition to basic insulation for fault protection”

    Now to a discussion of the term “ADDITIONAL PROTECTION”.

    Z.

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    Now Zoom has been thinking. ?

    Additional protection is additional to fault protection, i.e. it protects against direct contact as well as indirect contact. Supplementary protection (of an SRCD) protects against nothing else, but it does supplement the protection which is already there.

    Is there an analogy with vaccines? The covid booster supplements the previous doses, but it provides no extra protection against anything else; but the ‘flu’ vaccine gives you additional protection over the winter.

     

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    Timeserved: 
     

    Supplementary protection upstream to me means for example galv conduit surface run this way the cable from the distribution has additional protection via the conduit with the SRCD providing additional protection further down stream. 

    TS

    Agreed.

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    ‘Additional’ and ‘supplementary’ merely mean ‘extra’ - ‘Extra protection’ or ‘extra bonding’.

    The terms do not define any specific electrical methods or devices.

     

     

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    Chris Pearson: 
     

    Now Zoom has been thinking. ?

     

     

    NO, NO, NO young Chris. I can not be accused of thinking. I just plagiarize, it involves less work, and me being intrinsically lazy suits me better.

    Z.

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    It’s time to bump this post back to the top of the forum.

    Monday 28th March 2022.

    BS7288 RCDs now provide Additional Protection, but not Fault Protection.

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    So (and I too have not yet got a copy of the latest so I am making a bit of a leap) that sounds like we are back to being able to fit RCD sockets as an upgrade to metal trunking so long as the upstream hot wire fuse would clear the Zs and there are no bits of bare single or T and E between the socket and the fuseboard not inside trunking or conduit with holes less than 35mm diameter.
    If so this seems very sensible, lots of factories and so on will be very relieved.
    Mike

    PS can someone say if the 3m bathroom socket has now become 2,5m - it was in the draft but I'd like to know if it survived editing.

  • 0 in reply to mapj1

    Apparently so Mike, we get a whole extra 500mm to play with.

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    cheers. Well a few borderline jobs are probably compliant now then ;-)

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    Regards the three metre rule, I worked on a job where the customer has a walk in bath in a bedroom and there’s an existing double socket on the far side of the double bed which is now fully compliant rather than being on the wrong side of the limit.

    It was detailed on the certificate at the time, the lady said moving it 200 mm was ridiculous and I did agree, I had already removed the socket on the other side of the bed, between the bed and the bath as well as adding RCD protection to both the lighting and socket circuits.

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