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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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    To my mind “supplementary protection" is just the old words for what we now call “additional protection” (BS 7671 itself used to refer to ‘Supplementary protection by residual current devices’ back in the 16th Ed). The fact that they manage to use both the old and new terms in the same sentance doesn't really surprise me given the way such text gets repeatedly modified over time by committee after committee.

    SRCDs are only intended to provide supplementary protection downstream of the SRCD

    Pretty obvious of course, but sets the context for the next sentence…

    SRCDs are intended for use in circuits where the fault protection and additional protection are already assured upstream of the SRCD.

    I reckon that was intended to mean something like: SRCDs are intended for use in circuits where the fault protection and (any) additional protection (required by the circuit itself) are already assured upstream of the SRCD. - so acknowldging that SRCDs etc won't provide any protection to supply cables concealed in walls or needing 300mA RCD protection for fire reasons etc. etc..

    But standards committees being standards committees are bound to what is actually written in standards rather than what was probably meant, so the BS 7671 committee rather has its hands tied.

    That's just my guess though - I could well be entirely wrong.

       - Andy.

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  • 0

    To my mind “supplementary protection" is just the old words for what we now call “additional protection” (BS 7671 itself used to refer to ‘Supplementary protection by residual current devices’ back in the 16th Ed). The fact that they manage to use both the old and new terms in the same sentance doesn't really surprise me given the way such text gets repeatedly modified over time by committee after committee.

    SRCDs are only intended to provide supplementary protection downstream of the SRCD

    Pretty obvious of course, but sets the context for the next sentence…

    SRCDs are intended for use in circuits where the fault protection and additional protection are already assured upstream of the SRCD.

    I reckon that was intended to mean something like: SRCDs are intended for use in circuits where the fault protection and (any) additional protection (required by the circuit itself) are already assured upstream of the SRCD. - so acknowldging that SRCDs etc won't provide any protection to supply cables concealed in walls or needing 300mA RCD protection for fire reasons etc. etc..

    But standards committees being standards committees are bound to what is actually written in standards rather than what was probably meant, so the BS 7671 committee rather has its hands tied.

    That's just my guess though - I could well be entirely wrong.

       - Andy.

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