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Time delay RCD - BS7671

Apostolos
Good morning.

We have a feeder to a consumer unit that for the last 30cm of its route it goes into a metal stud wall partition.

This was not caught early so that we could use an earthed metal conduit for this length and the walls are all up. Done and dusted.

This practically means that the feeder needs an RCD protection in the beginning.

End of story since we cannot interfere with the wall now.

The problem is that the CU has an 30mA RCD incomer itself.

So the RCD we are going to install upstream will be in series with the RCD incomer of the CU.

Does BS7671 accept a time delay 30mA RCD for the feeder that goes within the wall partition?

If it would I could still have some kind of "discrimination" with the RCD downstream in the CU.

I have been looking through BS7671 to find a respective regulation but I could not find something.

My gut says that its unlikely that I can use a time delay RCD but I cannot substansiate this with a regulation.

Any help is more than welcome!

Cheers

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  • 0
    While I agree that the current wording in BS 7671 isn't clear on that point - the previous references to operating within 40ms at 5x IΔn having been removed - the new references to acceptable standards (BS 61008 etc - see section 531.3.4) have pretty much the same effect - those standards typically specifying that a 30mA RCD must operate within 40ms at 150mA (or sometimes 250mA). Similarly I think you'd have problems sourcing a 30mA RCD to recognised standards that incorporates a delay - even the adjustable ones seem to disable the delay mechanism on the 30mA setting. In any event you can't reliably discriminate between two 30mA devices on time alone - as both will have a wide range of possible trigger currents (anywhere between 15mA and 30mA) a residual current within that tolerance range could trip trip the delayed unit (after the delay) but not cause the undelayed unit to trip.


    In your case, if you have one RCD covering the whole board, would it not be just as easy to locate it at the start of the submain (and change the RCD incomer to a plain switchdisconnector)? Either that or substitute a SWA cable for the soft sheathed one (if needs be biting the bullet and re-doing some construction work), or change the run to surface mounted (in trunking say) for that part of the run.


      - Andy.
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  • 0
    While I agree that the current wording in BS 7671 isn't clear on that point - the previous references to operating within 40ms at 5x IΔn having been removed - the new references to acceptable standards (BS 61008 etc - see section 531.3.4) have pretty much the same effect - those standards typically specifying that a 30mA RCD must operate within 40ms at 150mA (or sometimes 250mA). Similarly I think you'd have problems sourcing a 30mA RCD to recognised standards that incorporates a delay - even the adjustable ones seem to disable the delay mechanism on the 30mA setting. In any event you can't reliably discriminate between two 30mA devices on time alone - as both will have a wide range of possible trigger currents (anywhere between 15mA and 30mA) a residual current within that tolerance range could trip trip the delayed unit (after the delay) but not cause the undelayed unit to trip.


    In your case, if you have one RCD covering the whole board, would it not be just as easy to locate it at the start of the submain (and change the RCD incomer to a plain switchdisconnector)? Either that or substitute a SWA cable for the soft sheathed one (if needs be biting the bullet and re-doing some construction work), or change the run to surface mounted (in trunking say) for that part of the run.


      - Andy.
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