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RCD selection and BS7671:2018+Amendment 2:2022


My question relates to the latest regulations (BS7671:2018+Amendment 2:2022) and RCD selection.

I have an existing installation with an upfront 30mA type-AC RCD.

I am quoting for some work where there will be some electrical alterations for a new kitchen. No new circuits! Mainly some new sockets and LED lighting and relocating a cooker cable. So DC components will be introduced.

Is there a requirement to upgrade the RCD to a type-A RCD to comply with 531.3.3?

Are others anticipating fitting type-A rcds on their minor works now e.g. 4 new downlights in bathroom?

The current board is obsolete so either I put a different manufactures kit in to the board or a new consumer unit. Understandably the customer is reluctant to spend any more money

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  • If you look at the figures in the new Onsite Guide they generally show consumer units with all RCBOs, which is the way we are being directed to provide circuit protection with AFDDs being the “ultimate” RCBO.

    This also allows over 100 mA of leak current to drain away out of the installation without tripping a single RCD such as the upfront 30 mA main switch mentioned in the original post.

    Nobody actually comes straight out and says that they are installing RCBOs to avoid tripping due to high leakage currents, but it’s actually happening a lot.

    The safest installation is the one with the single upfront DP 30 mA RCD, the most practical is the one with the all the DP RCBOs for a few reasons.

  • There is no shortage of solutions, it just the cost implications. On this job I could just fit an stand-alone 100A 30mA type A RCD in it own enclosure and swap the existing RCD for a used main-switch off ebay.

    Or you get a small job to fit 4 LED downlights in a bathroom and an extractor fan. 

    Is the guidance going to be that you must then fit type-A RCDs which might mean a CU change as well?

  • Each case is different - the simple swapping of one sort of light for another will be prohibitively expensive if it requires a CU change!

    The only advice can sensibly be to not make matters worse and to improve where it is easy and proportionate to do so - and what that means that will depend on the connected loads and what sort of  fault the RCD is supposed to be tripping for. 

    If we think the RCD is  mainly there to  trap damage to cables with picture hooks and so on, then the situation of a defective load blinding the RCD, and cable damage then going undetected  is a double fault to danger.

    If we want the RCD to trip on  an appliance fault that causes unsmoothed  DC in  the L- E loop however, then that is not always single fault to immediate danger,  because then even if the RCD does not get out of bed, the MCB should on a TN-x type installation, but maybe not with TT. 
    After all we have had plenty of dimmer switches that have potential to fail like that and in general faults in lighting circuits still disconnect perfectly well.

    A current limited unsmoothed DC L-E fault, is all the extra protection the RCD upgrade brings, and some robustness against false tripping on surges due to better design, and that is not all fautl cases by far.

    Lights on string switches are a lot less of a hazard than hand held plug in things, especially if bathroom bonding present as per previous editions.

    Tricky, and each case will needs some sucking of teeth.

    Mike

  • This bloke is talking about urf leekage and limiting socket numbers per circuit. He seems to know what he is talking about.

    https://www.youtube.com/watch?v=IGXto9zWmMc

    Z.

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