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Caravan RCD trip power connections

Dear all , a holiday question

I have an acquaintance who asks why his IET 18th Edition RCD tester when plugs it in to the Local RCD (<30mA/300mS) , the primary supply RCD trips (that is further up the AC supply line) , rather than the caravan’s, so in effect nullifying the test.

Is this because the Primary Supply RCD is monitoring a small standing current from other connections and the Local RCD needs to be tested by using the Local Earth at the unit ? ( which is not so easily accessible).

Are there any other thoughts please ?

Paul , Swindon LN IET.

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0 kfh over 4 years ago

It's a case of fastest finger first. They both see the same fault at the same time and the fastest wins, of course they sometimes both trip. The standing load on the main RCD may give it a head start as I notice that when there are two RCDs in series if only one trips it is usually the furthest away but I have always put that down to sods law but I could be wrong. I am sure there is someone on here who can give a more technical explanation ?
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0 kfh over 4 years ago

It's a case of fastest finger first. They both see the same fault at the same time and the fastest wins, of course they sometimes both trip. The standing load on the main RCD may give it a head start as I notice that when there are two RCDs in series if only one trips it is usually the furthest away but I have always put that down to sods law but I could be wrong. I am sure there is someone on here who can give a more technical explanation ?
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Caravan RCD trip power connections

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