RCD Spur tripping question..

[ellal]

I've installed an RCD spur onto a downstairs ring which feeds an outside IP56 double socket. However, when using the test button on the RCD spur (intending to use it as an on/off), the main RCD in the CU trips out. Both RCD spur and CU RCD are contactum 30ma devices. Is this normal, or am I missing something.

edit: this happens when there is a load on the O/S socket (low voltage lights), but not when there is no load

 

[kai]

Two RCD's in series (which you appear to have) do NOT always discriminate correctly, unless the upstream one is a time delayed type, that is why you need a Type (S) RCD as the main RCD in a TT system.

 

[andy]

Two RCD's in series (which you appear to have) do NOT always discriminate correctly, unless the upstream one is a time delayed type, that is why you need a Type (S) RCD as the main RCD in a TT system.

not really. thats if your testing with an RCD tester making an earth leak. RCD test buttons dont make an earth leak so it shouldnt trip an RCD upstream

 

[kai]

Yes they do sometimes cause an earth leakage- Certain RCD test buttons pass a small current , via a resistor, between Phase and Earth (i.e. some older powerbreaker fittings I've seen), which is in effect an earth leakage.

 

[andy]

Yes they do sometimes cause an earth leakage- Certain RCD test buttons pass a small current , via a resistor, between Phase and Earth (i.e. some older powerbreaker fittings I've seen), which is in effect an earth leakage.

i know older types do, but since this is a spur RCD i would assume that it works like newer RCD's, making an earth leak accross the RCD only

 

[Big_Spark]

Gents you looking at the cause of the broken leg, not the injury...

The fact that the RCD is not tripping when the the load is used normally means that it is almost certainly caused by the test button operation.

The RCD in the DB should have a higher rating or be of the time delay type as stated above.

However I would replace the outside socket for one of the MK seal type that already incorporate an RCD. This type is electronic and willnot trip an upstream RCD, even of the same rating. The RCD spur should be removed.

I would also point out that the Contactum equipment is crap, cheap and nasty tat, the reliability of which is questionable at best...

 

[securespark]

Miaow!!

 

[ellal]

Thanks for the replies.

The outside sockets are MK Masterseal, but the RCD spur is contactum. I put the RCD in the spur so as to protect against any possibility of damage to the cable, however remote (hoping it would trip independantly of the main RCD).

Do you have an example of the type of spur that would work as required? (if possible I'd prefer to avoid changing the outside socket) - I usually buy from TLC, but they only have 2 RCD spurs which appear to be of the same type. Cheers.

 

[ban-all-sheds]

Would it be worth double checking that the RCD spur has been wired the right way round, i.e. load & supply have not got mixed up?

However, in any event, as has been said there is no guarantee of discrimination and also you should not use the test button as an on/off switch.

 

[plugwash]

considering that rcd sockets usually tell you to test them every time you use them i'd imagine that such rcds (i can't imagine a rcd fcu is any different from a rcd socket of the same brand internally) are designed to be tripped with the test button frequently.

 

[ban-all-sheds]

 

[ellal]

Would it be worth double checking that the RCD spur has been wired the right way round, i.e. load & supply have not got mixed up?

I've double checked it.

I'll probably run a cable back to a spare MCB in the split load CU which isn't on the main RCD - That'll teach me to look for the easy solution!..

 

[BJS_Spark]

If the circuit is fed from an rcd at the CU why not replace the rcd spur with an FCU with a switch?

 

[plugwash]

presumablly he wan'ted a seperate rcd to avoid tripping the main house rcd with his equipment outside.

 

[ellal]

If the circuit is fed from an rcd at the CU why not replace the rcd spur with a FCU with a switch?

Because if the outside sockets cause a trip (maybe through moisture), then all the power circuits covered by the main RCD go off as well.