RCD trips too slowly - why ?

[mcquitmp]

Hi,

I have a Mem 2000AD consumer unit that is fitted with a 80A 30ma 30ms response time RCD. Its a A80HE.

I have recently added a new circuit for a Hot Tub and on testing this the sparky found that it took 50ms to trip and said my RCD was faulty and would have to be replaced. He tested on the ring main and it also took 50ms to trip.

It sounds reasonable to me - but how do I know it wasn't just his test kit that was faulty. I thought an RCD would rarely go wrong like this. It suggests I've been running for at least some of the last 7 years with a faulty RCD.

Any view ? Is there anything else I can try to save myself £80+ or do I just bite the bullet.

Thanks...
Maurice

 

[mcquitmp]

I should have added that all other tests on the Hot Tub circuit showed it as pretty damm perfect...so it does seem to suggest something other than that circuit.

Maurice

 

[Adam_151]

Firstly, I assume it was the 5x test that he's on about, which is required to cause the RCD to operate in 40ms (the 1x test requires 200ms/300ms depending on which BS/BSEN the device is made to, so 50ms wouldn't fail on this test)

Secondly they can fail over time, have you been doing the quaterly pressing of the built in test button?... it goes someway towards keeping the mechanical parts moving smoothly among other things!

As for accurarcy of the test... was he testing the device at the board with all loads off it? (some devices can cause the reading to be unnaturally high), moving onto the test kit itself, one would hope he is sending it off for calibration yearly

Anyway, assuming your board isn't totally obsolete, I can't imagine a 2P 80A 30ma RCD would break the bank!, I think the 2000AD range is still current, John D will be along in a second to confirm it though

 

[mcquitmp]

Thanks for the reply.

The new unit costs about £82. Not cheap I thought.

I haven't been doing the quarterly test you suggested - but thanks for the tip - I will do in future.

He did the test at 5X and it registered 50ms on both the Hot Tub circuit and the ring main. He did not test at the board - but there was no load on the circuit (apart from a small neon in the isolator switch - which makes me think). Also the fact he registered the same on both circuits would suggest a common RCD fault would it not?

The device is about 7 years old (fitted by the original builders) so yes - maybe it has just done faulty - but I had not heard of that before - hence my question - but if you say it happens then fair enough.

Regards...
Maurice

 

[Steve]

£82? If this is fully fitted and tested fair enough.

 

[mcquitmp]

The cost is £69 + VAT - making it around the £82 mark.

That does not include fitting etc - but as the spark is a mate - he'll do that for free.

Regards...
Maurice

 

[Adam_151]

Also the fact he registered the same on both circuits would suggest a common RCD fault would it not?

RCD tripping times relate to the RCD itself and not to any other part of the circuit, and should be the same regardless of where the test is carried out as long as its downstream of that RCD, however the reason the test is carried out at the RCD outgoing terminals at the board with the MCBs off is because having certain equipment connected can cause the reading to appear falsely high, the only way the tester can know the RCD has dropped out is when the volts fall to zero, things like power factor correction capacitors can maintain a dc voltage on the circuit for however long it takes them to discharge through other connected loads.

Failing that, some sparks would test the RCD many times to see if repeated tests free it up a little, but its considered somewhat of a questionable practice as if the RCD has started to stick, you don't know that it won't stick when its really needed!

 

[JohnD]

I think the 2000AD range is still current, John D will be along in a second to confirm it though

yes, it is. If it is a 2000AD, it may have the "left handed" RCD with poles round the other way (this matters because it has a functional earth). These are uncommon so not widely stocked so probably not easily available discounted from list. Most MEM RCDs are right-handed fit and might be available locally below list price.

The AD can be reconfigured to change how many of the circuits on a split-load are RCD protected, so this could be a good time to consider if you want it done. Will be easy if your friend is familiar with MEM.

If you are paying full price for it make sure you tell the supplier the correct part no and that you receive the correct part if you need a left-handed one.

Regular testing will prolong active life.

If you fancy it you could move to RCBOs, but this would also mean reconfiguring the CU so would take more time than a simple swap, so maybe your mate wouldn't do it free.