RCD adaptor plug and CU RCD



Originally posted this on the Appliance forum, probably by mistake since its more relevant here I think

I have an intermittent tripping with the RCD in my Consumer Unit. I think, but can't prove, its the dishwasher which is now about 5 years old and we have hard water etc.. I had thought, mistakenly I now think, that I could prove it was the dishwasher by using a plug in RCD adaptor for the dishwasher. Trouble is, now if there is a trip they both (the RCD adaptor the dishwasher is plugged in to and the main RCD in the CU). My house also has a main RCD between the meter and the CU, this rarely trips at all.

I've had a quick look through the forums but I can't see an answer explaining how I can prove 100% its the dishwasher, or the washing machine. Obviously we could stop using it, but sometimes its several week between trips and I would want the wife to ruin her hands by getting the Fairy out :) I could also plug in the dishwasher to a circuit on the non-RCD side of the CU and see if the main RCD trips, but again I wouldn't want to do this for weeks at a time.

I'm guessing that even if I get a spark in with his test equipment it would only show positive if the dishwasher had a continual earth leak, as opposed to an intermittent one caused by water on the wash/dry cycle.

Any ideas?
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As you have found, 2 RCDs usually means they both trip (or sometimes only one, but it is not really predictable which one it is).
However the adaptor plug for the dishwasher will only trip if there is a fault on whatever is plugged into it. A fault elsewhere would not cause that one to trip, only the main one in the consumer unit.
This suggests it is the dishwasher causing the problem, unless the adaptor is the type where removing the power causes it to trip.

To be certain - plug the RCD adaptor into a socket, plug a small appliance into it (e.g. a table lamp). Switch on.
Now switch off at the socket only, then switch the socket on again. Does the lamp come on - or do you need to press the 'reset' button on the RCD again?
Your best bet will be to find a socket that is not on the consumer unit's RCD. Dioes your cooker switch have a socket on it too?

Plug an extension lead into there with your RCD adaptor. Run the dishwasher. If the RCD trips its down to Comet for a new dishwasher.

OR (better). Find a sparky with an insulation tester or PAT tester and have him test the dishwasher.
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hmmm...just tested it. damn, the RCD needs resetting when the power is switched off so this isn't going to help me!

does anyone know of an RCD adaptor that doesn't require this?
In general we fit RCD's in series with a 3 times tripping current, so a 10 mA plug in is fed by a 30 mA in consumer unit and with commercial we could continue with 100 mA, 300 mA 1 amp etc.

As we go up we also have time delay.

However when a worker put a nail in the wall to hang his coat up and hit the cable it took them all out.

As to testing there are three testers. The RCD tester measures time as well as imbalance, but does not really find faults just tests the RCD, the insulation tester is the main tool, which uses 500 volt DC for the test, since DC this may not find all faults, and the clamp on ammeter can show the imbalance due to induction and capacitive links, but since it needs the circuit live to test, can't always be used.

Since the appliances use heating elements with mineral insulation it seems likely one of the elements is leaking to earth, and likely it is near the neutral voltage and neutral voltage varies to the earth voltage due to load, so turning an item one which causes it to trip, does not mean that item is at fault.

In the main testing earth to neutral with the insulation tester will show the faults, testing line to earth often fails as some timer or relay has turned off the faulty bit, but they tend to only switch line, so neutral to earth works better.

But one the testers cost around £40 plus, and second they use 500 volt DC which tends to hurt if you touch the wrong bit. But it likely takes less than 5 minutes to find out which item is faulty, so normally better to get some one in to test them.

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