RCD tripping


Postby simon_williams » Sun Oct 02, 2011 10:05 pm

I am having a new boiler installed and experiencing problems with the RCD tripping out. The boiler, fitted in the loft is being temporarily power by one of the ring mains. When power is taken from upstairs, it trips instantly. If I take extension downstairs if lasts around 2 minutes before tripping. Interestingly, If I just connect the Earth from the boiler to the ring main, it still trips without actually connecting live or neutral.
Glowworm came out to test the boiler and couldn't detect a fault. However, they did do a test where they disconneted the gas and water pipes and this was then fine.

I would be very grateful if someone recognises what this puzzle is trying to say.
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Postby jimmy_one_ball » Mon Oct 03, 2011 6:15 pm

Is it the RCD thats tripping or an MCB? The RCD will have a little 'Test' or 'T' button on it. The MCB won't, they detect different types of fault.
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Postby ericmark » Tue Oct 04, 2011 5:40 pm

If we have an earth to neutral fault and both earth and neutral are at the same voltage then nothing will happen to the RCD as not current will flow. However as we increase the differential between the earth and neutral voltages current will start to flow and the RCD will trip.

This normally becomes a problem when some heavy user is connected so a fault with an old TV will trip the system when the kettle is being used and of course the kettle is blamed not the TV.

The same situation could accrue with a volt drop across the earth as happens with volt drop across neutral although as yet I have not come across it. But seems likely this may be the case with your problem.

Something has an earth neutral and since switches and fuses don't always switch the neutral it could even be something not in use. When the water pipes are connected to the earth this changes the earth voltage just enough for the fault to start tripping the RCD.

So to cure first you must find the faulty item. As with most fault finding there are two approaches.
1) A systematic testing of all items.
2) A stab at most likely items.
Likely a little of both will find the problem. Assuming the boiler is A1 then we first have to disconnect as many items as we can from the RCD protected circuit. Switching off the MCB's is useless at it only switches line and it's a neutral fault we are looking for so all plugs unplugged and all fused connection units switched off. Is the fault still there?

If not then it's a switch back on or plug in one at a time until fault is found.

If the fault is still there then it's a meter job. The electrician will at the consumer unit remove each neutral in turn and using a special 500 volt tester measure the ohms between neutral and earth. He will be looking for a reading better than 1000000 ohms (1Mohm) he will then work out which circuit is faulty.
If it turns out to be something like the immersion heater then not so bad but if it's a ring main then he will need to split the ring and test each half. Then with bad half do same again until fault is found.

He may stab in the dark and do things like remove the supply to outside socket or lights as these are likely to have a earth - neutral fault if they get wet.

There is no magic cure and it can quick or very slow it's one of those jobs which luck plays a big part. Since it needs a tester which uses 500 volt it's not a DIY job.

So step one connect up so the RCD is tripping then go around removing every plug you can find and switching off every FCU you can find and hope you find it. If not sorry it's a job for an electrician as just not worth buying an insulation tester as also it takes some skill to use it.
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Postby simon_williams » Tue Oct 04, 2011 9:39 pm

Hi jimmy, the problem is due to a residual current and not an over-current, hence the mcb is fine. I have put a multimeter across the two earths(ring main and the boiler which is earth bonded at the gas meter), they measure zero voltage and almost no resistance as expected. But as soon as I connect them the rcd trips.
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Postby ericmark » Wed Oct 05, 2011 8:48 am

With the main switch off otherwise the supplier connection will give wrong reading measure between earth and neutral it should be open circuit.

Do remember this should really be read with a meter using 500v so using a multimeter it's only a rough reading.

Your reading between boiler and earth should be nearly zero but again with a meter using at least 200ma to test with. Zero would be impossible it will be something like 0.35 ohms or some other low figure. Zero means your using wrong scale or wrong meter.

I will guess you answered before my post was cleared so I will not continue.
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