I have been experiencing problems with a 30mA RCB on a DB tripping. Most of the wiring to this DB is SWA cable and is about 2 years old. Nothing has obviously changed since before the tripping problem started. The tripping is still occurring when nothing is connected to the power circuit and all the MCBs on that board are switched off and nothing is plugged in. When the lighting circuits (2 with soft start dimmer switches and GU10 bulbs and 1 a simple switched incandescent bulb) and the power circuit with neon indicators and outdoor heaters plugged in but not operating do not appear to trip the RCD when I have tried them. Initially I thought there was no obvious reason for the tripping but noticed that sometimes it trips when a load is switched on on a circuit that is connected to a different DB that has its own 30mA RCB which does not trip. This 2nd DB comes from the same supply on the main house DB. The whole system is also protected by a 100mA RCB which is also sometimes tripped when a load trips the 30mA RCB. I have done some tests and the RCB trips when various things on the 2nd DB are switched on - a garden shredder, fan heater and 2 different electric drills and a lighting circuit with 12v transformers. I have also tried a drill with variable speed and if I start it very slowly and gradually increase the speed it does not trip until almost at full power. When I do this last test on the problem power circuit it trips no matter how 'soft' the start. Any advice on how to cure this problem would be greatly appreciated.
Unfortunately I do not have a suitable tester but I do have a second RCB and have tried exchanging them - it does not significantly change the situation although I could guess that one is slightly more sensitive than the other. Of course they could both be over sensitive but that is not likely.
Accumulative earth leakage, neutral earth fault on fixed wiring or appliance, dodgy RCD or a mixture of all of them. Only real way to narrow it down would be to get a decent electrician who possesses the necessary test equipment.
You will have to travel many miles to find a DIYer who even possesses an RCD tester, let alone understand what the correct test parameters might be! Its funny, when an RCD or an MCB trips, the typical DIYer's thought is that the problem is the RCD/MCB and not that there is an underlying reason why the device has operated.
Disregard the MCB Baz - to avoid red herrings then a first instance course of action in my case would be to ramp the rcd - it just does not take very long to do tbqh..I have found a faulty rcd before - if I didn't ramp it then it would have been a long process to finally nail it down.
Pre-electronic RCD's you did get electrical storms tripping them. And yes not all 30 mA at 40 mS RCD's are the same, some do resist spikes better than others, and some will work between 15 and 30 mA and others 27 to 30 mA however in the main when a RCD trips on a regular basis there is some thing wrong. The basic problem is the MCB is single pole, and the RCD monitors all live wires both line and neutral, so switching off an MCB will not remove a neutral earth fault. Also the neutral earth fault is made worse the more power is used. Bit of damp toast in a toaster can cause a RCD to trip when the kettle is used. The reason is down to simple ohms law, as more power is used so the voltage drop increases and as the voltage increases so the current increases. So with a TN-C-S supply where neutral and earth are linked at the head and you have 16 mm tails feeding two RCD's as I have then they in turn supply two old fuse boxes with MCB's replacing the fuses, one may think the two RCD's are completely independent, however that's not the case the small volt drop on the 16 mm tails can be enough so with a neutral earth fault switching one a load to one fuse box will cause the RCD feeding the other box to trip. The test instrument is an insulation tester, these use 500 volt and with main isolator off so earth and neutral are not connected together you test the resistance earth to neutral. Some times you are lucky and a multi-meter using a couple of volts will find the fault, but also often the multi-meter will not show a fault, yet the insulation tester on the same circuit will show less than one meg ohm. As for DIY main thing to remember is do all you can to disconnect the neutrals. Don't simply switch off at socket, unplug instead, some sockets do have double pole switching, but not all, so unplug all you can. The real problem is items which auto switch on functions during a cycle, be it the washing machine, or the frost free freezer, switching on a heater during part of the cycle means testing can show all OK at the plug, but once running, as the defrost cycle comes in, the freezer trips the RCD. Some times testing neutral to earth finds it as inside the freezer it is single pole switching, but when used with German sockets it should not be single pole switching.
As Mr. MGW above. But you'll need an Insulation resistance test meter to get to the bottom of it. It will be quite a bit cheaper to get an electrician in for a couple of hours than to buy one. But ask about for an electrician who 'always finds the problem quickly' rather than one who doesn't know what he's doing.
I have managed to narrow down the problem by disconnecting various circuits totally in turn and now have one external lighting circuit off at the MCB with the neutral disconnected. So far it is not tripping anymore so it looks like that is where the issue is. Will now check all the junction boxes etc to see if any obvious faults. If that does not solve it my problem is knowing where to get a local electrician who will know what he is doing and solve it in an efficient manner. Thanks for all the guidance
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