RCD Tripping intermittently

Dal_Whinnie

We were suffering regular trips on our RCD so got an electrician in. They discovered a problem with the dishwasher which we got repaired. We have also tested all equipment by turning off particular circuit switch, unplugging all items on that circuit, turning circuit switch back on and plugging items back in one by one but no problems. The last such trip we had before Christmas when we were away.

However, and this is the puzzling one, occasionally when we switch the under unit lights on in the kitchen the RCD trips. We have had an electrician in to check this and they can't find any leakage between the lighting circuit and the power circuit. There does not appear to be any connection between the two circuits. I have arranged for a fixed wiring test to be undertaken in case this throws up anything and also because it is probably sensible to have this done from time to time.

The only suggestion for the cause at the moment is that the under unit lights are LCD 'strip' lights with a transformer (?) and this is somehow causing the problem. Has anybody else come across anything similar and if so, found a cure?

ic

The RCD could be faulty? Some devices might have a fault that only shows at particular points when they're in use - for example your central heating boiler and fridge freezer operate according to their thermostats.

zax47

Either faulty "transformer" or over-sensitive RCD.

I reckon it's the "transformer". These are not true transformers (hence my " " ) but Switched Mode Power Supplies (SMPS) and they naturally have a leakage current to earth - it's intrinsic to their design, nothing you can do about it. Your RCD is constantly looking for a leakage current, caused by a dangerous fault, to make it trip. This needs to be more than 30mA, but the "transformer" could naturally leak about 8 or even 10mA in normal use.

Two choices;

1. The "transformer" is leaking a varying amount of current and often much more than it should, more than 30mA, and tripping the RCD or,

2. The RCD is over senstive and the natural leakage level of the "transformer" is making it trip.

My money is on the "transformer", they are generally cheaply made bits of crap, with poor component tolerances, and fail with monotenous regularity. If it were the RCD then I'd expect far more tripping than you are reporting, and when other naturally leaky devices are used on the same RCD protected circuit (laptop SMPS, or phone chargers for example)

Any decent electrican should have the correct test gear to be able to test the RCD for correct operation, tripping level (30mA) and tripping time (<40mSec) and thus discount it from being faulty, or otherwise.

Dal_Whinnie

Thank you for those comments. What intrigues me is, if it is the "transformer" then, since it is on the lighting circuit and not connected to the power circuit, why does it trip the RCD?

27col

The RCD is to protect all the circuits and not just the power circuits.

bungle4by4

Your sparky couldn't find the fault??? well then, he's not a sparky. get someone else in.

rrf494g

If the RCD protects the whole house (lighting circuits and ring mains etc) then it will trip whenever it detects any earth leakage.

tartanterra

bungle4by4 wrote: »

Your sparky couldn't find the fault??? well then, he's not a sparky. get someone else in.

I would ignore that comment.
Your sparks can only investigate a fault if it is actually present. As you stated, it's an "occasional" fault and doesn't happen all the time.

If your sparks is going to carry out a full inspection and test of your installation, ensure that they issue you with an electrical installation condition report which will detail all testing carried out along with all the relevant figures.

fto

You may have a small water leak......does it trip after someones had a bath or shower?.

Dal_Whinnie

fto wrote: »

You may have a small water leak......does it trip after someones had a bath or shower?.

Good thought, but no, it doesn't

bungle4by4

!!!!!! get your sparky to test the RCD and perform an insulation resistance test to ensure your wiring is actually ok. If he runs through his tests correctly, he'll find out if its a wiring fault. The beauty of electric wiring is that it doesn't move. so you sparky can find the fault if he works his meter correctly.