DIY Doctor

Problem circuits and RCD

Postby ewallace » Tue Jan 15, 2008 12:23 pm

I have lost power to all the sockets in my house 3 times now. When I check the circuit board no MCBs are off, nor is the RCD. The lights still work. When I push down the RCD at first it will not stay on. Eventually it does . I thought it might be because I had turned off the faulty appliance but this morning it came back on without turning any sockets off.

My boiler has been off but it has still happened. At first I thought it might be the pump. It does not seemed to be related to turning on an appliance as it has happened when I am not in and during the night.

I have had an electrician around twice but he cannot find a fault so far. Please can anyone help. I cannot afford to lose more food from my freezer.
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Simply Build It

Postby ericmark » Wed Jan 16, 2008 12:43 am

Your RCD compares the power out to power returned and if the two are within 30ma of each other it should hold in. That’s theory but when tested three tests are made.
1. It must hold in with 15ma differential.
2. It must trip with 30ma differential.
3. It must trip in 40milliseconds with 150ma differential.
15ma is not much so if for example the wiring leaks 4ma and fridge 3ma and freezer 3ma and microwave 5ma then it can trip because one it is too sensitive and there is also a build up of many different items all adding up together, non on their own really being faulty. Also there are more complicated problems where capacitors in switch mode power supplies modify the wave form. As a result I do not trust RCD testers and when I get a fault like yours I change the RCD. OK for me if that’s not the fault it can go back to stock but 9 times out of 10 it is the RCD that has become over sensitive when no other faults can be found.
As to fridge and freezer these can be supplied from the non RCD protected side of the consumer unit but this can also cause problems as if it trips you may not notice. But supplying the freezer from the cooker socket one often finds on the cooker switch could help as that would not normally be through the RCD.
One problem is not only does it trip with an earth to live problem but also earth to neutral and with some items only the live is switched so even though the item is not switched on it can still cause the RCD to trip if plugged in.
Any lose connections can in practice cause the RCD to trip. As can thunder storms. The in rush does sometimes make it hard to reset an RCD normally you should turn off all the RCD protected MCB’s before resetting RCD if then it will not reset you either have a neutral earth fault or the RCD is faulty and if an electrician as you state has already checked then only thing left is faulty RCD.
Computers, Video’s and other switch mode supplied items do tend to trip RCD’s when first switched on. Any lose connections on those types of appliance are likely to trip an RCD for example the plug in the computer it’s self not being fully home.
Hope you find fault soon all best Eric

Postby ewallace » Wed Jan 16, 2008 9:32 am

Thank you Eric that is a huge help. I will try this. Thanks again Emma
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Postby 333rocky333 » Wed Jan 16, 2008 7:55 pm

The electrician that came twice what did he do to try to find the fault .

Did he use a megger or similar and test the circuits and also if possible any appliances on the circuit.

The rcd is proberly doing it s job ok.
With your type of rcd it only goes to off if done manually
An earth fault will trip it to the centre position.

As eric says it does not take much to trip these rcd's.
The mcb needs a lot more so is unlikely to go off with the problem you have.

If you are concerned, depending on what board you have and if any spare ways consider having an rcbo fitted to that one circuit and have it moved to the non rcd side,
Then the circuit will Only go off if fault on that area and not elsewhere in the house.

If it does go off turn off the mcb's except the lighting ones reset then turn mcb 's on one by one , hopefully it may trip and you can pinpoint down to a smaller area

The rcd moniters 24 hours a day and if its on then there is no visible fault for the electrician to see,
so he needs really to test for low readings that may be causing it.

Rcd's dont like to be switched on under load you were proberly not turning the fault off,but most likely reducing the load thereby it switched on.
FirstlyFind out If the actual circuit tested ok.
IF so then you need to concentrate on what was actually plugged in at the time.
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