DIY Doctor

Power Cut Several Times in Last Few Days or RCD Issue

Postby rustang21 » Tue Jun 06, 2017 3:11 am

Looking for some advice.
In the last day or so our house has experienced what initially appears to be a power cut. Each time an initial inspection of the RCDs suggests that's they haven't tripped and so it's a supply issue.
However after the third cut we checked with neighbours and discovered only our house had an issue. A closure inspection if the RCD still suggested no issues but when I touch the master RCD with the lightest of touch the power restores. Does anyone know what might be the issue? The RCD doesn't appeared to have tripped and I'm literally touching it and not moving it all?
What could be a fault? There have been no new appliances installed/plugged in and no building work carried out recently

Thanks in advance
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Simply Build It

Postby ericmark » Tue Jun 06, 2017 9:21 am

Not all RCD's are the same, with most they are designed so you can't hold them on, so you can get them trip without the lever moving, the test button on the RCD is there to test the physical movement of the RCD and often pressing it will allow a hard reset removing any dust which may impede movement.

For 100 mA RCD's designed to allow use of a TT earth system it is common to have one for whole house, however with the more sensitive 30 mA it is considered to comply with:-
314.1 Every installation shall be divided into circuits, as necessary, to:
(i) avoid hazards and minimize inconvenience in the event of a fault
(ii) facilitate safe inspection, testing and maintenance (see also Section 537)
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation
(v) mitigate the effects of electromagnetic interferences (EMI)
(vi) prevent the indirect energizing of a circuit intended to be isolated.

Then you need more than one RCD, the exception is things like caravans, but in a house at least two RCD's should be installed, this house has 6 RCD's two in main board and four in the form of RCBO's in the kitchen board.

Although you can get 100A RCD's many are 63A and unless there are two there is an overload problem.

Although you have a test button on the RCD it does not really test it, it only checks the mechanical linkage, you need a special tester to test it, since it needs to trip in 40 mS clearly that is too fast for any stop watch, and it needs to test both positive and negative cycle, one actually presses the test button 6 times, two prove it will not trip under 1/2 ratted value, two it will trip at rated value, and two it with trip within 40 mS at 5 times rated value.

The meter is expensive, so although you may be able to physical change the RCD, you could not test it, I came to one with a faulty earth rod connection, button worked fine, but it would never trip with real fault, so there is really no option, you need to call an electrician to test and if required change the RCD, and likely if only one RCD there is some other work required.
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Postby rustang21 » Tue Jun 06, 2017 6:52 pm

I thought it might be a job for an electrician, thanks for the information, hopefully it is just the one RCD causing the issue
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