RCD wont trip on Neutral to earth


Postby collectors » Tue Aug 02, 2011 6:05 pm

Hi, was helping a friend out the other day & a new consumer was fitted with a 80 amp 30ma rcd controlling the whole flat.
He tested everything & all seemed to be fine, with the exception of when the neutral & earth was shorted out directly the RDC didn't trip, yet it tripped with the test meter & also via the RCD test button. So he thought it was a faulty rcd & changed it but still getting the same results.
The intake is a 1980's PME.

Any suggestions??

Thanks.
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Postby Brickworkz » Wed Aug 03, 2011 9:06 am

A link that you may find useful: electrical.theiet.org/wiring-matters/16/earthing-questions.cfm?type=pdf
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Postby kbrownie » Thu Aug 04, 2011 8:40 pm

When you connected the neutral and CPC together what positions were breakers and main isolator in.
The theory behind RCDs are that they measure for an imbalance between Line and Neutral so if no current was flowing within the conductors to start with, there is a chance that this did not create an imbalance.
What test have you carried out on the RCD other than the functional one for the mechanism on the unit?
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Postby ericmark » Fri Aug 05, 2011 10:38 am

The RCD measures balance as already said between line and neutral. With a TN-C-S supply very likely it would not trip. But that's not how an RCD is tested.

The meter will test the RCD 6 times.
1) Half rated value on pos cycle should not trip
2) Half rated value on neg cycle should not trip
3) Rated value on pos cycle should trip
4) Rated value on neg cycle should trip
5) Five times rated value on pos cycle should trip in 30ms
6) Five times rated value on neg cycle should trip in 30ms
The button marked test is only to check the mechanism is not stuck and does not test the time it takes to trip you need a special meter to test a RCD.

To change a consumer unit will require an installation certificate to be completed and to fill in the values a full test set will be required. As well as the RCD tester you will also need an earth loop impedance tester and a high ohm meter the latter testing at 500v i.e. not a multi-meter.

The whole set can normally be hired for around £75 for min of a week. Or bought for around £750.

Unless the guy is a member of a scheme then before starting you need to inform the LABC and pay their fees of over £100 so they will issue the completion certificate.

So by time one adds the £200 for LABC charges and meter hire just not worth doing as a DIY job.
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Postby collectors » Sun Aug 07, 2011 9:28 am

Thanks for the info guys, will be going back to fish off soon, so will double check the testing. The guy i am helping (Qualified) has all the up to date test equipment but was in a bit of a rush that day & will have a bit more time on the next visit.


Cheers.
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Postby russell fields » Fri Nov 11, 2011 11:36 pm

just to clarify RCD test
tested at half times the RCD rating should not trip when tested for 2 seconds (both half cycles)
tested at 1 times the RCD rating should trip within 200mS if it is an old bs RCD or 300mS for a newer bs-en RCD(both half cycles)
if the RCD is used as additional protection then tested at 5 times the RCD rating, which should trip within 40mS (both half cycles)
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