RCD keeps tripping...

[Konradvg]

Hi, I wonder if anyone could help me. We have recently done some renovations work and my electrician has installed a new consumer unit with RCDs etc. Separate to this is a small older unit which must have been installed when the previous owners had an electric shower installed - this is the only thing that is fed from it and it is not connected to the new consumer unit. We've left this untouched as the shower is still being used but eventually it will go so then my electrician can strip this out.

There are 2 RDCs installed and the left one keeps tripping. There are two definite scenarios - one is when I switch the electric shower on, the other one is when I switch the main switch on the consumer unit off and on again. The RCD has been tested and isn't faulty.

To eliminate anything being wrong with the shower my electrician has disconnected the small unit but still the RCD will trip when I switch the main switch off and on again.

My electrician is scratching his head and said he'll have to come back and spend a day or more trying to figure out what's going on but I wonder if anyone could point us in the right direction?

Any questions please ask!

Thanks v much,
Konrad

 

[Spark123]

When he replaced the consumer unit he should have tested the circuits. What are the insulation resistance test results?
With a separate consumer unit causing it to trip it smells of a N to E fault on one of the circuits attached to that RCD (or an appliance plugged into a socket circuit on it) and a TN-CS supply.

 

[Echo the husky]

My electrician is scratching his head and said he'll have to come back and spend a day or more trying to figure out what's going on

A day or more?!?! 10 minutes with an insulation tester should at least narrow it down to one circuit.

 

[Konradvg]

Thanks for your feedback guys. Could I narrow this down myself by switching off one by one each of the trip switches that fall under that RCD and see if the RCD trips when I switch the shower on? Or is it not that simple?

Thanks!

 

[Echo the husky]

As Spark said it is likely to be a neutral to earth fault. Switching off the trip switches only disconnects the line side, not the neutral so will not narrow down the fault. It really does only take a few minutes to test through each of the circuits with an insulation tester, and is the only really practical way of doing it.

I do the out of hours call outs and get called out to this type of fault frequently!

 

[ericmark]

My RCD will trip when the other one is reset, nothing wrong except I have very old RCD's totally mechanical and prone to tripping with spikes on the supply. New better quality RCD units would cure my problem, I am just too lazy to change them. Not all RCD's which claim 30 mA at 40 ms are the same. Some high quality types are claimed to trip at 90 ~ 100% of tripping current where cheaper types trip at 50 ~ 100% of tripping current.

There is always some leakage, the more wiring on each circuit the more likely to trip. My mother has 4 RCBO's in the kitchen which have never tripped, replacing the RCD with 4 RCBO's must be cheaper than a days work trying to find the fault, which if he has done his job correct should not exist.

To trip on inrush when you first switch on is common, as I say with my pair I often need two or three goes to reset them. But also had the same at my mothers house, I used the RCD tester no fault, I used the insulation tester and no fault, I swapped the RCD anyway and did not trip again, well at least only tripped with a real fault.

In early days of RCD many of us had problems, but they have been around for at least 20 years now, mine are over 20 years old, so electricians should have come across the faults many many times, and by now they should be able to fault find most faults within 1/2 an hour. The faults are now well know, from borrowed neutral on the stairs lighting to the outside light with water in it we have seen all the faults many times.

20 years ago yes they caught out many electricians. But today they are common place no excuse.

 

[Konradvg]

Found it! There was a light in the utility room that hasn't been working for ages (before the consumer unit was installed) so I never thought to look at that but there was an earth leak there. The light still doesn't work but at least it's no longer making thing trip - very happy. Thanks for your help everyone.