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Posted By Topic: Faulty RCD...?

May 26 2015 20:15

I installed an RCD at the switchboard protecting 2 lighting circuits and a couple of Under carpet heating circuits. It was working fine for a few months, with no issues.

My client contacted me yesterday saying they lost power to the RCD protected circuits i had installed, but neither the RCD or MCB\'s had tripped. 30 minutes later these circuits came to life again and worked for the rest of evening. I investigated the next day and found everything working fine.
Activated all the protected loads on the RCD, tested the RCD with RCD tester, activated the test button. all good
Checked terminations at the board. all good
Checked neutrals. all good.
Tonight, I received a call saying the same thing happened again. Lost power to RCD protected circuits for 20-30min without tripping the RCD or MCB.

Never seen an RCD fault like this before. As this was all working fine before leads me to think it must be an issue with the RCD as other circuits in the house are not affected.

I plan to return tomorrow and replace the RCD, but need a backup plan in case its something else....Any suggestions.


May 26 2015 20:20

if it\'s in an area you regularly go why not hook a batten holder up to the output with a bulb, next time it goes faulty ask whether the lamp is on or off.

May 26 2015 20:54

All on the same phase?

May 26 2015 21:10

Single phase.

The lamp theory would work, means another couple of site visits though.

May 26 2015 21:32

Logic would suggest that it must be the RCD if it affects all three circuits, and you have checked all connections.

Did you do a loop test to check for any resistance in the RCD contacts?

Otherwise just replace it and put it on test at home if you want - although without actually installing it, it wouldn\'t be very convenient trying to test it with 5-10A of load over a long period of time.

May 27 2015 08:58

My money would be on a connection problem rather than an internal fault.

Temporary faults can be a real bugger to locate, never there when you go looking.
A loop test using the low-current (so as not to trip RCD)won\'t have enough current to help track down a high-resistance connection.

Losing all 3 circuits indicates either RCD input, RCD output, or mcbs input; so double-check all these points

If you can\'t find a connection issue, suggest you set a trap, eg a relay supplied directly from each side of RCD. Any temp power loss at point of connection will drop the relay out.


May 27 2015 12:12

Would need a battern holder on the input as well incase its not the RCD.

May 27 2015 17:12

Just a shot in the dark but have you perhaps got it on the load controlled circuit? Being off for that length of time and then returning ok. And wouldn\'t have been an issue over the summer months.