RCD Failure modes

[Andrew Mawson]

Been having lots of trips on my main RCD, mainly in the small hours of the night. I've isolated each individual circuit, and checked with a megger to earth for leakage and there is zilch. On the last couple of nights I've manually tripped all the MCBs in the box before trying to reset the RCD to eliminate external issues and it is very reluctant to hold in, taking several attempts before it holds.

I suspect the RCD itself is knackered, and I've a replacement on order (Wylex 30mA/100A) but is this a common failure mode for RCD's?

[Andrew Gabriel]

Not common, but not completely unheard of. Has it been operated lots of times? Could be the mechanics are worn out, which can happen with MCBs too.

The other possibility is that there is an earth leakage in the CU.

Most common failure IME is a failure to trip when it should.

[ARWadsworth]

RCDs can fail like that. You will soon find out if yours has when you put the new one in. Let us know what happens.

[Andy Wade]

Did that include checking N to E? The quickest test is to isolate the whole installation, strap L&N and megger that to earth.

Otherwise hire/beg/borrow/buy/etc. a 'leakage clamp meter'. I bought a Fluke 360 a year of so ago and have found it extremely useful, for general diagnostic work as well as leakage problems.

[Andrew Mawson]

bought a

Postie brought the replacement this morning., and it's now installed and tested to trip as it should. Removing the old one, it trips with the very slightest mechanical tap, and the toggle action is far less definate than the replacement. Tonight will tell ! Andy Wade, yes I meggered both live and neutral to earth on each circut.

AWEM