RCD and replacement fails to operate

[westward10]

If you have a neutral/earth fault on a TT system the impedance of the earth path maybe such that the current will choose the neutral route and hence no imbalance is detected.

 

[Adam Lench]

I'd definitely recommend disconnection of the load. Would be interested to know how you get on. Good luck

 

[weevilward]

Yeah I learnt that the hard way too, in a hurry to get out of a pub (!) before they opened - did the live RCD testing without fully unplugging everything - found the trip times were really slow - way past the limits, so I thought I should change it. Came next day with a new RCBO. When I powered everything down and removed the outgoing conductors I thought "I'll just do one more test before I remove it..." and it tested spot on. That was a Merlin RCBO, and the cause was a fridge. I've seen the same effect a few times and it only seems to be things with motors that cause it ???

 

[jibjob]

I had a similar problem on a TT a few years back due to a N-E fault. I can't remember the details but I'm sure it was a single split load board with a main switch & a standalone 100mA RCD. The fault was on the non-RCD side of the board which added to the confusion. Once I'd located the faulty circuit I left the neutral disconnected & then retested the RCD & hey presto.......!
This may not be the issue here but it sounds very familiar to the one I had, hope this helps if you haven't already sorted it.