RCD and replacement fails to operate

[John Bailey]

On completion of some recent minor work , I carried out the usual testing and noticed the incoming 100A 30mA rcd on the Consumer unit was failing to trip at X1 and X5 at both 0 & 180 degrees Further investigation revealed that the test button on the device failed to trip it as well . All well and good so far faulty RCD me thinks . How often do these devices actually get tested by the end user (despite the quarterly test label ) Replaced it . Checked again and although test button now trips, it still fails to pass RCD ramp or time tests ? Just in case the replacement unit was faulty ( however unlikely ) , I had another unit on the van changed that out with the same result .
Earth loop impedance testing on both on the ring final and incoming Ze, very low reading less than 1 Ohm bearing in mind this is a TT installation ( does seem exceptionally low,) I have checked all round the incoming supply head no sign any PME link I may have missed ????

Any ideas fellow sparks

 

[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.