Help needed with an RCD question

[vickery1971]

Hello again, as usual I need some advice from the learned amongst us. I went to a job today with the customer saying he was reading a book and the RCD tripped. When I got there he had sussed that it was the RFC in his flat and had turned that circuit off. I unplugged all appliances and it was still tripping. I then began to check the circuit out to check for a faulty socket or water ingress. To cut a long story short I ended up sussing that the RCD stays on until it is under load, it seems that the oven circuit (on its own circuit and working fine with the oven) but if you plug any appliance into the cooker switch it trips as soon as there is a load. I then thought it may be the RCD. When I tested the RCD I got >51ma on a ramp test and 1x >40 and 5x >400. I then changed the RCD for a new one and got the same readings. I have tested with all loads disconnected and still the same.


Answers on a postcard

 

[robsparky1975]

Guidance note 3 has some good examples of rcd testing,what was the ma rating and what setting did you have the tester on?

 

[tony mc]

With those results it looks like the RCD is faulty but you must have your readings the wrong way around. What other tests did you perform as I have my doubts re your results. How did you leave it if the new RCD gave the same readings ?

 

[zumber94]

Have you done visual inspection to identify any faults?

 

[vickery1971]

30ma RCD tester on 30 . . . . I didnt get there late and after a 11 hour day so after scratching my head for three hours i have given up and will have a go Saturday. Just couldnt siss it out. It is only a single RCD board, someone aparently replaced all the MCB's and the RCD with CED oned 3 years ago and there have been no problems. Ze 0.01 ohms, 241v supply. Just trips if you breathe on it.

 

[telectrix]

suspect a N-E fault on either the cooker circuit, or more likely the RFC. i've had similar readings due to that fault. and my crystal ball says your tester is a megger 1552/3.

 

[tony mc]

My crystal Ball say the customer will be lighting candles until he gets back if its a RCD main switch !

 

[nickblake]

with the readings you have given i would place my money on the RCD being faulty , having said that it could well be a neutral to earth fault that has developed which without a load could well be ok its when you add the load with that sort of fault that the RCD will trip , its get the test kit out and start testing insulation reading , if you get the fault intermitantly you could well have a neutral to earth fault that is effected by thermal changes , i've come across this on numerous occasions

 

[vickery1971]

Done a visual check and all seems ok. At first i thought it was a neutral issue (had same readings on a twin RCD board a few years ago and someone had got the neutrals wired up wrong)
Readings were definately >51ma on a ramp and >400ms . . . . same on two differend brand new Wylex/MK RCD's.

 

[vickery1971]

I wish I had a crystal ball!

 

[tony mc]

Disconnect the neutral on the load side of the RCD and isolate all the MCB's (test) to eliminate the RCD. Then get IR resting each cct making sure the neutrals are removed !

 

[vickery1971]

I was thinking N-E too but every socket is fine, nothing hard wired, all appliances unplugged and the customer is a pretty smart guy (ex RAF radio technician) and promised me he hasnt been playing . . . . got a headache now!

 

[nickblake]

If your RCD is tripping at >51ma and its a 30ma RCD then its faulty it only takes 50ma to kill you hence the reason RCD's are set at 30ma , change the RCD and re check the tripping times , it could be a circuit fault so you'll need to check each circuit , dont forget you have a common link neutrals are all linked to gether on each bus bar connecting each RCD if you have a neutral to earth fault on any of the circuits can cause and RCD to trip when you plug an appliance into a socket, the fault could well be on a lighting circuit

 

[markythesparky]

30ma RCD tester on 30 . . . . I didnt get there late and after a 11 hour day so after scratching my head for three hours i have given up and will have a go Saturday. Just couldnt siss it out. It is only a single RCD board, someone aparently replaced all the MCB's and the RCD with CED oned 3 years ago and there have been no problems. Ze 0.01 ohms, 241v supply. Just trips if you breathe on it.

Ze of 0.01!
Blimey you could have some massive PSSC. Probably nothing to do with your problem but that is an extremely low Ze. What's the breaking capacity of the mcbs etc?

 

[Octopus]

Have you done any IR tests?