RCD Problems. Serious Head Scratching!!

[BangTidy]

Loooking for a bit of ideas on a issue if have with a RCD. 4 circuits on a submain that i recently changed, above the board was a leak so intally thought the problem was related to that. When carrying out the RCD auto test when it came to x1 the meter read error and would not trip. If i set it to x5 it tripped.

So put it down to a faulty RCD and changed it today, so when i auto tested this time it only goes and trips on x 1/2.
First off i checked my meter on another circut testing an RCBO on the main board which went fine.
So i disconnected all the loads on the problem board and tested individually but still the problem of trip on x 1/2. Next i disconnected the incoming supply and carried out a insulation resistance test 500v between conductors, no problem there.

I carried out a insulation test on the lighting circuit and had a reading of 0.04M between Live and Neutral but with all the lights as GU10 fluorescents would i be right in thinking the reading is down to this?
Anyway with the lighting circuit disconnected from the board still the RCD trips on x 1/2 so this has got me seriously scratching my head.
What i wondering is the board is the 3rd submain from the main panel and there is a 2 way RCD unit supplying a outside socket in the middle which im unsure where is fed from at this stage. Could this be a problem having another RCD downstream?

In the end put the old RCD back in tested that and is ok on x 1/2 but get the error message and no trip on x 1 again!!

Came away to get my thinking cap on and any help on what the problem may be?
The tester is a megger 1553 and was calabrated 6 months ago.

Any ideas gratefully accepted?

Cheers

 

[Kiers1970]

Cheers for the replys, ive got hold of another RCD from the wholesalers and give that a try and see tomorrow. Ill put it through a ramp test aswell and get back with the outcome.
Hoping it was just another faulty RCD.

Cheers again for the ideas.

It wont be mate, guarantee you have a fault somewhere, like i said earlier, I had the same thing this week.

 

[Engineer54]

It wont be mate, guarantee you have a fault somewhere, like i said earlier, I had the same thing this week.

So which of the faults did you have this week then?? Because our guy here has experienced 2 different faults on two different RCDs. So if he has an installation fault, it's a fault that affects 2 RCDs at two different functional levels!!

I'm not saying that it isn't an installation fault, because RCD are, or can be fickle things.... But it is more likely that these two RCDs are faulty units...

 

[Kiers1970]

be a hell of a coincidence.

 

[Kiers1970]

this week this happened
a fellow electrician changed his consumer unit to a twin RCD CU
when using appliances in his kitchen, RCD tripped
this led him to believe logically that he had a faulty appliance/ wiring
tested wiring, ok, changed the kitchen ring over to the adj rcd, all ok.
so he as the OP wazs left head scratching
called me in.
reconnected and tested, rcd set no probs, all lights and power ok
used a few kitchen apps, rcd goes.
test 1st rcd, no probs
test second rcd, no trip.
all circuits test out fine!
so , disconnect all neutrals, switch off mcbs, rcd trips fine. within times.
go thru all circuits one by one, remove and replace neutrals as we go, rcd testing as I go , bingo, fault on lighting circuit, strapper crushed behind switch, made good, tested all ok.
OP , try this, I've not read if you've tried this, or if youve tried the rcd test with no circuits connected, if it operates fine without any circuits connected, its more than likely ok.

 

[Engineer54]

be a hell of a coincidence.

Not as uncommon as you may think Kiers, we've had, ...on more than occasion whole batches that were U/S that had to be returned. I'm not talking about the cheapo variety but top of the range stuff. You only have to look at the forum posts here, to see just how common RCDs actually do fail. As i stated they are, or can be, fickle things. Which is why it is always better to have a decent Ze, Ra, Zs on an installation and not rely on RCDs as many are doing these days, especially on TT systems where they become the sole means of earth fault protection...

 

[Deleted member 9648]

While RCD's clearly fail from time to time...I can honestly say that of the hundreds I have installed as far back as I can remember,there has only been one faulty one that I recall,a Proteus that literally went up in smoke as soon as it was energised. We fit mostly crabtree,MEM or proteus.....they have all tripped within the specified times,and if they havent it has always been because of connected loads....I have occasionally come across existing units that fail to trip,but this has always been down to 'sticky' operation because the test button is never operated...a few trips and they are OK again.
Perhaps I have been lucky...but hand on heart that has been my experience with RCD's.

 

[Engineer54]

While RCD's clearly fail from time to time...I can honestly say that of the hundreds I have installed as far back as I can remember,there has only been one faulty one that I recall,a Proteus that literally went up in smoke as soon as it was energised. We fit mostly crabtree,MEM or proteus.....they have all tripped within the specified times,and if they havent it has always been because of connected loads....I have occasionally come across existing units that fail to trip,but this has always been down to 'sticky' operation because the test button is never operated...a few trips and they are OK again.
Perhaps I have been lucky...but hand on heart that has been my experience with RCD's.

Maybe because of the size of projects i work on, i'm going to see far more RCD failures than most here, after all we will be installing literary thousands of the things lol!! Having said that, we are seeing an awful lot of RCD failures of one type or another here on the forums. Yes some will obviously be nothing to do with the RCD devices themselves but installation faults or inexperience of the testing electrician. But there is still, a large amount that will be down to duff RCD devices...

 

[JD6400]

I know that a fair majority of our installs are in a more aggressive atmosphere than most - agricultural , plant rooms and equestrian .
But we do tend to have on average at least one newly installed (straight out of the box) and several existing rcd's that we find have failed each year .
As stated rcd's are a useful bit of kit , but not the be all and all of it .

 

[BangTidy]

Hi all,
Update on things. This morning i tryed out the 3rd RCD which i got from the wholesalers and what i can say is the tests went fine with good tripping times.
Ive read a few replys since my last post and im very surprised myself at the problem just being a simple RCD unit fault all along, all being with 2 new different units.
What i can say is the consumer unit i installed was a cheap Nigilon as it was only for a friend and not a customer. This really has made me think twice on buying cheap materials again, as its bit me in the arse with the running around and wasted time looking for faults that were not there.
Cheers again for the feedback.

 

[Engineer54]

Sorted then!!