How can I tell if an RCD in the consumer unit is faulty

[GaryM90]

Hi, question regarding RCDs in CUs.

Even if there is a fault in a circuit or with an appliance plugged into a circuit that is preventing an RCD in an CU from being reset should the RCD reset if the isolator switch on the CU is switched off?

Late last night about 30 seconds after turning on my bathroom lights the RCD protecting that circuit tripped. All the MCBs on that side of the CU were still up, so I turned them all off then tried to reset the RCD but it won't reset. Now I cud spend some time investigating what has caused the trip but I thought that perhaps I cud first eliminate the RCD itself as the problem so I switched off the isolator switch on the CU thinking that if the RCD was ok I wud then be able to reset it but it still won't reset. The CU has 2 RCDs & the other one stays up when the isolator switch is off so I am assuming that, since RCD detect current inbalance they don't actually need a current running thru them to be capable of resetting.

So am I right in assuming that because the RCD won't reset even when the isolator switch is off that the RCD itself is faulty & needs to be replaced.

Thanks

 

[Adam_151]

That would normally be the conclusion, but be aware that there are some brands of RCDs that are designed so that the lever needs to be pushed fully to off (having tripped to a position, near to, but not quite off) before reseting. But other than that, it should mechanically latch without power applied with the exception of active RCDs (which DIN rail/comsumer unit RCDs generally never are* - I've never seen one in this format)


*RCDs built into plugtops/sockets and fused spurs are generally available as both active or passive, depending on whats required

 

[GaryM90]

Hi Adam, thanks for your prompt response.

You mentioned that some RCDs need to push pushed fully off b4 resetting, I don't think that's the case with the model I have ( I have attached an image). I have had a few RCD trips over the years when bulbs have blown, etc & it always just been a case of flicking the switch back up to reset it.

So it looks like it's a new RCD that is needed. I assume like all electronics these things have a limited life, mine is about 10 years old. Wud u recommend having both RCDs in the CU replaced at the same time? Are they likely to both have deteriorated at the same rate so the other one will fail soon?

 

[ericmark]

I agree with @Adam_151 never seen an active RCD fitted in a consumer unit, so with isolator off it should reset. However I have found Isolators that have been bypassed in the past, and the MCB only switches the line.

 

[ericmark]

I have not had many RCD's fail, they seem to last years, however I have had a RCD fail due to excessive strain on the cables, not often, but it has happened, so it is important to test a RCD after it is fitted, but to show it will not trip prematurely, and it will trip with the 30 mA, this means using a RCD tester, which are expensive, also today I would not use a type AC like you have, I would use type A, as DC can freeze type AC RCD's.

But because it needs testing, and the testers are so expensive, I would say this is not a DIY job, cheaper to get an electrician to swap it and test than to buy a tester.

 

[GaryM90]

Hi guys,

thanks for all your feedback. Coincidentally I have an electrician coming next weekend to connect a new garden circuit into the CU & give tthe electrics a healthcheck. I can probably just about survive without power upstairs for a week by using extension cables & a couple of rechargeable LED worklamps for lighting so he can replace it then.

Thks

 

[ericmark]

This is why when I came to this house I went all RCBO (MCB and RCD combined) as if one fails, only lost one circuit. Also this house sockets wired front and rear not up/down so loosing a circuit I can use extension leads without them going up/down the stairs.

Personally I would fit a new RCD and ask the electrician to test it, but of course tell him before he comes, so he will bring the tester with him. A quick google and seems £20, but I am an electrician, so not worried about working in a CU, although when I retired son took most of my test equipment, so don't have a RCD tester any more.

 

[Taylortwocities]

@GaryM90 You could save time fore your electrician by buying a new one. But it must be the same make as the existing one.
Toolstation sell Contactum RCDs. Go buy one. You can always take it back for a refund if its not needed.

Contactum Incomer Devices 80A 30mA RCD | Toolstation

www.toolstation.com

 

[JohnD]

I don't know Contactum, but I'd suggest getting one with the same partno. Some brands change the design over time and new ones do not fit.

Edit
I see now the Toolstation one is marked CPR80 so should be the same.

 

[Risteard]

Shouldn't really be fitting a Type AC RCCB to replace the existing AC. It would be more sensible to fit something actually appropriate.

 

[flameport]

If you really must buy it for someone else to install, it's this one: https://www.toolstation.com/contactum-incomer-devices/p58826
Part number CPR100A, which is a Type A and rated 100A.

 

[GaryM90]

Hi all,

thks for all your help. Problem now resolved. It was the RCD that was faulty, been replaced & all working fine. The one that was removed looks like it has suffered some sort of mechanical failure that is stopping the switch from staying in the ON position.

Thks again

Gary

 

[Agile]

A simple plug in RCD tester should not be very expensive.

I used to have some that I sold for less than £10.

 

[oldbutnotdead]

A simple plug in RCD tester should not be very expensive.

I used to have some that I sold for less than £10.

Yeah I've got one, think it was more like £25 but serves its purpose

 

[ericmark]

I looked for cheap RCD tester on Amazon, set low to high price, this was first proper tester at £199.12, I am sure there must be cheaper, I found a cheap insulation tester, but £10 and £25 think your living in cloud cuckoo land.