UK RCD TRIPS BUT MCB DOES NOT

[Tommy29]

I have isolated the MCB feeding my garage when switched on trips the RCD. There is no load on the garage I have an internal switch in the garage that switches all circuits off. So the fault must be between the MCB and the garage switch. The armoured cable is under the patch so to dig up would be horrendous. Could it be a faulty MCB?

 

[telectrix]

you need the services of an electrician to test the installation to the garage and identify thre fault.. otherwise youmay end up digging up a perfectly good cable.

 

[ruston]

As above .
Is it a double pole switch ?

 

[James]

In short, the chance of this fault being a faulty mcb is so small, it’s verging on the impossible.

All this wet weather lately if I had to guess I would say water ingress is most likely.
You need an electrician to insulation test it first .
At least you will have a definite answer.

 

[Tommy29]

In short, the chance of this fault being a faulty mcb is so small, it’s verging on the impossible.

All this wet weather lately if I had to guess I would say water ingress is most likely.
You need an electrician to insulation test it first .
At least you will have a definite answer.

Thanks that’s what I has thought. I have checked insulation between Earth and Neutral and there is no indication of a short. I did not megger it only a multi meter.

 

[Tommy29]

As above .
Is it a double pole switch ?

How do identify if the MCB or RCD is a double pole switch?
[automerge]1583250595[/automerge]

As above .
Is it a double pole switch ?

the MCB is single pole

 

[MJPD29]

I have isolated the MCB feeding my garage when switched on trips the RCD. There is no load on the garage I have an internal switch in the garage that switches all circuits off. So the fault must be between the MCB and the garage switch. The armoured cable is under the patch so to dig up would be horrendous. Could it be a faulty MCB?

Highly unlikely to be an mcb. I'd get someone to do a bit of testing for you, as changing random parts such as the mcb would be false economy. Is the switch in your garage single or double pole? If only single the fault cou,d be upstream of that rather than underground. But again testing would find this out.

 

[ruston]

How do identify if the MCB or RCD is a double pole switch?
[automerge]1583250595[/automerge]

the MCB is single pole

It was the main switch in the garage l was referring to. @Matthewd29 has answered your question in post 7

 

[Tommy29]

It was the main switch in the garage l was referring to. @Matthewd29 has answered your question in post 7

The switch in the garage is a fuse spur switch.

 

[radiohead]

I'm confused as to how a 'faulty' mcb could possibly be thought to be the cause of an RCD tripping??

 

[ipf]

Thanks that’s what I has thought. I have checked insulation between Earth and Neutral and there is no indication of a short. I did not megger it only a multi meter.

You haven't 'checked insulation' just lack of continuity. Multi meters tell fibs to the uninitiated, when it comes to IR. Needs L involved, too.

 

[MJPD29]

The switch in the garage is a fuse spur switch.

Highly doubtful that it is double pole then

 

[James]

I'm confused as to how a 'faulty' mcb could possibly be thought to be the cause of an RCD tripping??

Read post 4, beat you too it!

 

[Floody]

I'd be inclined to say you have an earth to neutral fault somewhere. As said above, wet weather. Has anything in the installation changed recently?

 

[Carlspark]

I have isolated the MCB feeding my garage when switched on trips the RCD. There is no load on the garage I have an internal switch in the garage that switches all circuits off. So the fault must be between the MCB and the garage switch. The armoured cable is under the patch so to dig up would be horrendous. Could it be a faulty MCB?

If the SWA cable is in fact under the patio, then the cement holding the paving down could have eaten through the cable. It happens!

 

[davesparks]

If the SWA cable is in fact under the patio, then the cement holding the paving down could have eaten through the cable. It happens!

Do you have any evidence of this, it sounds highly unlikely unless the SWA was already damaged.

 

[James]

15 minutes work, disconnect swa from both ends.
IR test the cable.

You will then know if the fault lies with the cable or in the garage installation itself.

At a guess, if it’s a normal garage with a socket circuit and a lighting circuit only, I would expect a decent spark that is used to fault finding should have a diagnosis within an hour.

If it’s an easy to find one, probably have it diagnosed and fixed within the hour.

 

[Intoelectrics]

If the SWA cable is in fact under the patio, then the cement holding the paving down could have eaten through the cable. It happens!

As davesparks stated the cement is highly unlikely to damage the cable. I've seen people get chemical reaction burns from cement/concrete its nasty stuff if it gets on your skin and reacts.