40 amp MCB live when off? Faulty?

[AJB Temple]

I have a number of circuits in a not very old Hager sub CU, with split RCDs. All of the active circuits (8 used in a 12 slot dual RCD CU) test fine. RCDs tripping fine under test with Megger MFT. However, I have a MTN B140 (40 amp obvs) MCB in the board that has nothing connected to it. ie unused. It's live (230v) at the busbar terminal as expected, but also live at full voltage at the top terminal even if the MCB switch is off.

All of the other MCB's behave as normal, with the switches operating as expected. It's not tripping the RCD, presumably because it is an unused circuit breaker.

Is this just a faulty MCB? Obviously I can swap it out for another one, and I don't want a faulty component in the board.

I'm more curious than anything. Any thoughts on causation?

 

[westward10]

So it has voltage at the outgoing terminal when off but what do you mean it is not tripping the rcd.

 

[AJB Temple]

Yes, it has 230v live at the outgoing, irrespective of whether the switch on the MCB is in the on or off position. Toggling the switch does not trip the RCB (nor would I expect it to in normal use). There is no circuit connected to this MCB and I have no need or present intention to connect one - it is just a spare.

 

[Taylortwocities]

Remove the MCB and fit a Hager blank.

Hager JK01B Single Module Blank

Buy online Hager JK01B Single Module Blank with 24 hour delivery from Edwardes

www.edwardes.co.uk

 

[littlespark]

Simply a faulty mcb. Can happen.

If there’s no circuit connected, it’s not getting a back feed from somewhere else like you would with interconnected rcd’s… so only explanation is a faulty breaker.

Would be good to know history of installation… was there ever an electric cooker or shower on this breaker? Is there an unused cable cut back in the board of decent size that may have one been connected?
It may even have been faulty from new, came with the board but never used… so never discovered it was faulty.

Yes, take it out, fit a blank.
If you leave faulty breakers in place, then it’s an increased risk of someone in the future using it for something.

 

[snowhead]

What test instrument are you using that shows 240v out of the 40amp mcb?
Have you tried removing the mcb and testing continuity?

It could be jammed mechanism or overloaded and contacts welded together.
If when removed it still has continuity then tap the mcb onto a hard surface and retest.
If that doesn't clear the continuity then carefully open it up and see if it's welded together.

But as others above if it's not needed then remove and replace with blank.
And before disposal destroy the MCB so it can't be picked up by someone and used.

 

[pc1966]

Most likely reason is it had been closed on a few too many high current faults, though it might just have been faulty from new (e.g. bit of swarf stuck inside).

Remove and destroy!

 

[AJB Temple]

Thanks guys. It is a brand new and previously unused MCB so has never been abused in any way, or had too many trips etc. Anyway I've popped it out and replaced with a spare B132 as I don't have any blanking plates and I want to use the slot for a radial circuit to a utility room in due course.

Cant see anything physically wrong with the MCB but putting a test current through on the bench, it is clearly by-passing the switch as if it is not there. Quite dangerous really as someone could think they are switched off when they are not.

I decided against tapping and such like, as if it is faulty once with swarf or whatever, it can go faulty again as the switch is a sealed unit. Should not happen with a good brand really.

I identified it with a quick live check all round the CU. I use a Megger continuity tester with proving unit, and for testing a Megger MFT 1731. (I know - I am a DIYer but I still understand testing and circuits). Obviously I am not domestic trade so I don't see many faults like this. The MCB unit was actually bought from Amazon (who are a lot cheaper than CEF !) and they are only a few pounds each, but I will return it as faulty anyway.

 

[pc1966]

The MCB unit was actually bought from Amazon (who are a lot cheaper than CEF !) and they are only a few pounds each, but I will return it as faulty anyway.

Suddenly I am not surprised! Possibly fake?

I would open it and check, or better still report them to trading standards.

 

[James]

Suddenly I am not surprised! Possibly fake?

I would open it and check, or better still report them to trading standards.

Trading standards would be where I was sending it.

also, if you have any others from there, I would replace them with ones sourced from reputable supplier immediately.

 

[Moley]

Remind me of a Big Clive teardown

 

[Moley]

Okay, try again. Reminds me of a Big Clive teardown

 

[Moley]

There are times when I hate my phone

 

[AJB Temple]

Yes, it could be fake I suppose. All of the others came from TLC in Tunbridge Wells - I buy nearly all of my electrical stuff from them. The Amazon one looks absolutely identical in every respect: same plastics, weight, printing etc and same packaging. Hardly seems worth counterfeit fakery for things as cheap as this. I doubt trading standard would take any interest in something under £10 like this.

 

[pc1966]

I doubt trading standard would take any interest in something under £10 like this.

It is not the price, it is the risk to safety.

At the very least Hager would want to know if there is a manufacturing fault or there is a dodgy side-trade in fake/failed ones.