One for the Double D club.

[The Baker]

I had a job the other evening repairing a Boiler where an RCBO tripped in a customer’s house whilst I was out at the van.

The first I new about this was when the customer asked if I had turned off some of the electrics.

My initial thought was that although I had isolated boiler I may have allowed the N & E to touch so causing an imbalance at the RCBO.

However after a couple of minutes unsuccessfully trying to reset the RCBO I realised that as the connection between the boiler and the electrical supply was via a three pin plug which I had removed from the socket outlet,

So if understand things correctly I had not only switched off the boilers electrical supply but also totally isolated the circuitry of the appliance from the domestic wiring,

After trying to fathom out what was causing the Double D RCBO to fail for about an hour, I removed the ring main conductors from the RCBO (removed the load), and as the RCBO still would not reset I recommended the customer get an electrician.

I ran a temporary supply to the boiler via an extension lead.

Later the customer rang to say their electrician explained that not using the test button on the RCBO regularly had contributed to its demise.

I am not a proper Sparkie like what you lot are, so would appreciate any crumbs of info you might see fit to throw my way.

Tim
________________________________________

 

[ban-all-sheds]

It's known that RCDs (and presumably RCBOs?) become "sticky" if not regularly tested, i.e. they don't trip as fast as they should.

 

[JohnW2]

It's known that RCDs (and presumably RCBOs?) become "sticky" if not regularly tested, i.e. they don't trip as fast as they should.

That is certainly said of RCDs, but that is obviously a bit different from what the OP is describing (tripped and wouldn't reset, even with load disconnected), and doesn't explain why it tripped in the first place.

As for RCBOs, I don't know. SP RCBOs look as if, they may be mechanically similar to MCBs with the added RCD functionality. We certainly don't hear much about MCBs 'sticking' in the same way as RCDs but, of course, that may simply be because they are not tested.

Kindest Regards, John.

 

[Adam_151]

If by Double D, you mean Square D?

Then some of the older QOE rcbos need to be pushed fully to the off position before reseting, it can easily catch the unwary out as it doesn't even really look like its sitting in a middle tripped position

 

[RF Lighting]

I was just going to say that. It's caught me out before now!

 

[The Baker]

Yes it Square D, but I thought Double D would get more attention.

Anyway thanks for the info.

Tim

 

[ebee]

I`ve often wondered if we did have a tester to readily test MCBs what the results might well be and how concerned we might become.

We always accept them as functioning (correctly) unless we know different.

 

[ericmark]

I was helping the owner of a caravan park when his main moulded breaker was tripping far too often. All caravans supplied from a 6A MCB in the main the old Loadmaster type. So I fitted a 16A plug to a 1 or 2Kw fan heater clearly at 1kW it should hold and 2kW it should trip. Of 100 MCB's tested in that crude manor 34 failed and did not trip at 2kW and were replaced and this did stop the main moulded breaker from tripping.

As to if this was due to manual interference or just old age we can only guess but either way 1/3 of the MCB's failed that can't be good.

 

[ebee]

When you say 1/3 failed on your test then are you accounting for a tolerance on your 2KW heater, whether voltage is 230 or 240 and the fact that on overload 2KW/230V puts you pretty much on the limit of a trip within 2 hours . All of your MCBs might have in fact been within tolerance

 

[JohnW2]

When you say 1/3 failed on your test then are you accounting for a tolerance on your 2KW heater, whether voltage is 230 or 240 and the fact that on overload 2KW/230V puts you pretty much on the limit of a trip within 2 hours . All of your MCBs might have in fact been within tolerance

Indeed so. To test a B6 MCB you'd really need a load of ~3kW (which should cause operation in a minute or two) but, even then, you'd only be testing the thermal part of device. If you wanted to also test the magnetic bit (which is what matters in terms of disconnection times etc.), you'd obviously need at least 30A for a B6 and increasingly impractical (probably 'unthinkable') test currents for MCBs of higher ratings and/or Type C/D ones.

That's obviously the main reason why routine testing of MCBs is not undertaken but, as has often been discussed before, it is rather worrying that this means that we haven't a clue as to how many faulty ones are in service.

Kind Regards, John.

 

[ebee]

Absolutely,
I only accounted for the overload characteristic here because I felt that was what the OP was "Testing" with his heater test rig set up.

You are correct that the same testing (lack of) applies to fault characteristics too.

We always take it a face value that it does what it says on the tin.

I am not aware of any major testing procedure done on a decent sample in service MCBs (or fuses either).

I wonder if we might be fearful if we had such results?

 

[Adam_151]

After hearing about what happened regarding federal stab lok breakers in the US, I do wonder about the older range of federal we had here (the ones that had a black bakelite type casing).

I have seen a case where a crushed 1mm t+e blew the copper clear around the fault rather than tripping the breaker. However it was a type 4 and a very long circuit. Didn't get the chance to measure the zs though, it was a case of joint it through and get it back on

 

[JohnW2]

I am not aware of any major testing procedure done on a decent sample in service MCBs (or fuses either). I wonder if we might be fearful if we had such results?

Exactly. The problem is that we just don't know and, as you say, it's far from impossible that we would be fearful if we knew the truth.

Because we can test them, we know that RCDs can often fail to do what it says on their tin, particularly if they are not regularly operated. On the face of it, the magnetic part of an MCB mechanism is not all that much different, so it wouldn't surprise me at all if they suffered to at least some extent from the same problem.

As RCDs (which we can/do test) become an increasingly common part of the equation, there is a backup for at least some of functionality of an MCB (in TN installations), but we just don't know how many installations are suffering from a lack of effective overload protection - the cutout fuse obviously does not provide adequate overload protection for individual final circuits within the installation.

Howver, I can't really see any solution. Routine in-situ testing of MCBs is unlikely to ever be practical or even safe - and, of course, it would probably be impossible with higher-rated MCBs in the presence of a cutout fuse.

Kind Regards, John.