Square D QOE MCBs

[RF Lighting]

I am currently rewiring a house which has an old QOE CU installed. Following two entirely seperate faults on the upstairs and downstairs lighting, which were both a short L>E, both MCBs have been destroyed.

I can't seem to find a kA rating on them, but the Zs was nothing special, so there will not have been a huge fault current.

The QOE CU is currently temporarily fed from the new CU on a 40A type B which has also tripped on both occasions, so the discrimination doesn't seem to work very well there

Just wondered if this was a common thing with these MCBs? I suppose it's better than them failing closed

 

[333rocky333]

Seen panels full of them in Tescos fitted in the 80's , do not recall ever changing any.
Even the previous black ones still working ok

Think they have M ratings on them somewhere

Whats that below the green window

 

[RF Lighting]

There was no M rating that I could see which was a bit odd. I'll have to go have another look.

The one in the photo above does, but that was just from google images.

 

[Adam_151]

The QOE CU is currently temporarily fed from the new CU on a 40A type B which has also tripped on both occasions, so the discrimination doesn't seem to work very well there

It never does between MCBs! You'd need your Zs at the point of the fault to be >1.91 to avoid taking out the B40, its not as simple as fuses where you can rely on the final circuit fuse clearing before the pre-arcing of the submain fuse is reached.... if a fault is enough to clear an MCB, then it will 'unlatch' it as quick as the mechanism can move... and largely indepandant of the magnatude of the fault, and once its unlatched thats it, no matter that the downstream breaker is clearing the fault, the submain one is still going to trip

 

[RF Lighting]

Just been and got one of the dead ones. It was just me being blind

It is printed in tiny letters at the bottom of the MCB

It was M6 rated so should have been able to safely clear the fault.

Maybe just one of those things.

I might open it up and see which bit failed.

 

[333rocky333]

Does the switch still go over.
Seem to recall there the ones with a pretty slack mechanism.

 

[RF Lighting]

The switch still moves, but doesn't actually latch on, and the little indicator stays on green.

OOI, it is one of the rare breed of 6A BS3871 MCBs

 

[333rocky333]

Try pushing it off a bit more, sometimes works.

 

[RF Lighting]

Tried everything including taking it out of the CU and bashing it

It's not too much of a problem. All of the circuits are getting transfered across to the new CU eventually anyway.

It was more of a post out of curiosity. For one MCB to fail like that would be no huge suprise on older equipment, but thought two doing the same thing within a week of each other in the same CU was maybe more than just coincidence.

 

[Adam_151]

Just looking at that pic... it struck me...

spot the federal influence on those...

(though going by the reputation of some federal devices made for the us market, then perhaps you are lucky it didn't fail in the on state...)

 

[plugwash]

I seem to remember reading somewhere (don't remember where so I may be wrong on this) that the breaking capacity of a MCB is the maximum fault they can safely break which is sometimes higher than the maximum they can survive breaking.

 

[Spark123]

The switch still moves, but doesn't actually latch on, and the little indicator stays on green.

OOI, it is one of the rare breed of 6A BS3871 MCBs

What type? I have one or two of these spare if you need any.

 

[ricicle]

I seem to remember reading somewhere (don't remember where so I may be wrong on this) that the breaking capacity of a MCB is the maximum fault they can safely break which is sometimes higher than the maximum they can survive breaking.

IcS / IcU

 

[VanSolo]

Those things were the worst Square D breakers, even the old black ones were better than them. The rcbos were even worse, practically every one I test now fails. Mind you they're a fair age now.
I keep a good stock of the more modern breakers as I have come across quite a few that have failed in the way RF has described.

 

[RF Lighting]

The switch still moves, but doesn't actually latch on, and the little indicator stays on green.

OOI, it is one of the rare breed of 6A BS3871 MCBs

What type? I have one or two of these spare if you need any.

Thanks for the offer, but as I work on each circuit, I'm transferring it over to a new CU with BSEN 60898 / 61009 devices