MCBs Busbar and magnetic screwdrivers

[bernardgreen]

When fixing MCBs in a CU do not use a magnetised screwdriver for the bus bar clamp screw. Or if you do use something non magnetic to push the screw back into the MCB if it has come forward stuck magnetically to the screw driver as it is removed.

If the screw isn't pushed back then the clamp in the MCB remains closed and the tongue of the bus bar goes behind the clamp instead of into the clamp. As a result the tongue is not clamped when the screw is tightened.

The bus bar is pulled forward by the clamps in adjacent MCBs ( assuming those tongues are in the clamps ) so the tongue that is not in the clamp is pulled forward and presses against the back of the clamp. It makes electrical contact and thus the circuit appears to work and it seems also test as being properly clamped.

 

[PrenticeBoyofDerry]

I normally do a visually on the MCB clamps on busbar to make sure they are gripped correctly.
I have come across some army installations and others, where the engineer/installer in question had on many occasions made bad connection as described above, whether it was a magnetic screwdrivers faulty or incompetence of the installer, I will let others decide.

 

[Lectrician]

One of the largest causes of burning out.

 

[PrenticeBoyofDerry]

Nasty!

 

[JohnW2]

I normally do a visually on the MCB clamps on busbar to make sure they are gripped correctly.

I think that's very wise - I've always been a bit paranoid about this. Whilst what bernard says about magnetised screwdrivers is one possible mechanism, it is very possible to 'get it wrong' when not using a magnetic screwdriver, particularly with some makes of device.

I can't help feeling that it ought to be possible to improve the design of some of these clamp terminals, to make it much more difficult (ideally, impossible) to get things wrong.

Kind Regards, John

 

[Lectrician]

I normally do a visually on the MCB clamps on busbar to make sure they are gripped correctly.

I think that's very wise - I've always been a bit paranoid about this. Whilst what bernard says about magnetised screwdrivers is one possible mechanism, it is very possible to 'get it wrong' when not using a magnetic screwdriver, particularly with some makes of device.

I can't help feeling that it ought to be possible to improve the design of some of these clamp terminals, to make it much more difficult (ideally, impossible) to get things wrong.

Kind Regards, John

Old memshield 2 used to have a plastic wigit in behind the terminal to force the bus far onto the correct side.

 

[bernardgreen]

I normally do a visually on the MCB clamps on busbar to make sure they are gripped correctly.

Short of using a dental mirror it seems almost impossible to get a clear view of the under side of the MCB once installed in the CU.

One "old hand" when confronted with these "new fangled" consumer units would remove the DIN rail, assemble the MCBs and bus bar and then fit the DIN rail back into the CU.

 

[JohnW2]

Old memshield 2 used to have a plastic wigit in behind the terminal to force the bus far onto the correct side.

I've seen some which are pretty good, but, in my limited experience, most of those around today, even the 'trusted' major brands, seem to be far from foolproof.

Kind Regards, John

 

[Lectrician]

Some don't actually clip onto the din rail, they sit onto the top of the rail, the bottom just secured by the busbar. If the MCB is not secured to the busbar, you can still lift the MCB up.

 

[JohnW2]

Short of using a dental mirror it seems almost impossible to get a clear view of the under side of the MCB once installed in the CU.

In many CUs it is. That's one of the reasons why there are always inspection mirrors in my toolboxes!

One "old hand" when confronted with these "new fangled" consumer units would remove the DIN rail, assemble the MCBs and bus bar and then fit the DIN rail back into the CU.

I've sometimes been tempted, but have never actually done it! Whether it can be done obviously depends upon where the DIN rail fixings are.

KInd Regards, John

 

[JohnW2]

Some don't actually clip onto the din rail, they sit onto the top of the rail, the bottom just secured by the busbar. If the MCB is not secured to the busbar, you can still lift the MCB up.

True, but many (most?) do clip onto the bottom of the rail.

Kind |Regards, John

 

[RF Lighting]

Just fit hager

Untitled

• RF Lighting
• 3 Feb 2015
• 2

Hager on the left, mem on the right.
It's impossible to miss with hager.

 

[Lectrician]

Three phase MEM had the plastic wigits. I assume as with TP you were more likely to miss a terminal without knowing. You can't look under a TP boards busbar after all.

 

[Taylortwocities]

You don't need to be careful anymore, as everything will be housed inside a fire-resistant enclosure

 

[JohnW2]

You don't need to be careful anymore, as everything will be housed inside a fire-resistant enclosure

I still worry a bit about that. This change, seemingly the result of lobbying by 'fire investigators' and fire brigades may possibly have an (I would think extremely small) effect in relation to fires, but I'm not convinced that enough thought has necessarily be given to the totality of the situation. I may be wrong, but I do fear that, particularly in relation to 'less-than-fully-competent' DIYers, metal CUs may well increase the risk (and incidence) of electric shock.

Kind Regards, John