Consumer Unit Blanking Plates

The MCBs are not metal so why have metal blanks?

The idea is the blanks stay in place, so modular blanks, that take the place of the MCB are the way to go, as they, mostly(!), stay in place when you remove the cover.

Qbit: 
 

It is conceivable that a loose live conductor could come into contact with a metal blanking plate.  Some of the twist-on  types do not make electrical contact with the (earthed / powder-coated) case in which case they thus constitute exposed conductive parts.  This means that they should be individually earthed. But there is no means provided to do do this.  So the question is: does the use of metal CU blanking plates contravene the Wiring Regulations if fitted but not earthed?

The Niglon consumer units that I use have metal blanks to fill the gap when there are spare ways. They fit very tightly and sometimes have to be bashed into place. The cables are normally well behind the front cover and metal blanks. But, theoretically, a live cable could make the front cover live if it is damaged or gets crushed between the cover and consumer unit body. At that point the circuit R.C.D. or M.C.B should automatically disconnect the damaged cable and render the situation safe.

Z.

Have to say, despite having fitted all-metal consumer units for the last couple of years, I have never come across one either supplied with, nor fitted with any metal blanks. If any are missing I just fill in with the usual plastic items.

Alcomax: 
 

The MCBs are not metal so why have metal blanks?

The idea is the blanks stay in place, so modular blanks, that take the place of the MCB are the way to go, as they, mostly(!), stay in place when you remove the cover.

 

That is what  some manufacturer supply with metal-cased CUs.

Zoomup: 
 

Qbit: 
 

It is conceivable that a loose live conductor could come into contact with a metal blanking plate.  Some of the twist-on  types do not make electrical contact with the (earthed / powder-coated) case in which case they thus constitute exposed conductive parts.  This means that they should be individually earthed. But there is no means provided to do do this.  So the question is: does the use of metal CU blanking plates contravene the Wiring Regulations if fitted but not earthed?

The Niglon consumer units that I use have metal blanks to fill the gap when there are spare ways. They fit very tightly and sometimes have to be bashed into place. The cables are normally well behind the front cover and metal blanks. But, theoretically, a live cable could make the front cover live if it is damaged or gets crushed between the cover and consumer unit body. At that point the circuit R.C.D. or M.C.B should automatically disconnect the damaged cable and render the situation safe.

 

Z.

my point is that the blanks are not earthed.  So anyone touching a blank which has a live conductor touching it would receive a shock.  Yes an RCD might provide adequate protection but you are assuming one is fitted.  That is not always the case.  

Qbit: 
 

Zoomup: 
 

Qbit: 
 

It is conceivable that a loose live conductor could come into contact with a metal blanking plate.  Some of the twist-on  types do not make electrical contact with the (earthed / powder-coated) case in which case they thus constitute exposed conductive parts.  This means that they should be individually earthed. But there is no means provided to do do this.  So the question is: does the use of metal CU blanking plates contravene the Wiring Regulations if fitted but not earthed?

The Niglon consumer units that I use have metal blanks to fill the gap when there are spare ways. They fit very tightly and sometimes have to be bashed into place. The cables are normally well behind the front cover and metal blanks. But, theoretically, a live cable could make the front cover live if it is damaged or gets crushed between the cover and consumer unit body. At that point the circuit R.C.D. or M.C.B should automatically disconnect the damaged cable and render the situation safe.

 

Z.

my point is that the blanks are not earthed.  So anyone touching a blank which has a live conductor touching it would receive a shock.  Yes an RCD might provide adequate protection but you are assuming one is fitted.  That is not always the case.  

If the blanks are painted and not earthed they are electrically insulated from the metal earthed cover. If so they can not become live either, as the paint is insulating them. B.S. 7671 does not accept that paint can be used as an insulator but it may well happen in reality. (416.1).If their paint is scratched off and they fortuitously become earthed by being in intimate contact with the front cover then a protective device will operate and disconnect  the fault. If you are really worried then why not file off the paint on the metal blank locating point and on the opposite cover location to earth the blank? Then you have clean metal to metal.

I think that the risk of a single metal blank becoming live is very unlikely indeed.

Z.

Presumably you will have already opened the metal door to access the blank plate?

I think that this is an interesting point. (Some) Wylex boards have galvanised blanks, but painted covers. However, how do you get a line conductor flapping in the breeze? A neutral conductor could become detached and present a threat. I suspect that in reality, there is sufficient contact to trip an RCD.

Sparkingchip: 
 

Presumably you will have already opened the metal door to access the blank plate?

As any householder could. So you could have one hand on the earthed enclosure door, presuming that it is conductive, and you could touch  a live metal blanking plate with the other.

Discuss.

Z.

Chris Pearson: 

If a compact / miniature rcbo is used, the line and neutral are both quite close to the front panel At installation, human error in making live conductor connections could result in one of them touching (unpainted) Blanking plate when front cover is replaced. I realise that I am dealing with relatively low probabilities but the aim is to ensure fail safe.  For example, for the same reason, great care is taken to ensure that socket fronts and boxes are earthed.  I see no difference for consumer unit fronts.  The case itself is earthed / protected but unpainted blanking plates are exposed, conductive and unearthed.  I am questioning why some manufacturers supply uninsulated bare metal blanking plates.  I suggest that plates should either be made of non-conductive material or they should be designed in such a way to assure that they make full and reliable contact with the case (a simple screw would suffice).