Discuss Metal Clad board on TT system in the Electrical Forum area at ElectriciansForums.net

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Hello all,

Currently pricing up a job, which is to install a new ring, light and shower circuit for a small extension. The exsiting board is full so I was considering putting in a second board just for these circuits. After flicking through osg i was going to use a main switch and individual rcbos. However just realised that it is 2016 so would have to use metal clad board. Doe this mean that I have to put an s type rcd in a seperate enclosure before the new board, if so can the rcd be in plastic as it is not classed as switchgear. Or is there another alternative I am missing.

Many Thanks
 
i'd be looking at upgrading the xisting CU to a bigger unit.
 
From what I've read I think the general consensus from the scams is to use a plastic stuffer like the wiska tail gland kit so the tails are nice and secure and if you can get a board with the tail clamp like hager do then even better.
 
He is talking about using one to protect against the dis board becoming live.

That's right, I did a board change just before Christmas and had to look it up,
If you are using all RCBOs then you are ok to just use a main switch.
If you are using a dual rcd board then you should use an rcd to protect against the board becoming live.

The theory is that with 2 or 3 rcds in the board there is a greater chance of one of the cables feeding the rcds coming loose and causing the metal board to become live.
If you are just using RCBOs then the risk is negligible.

Of course, the tails need to be securely fixed and enter through an appropriate gland.
 
The theory is that with 2 or 3 rcds in the board there is a greater chance of one of the cables feeding the rcds coming loose and causing the metal board to become live.

but that's like saying that a CU will only catch fire if it's plastic. a poor termination is the root cause of both scenarios, but the cure is based on treating the symptoms, not the disease.
 
but that's like saying that a CU will only catch fire if it's plastic. a poor termination is the root cause of both scenarios, but the cure is based on treating the symptoms, not the disease.

You're probably right Tel. it's not me saying it, it's the IEE.
All I'm saying is that I was unsure so I consulted the on site guide.

But to play devils advocate for a while, if you have a live conductor firmly fixed going into a board and into a switch and coming out of the other side of the switch is a solid copper busbar firmly fixed by the switch and several MCBs then there is very little chance of any live conductor coming into contact with the metal consumer unit.
If however the other side of the switch has a loose cable coming out and going unsupported to the top of an RCD and then from there looping out to another RCD and who knows maybe to a third RCD sometimes. Then the risk is higher.
 
You're probably right Tel. it's not me saying it, it's the IEE.
All I'm saying is that I was unsure so I consulted the on site guide.

But to play devils advocate for a while, if you have a live conductor firmly fixed going into a board and into a switch and coming out of the other side of the switch is a solid copper busbar firmly fixed by the switch and several MCBs then there is very little chance of any live conductor coming into contact with the metal consumer unit.
If however the other side of the switch has a loose cable coming out and going unsupported to the top of an RCD and then from there looping out to another RCD and who knows maybe to a third RCD sometimes. Then the risk is higher.
agrre, but if the bloody thing is installed properly in the first place........
 
I'll bet that not one forum member has ever come across a situation where a metal cased CU has had a short circuit from tails or busbar....or become 'live' because of the same on a TT system.
The risk is absolutely zilch if the CU is properly installed by a skilled person.
 
I'll bet that not one forum member has ever come across a situation where a metal cased CU has had a short circuit from tails or busbar....or become 'live' because of the same on a TT system.
The risk is absolutely zilch if the CU is properly installed by a skilled person.

agrre, but if the bloody thing is installed properly in the first place........
Well I've just tried to answer the op as to what I think is correct. I'm more than happy to be corrected or proved wrong.

Help me out here, you're saying to the op that he does not need to install an up front 100ma rcd regardless? Whether he fits a dual rcd board or not?

Is this what you are advising?
 
He is talking about using one to protect against the dis board becoming live.
Are you a medium? have you got a crystal ball? I cannot work out how you know that from the post submitted, it doesn't say that at all, apologies if a mod has edited a post out I haven't seen.
 
Are you a medium? have you got a crystal ball? I cannot work out how you know that from the post submitted, it doesn't say that at all, apologies if a mod has edited a post out I haven't seen.
To be fair, I took the op to mean protection from the board becoming live.

He does mention the posibility of using plastic.
 
Well I've just tried to answer the op as to what I think is correct. I'm more than happy to be corrected or proved wrong.

Help me out here, you're saying to the op that he does not need to install an up front 100ma rcd regardless? Whether he fits a dual rcd board or not?

Is this what you are advising?

In order to protect a metal CU with a high Ra using an S type RCD upfront on the tails it would be necessary to install the RCD in a metal enclosure as it is classed as associated switchgear. Clearly this creates another problem!
Personally I have no issue with using a metal cased CU on a TT system without upfront RCD protection regardless of internal components because as previously stated, properly installed by a competent electrician there is zero chance of a fault to the metal case from non protected tails. My preference would be a metal cased CU with a 100ma S type main switch and either duel RCD or RCBO internals. With an S type RCD main switch the only risk of a fault to the metal case is from the tails and live side of the RCD,and this risk is easily totally eliminated.
 
Are you a medium? have you got a crystal ball? I cannot work out how you know that from the post submitted, it doesn't say that at all, apologies if a mod has edited a post out I haven't seen.

Why would I need a crystal ball? It's extremely obvious that this was the reason for his question.
 

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