TT install with metalclad CU

[kingeri]

So I went to a job today on a TT install, just to install a new shower and move an aerial socket. Noticed that the CU was metal.....now I was always taught this was a big no-no on TT, simply due to the fact that if the L tail popped out and touched the enclosure, there would be nothing to clear this fault. The only thing standing in the way would be the main cutout fuse, and with the impedances typical on TT systems, it's not gonna blow. Most concerning thing was that this CU had been recently installed. Any views on this?

 

[PEG]

Inspected one only tonight with the same issue. Unfortunately,not its only problem,NO upfront RCD,7 blanks missing and not a label in sight. TP + N and installed approx 8 years back.Marvellous...

 

[Des 56]

I think the IET permit the tails for a TT to be fitted to a metal clad, there is guidance
Something like they must be secured and the entry must be enhanced.use of a plastic or rubber gland or something else

But yes they can be used on metal TT

 

[Sintra]

Or protected by 100mA s type RCD.

 

[markc123]

an see your point Kingeri, and thats what i've always based myself on...but i don't think theres actually anything wrong with it.

This is a tricky one actually, not quite sure and i too am wondering what everyones opinions are, i do love a good debate.

Could you not fit a time-delayed RCD before the CU lol? I know then uve got nuisance tripping, but then you could bang everything on an RCBO and surely that would go first in a minor individual circuit related fault...but that would cost a bomb lol. This would hopefully not blow the main fuse. I know you're not supposed to have a time-delayed as a main switch anymore, but im sure i've read somewhere its different for TT...dammit i need my books out the van! Someone please correct me if im wrong (which i think i might be in this case)

Or, theres the aslong as it is suitably earthed then its suitable for the environment, however this would take out the main fuse if you had the main live touch the CU.

But yeah, to avoid all this confusion fit a plastic cu where possible lol.

- - - Updated - - -

and now on posting that, ive just seen Sintras last post...maybe i was right about the RCD before, someone please please clear this up for me lol.

 

[markc123]

E54 on here now, im sure we've all got the answer coming and we're all gonna be wrong come on fella, hit me where it hurts lol...

 

[Engineer54]

If you really want to go the FULL hog, then all TT systems should also have an upfront 100mA S type RCD, whether metal clad CU or not,...and all MCB's/RCBO's should be DP. Nothing stipulating any of this in the BGB though!! lol!!

 

[Des 56]

. I found a post where it explains things much better than my feeble efforts

I will assume the reg is still in the 17th,
If so,I will say again,you can fit a metal clad board in a TT system,

I also believe the supplier may not permit the supply unless there is a S type RCd in a seperate enclosure,so probably best to fit one,but not strictly necessary

The 16th Edition introduced a regulation to normalise the situation [531-04-01] in 1991 when it was first issued. This regulation is still in use [531.4.1]. It allows a metal enclosure to be used provided that the part of the installation between the origin and the input terminals of the RCD meet the requirements for class II. In the manufacturer's view, this can be achieved by improving the bushing around the cable entry, using PVC/PVC tails, and securing those tails. All of this does indeed reduce the probability of a fault between the tails and the metal case.


1) if you intend to use a metal clad ccu on a TT installation make sure that the manufacturer makes (still makes) an insulating kit.

 

[JD6400]

I was taught that you could only use a metal clad board with no upfront rcd , as long as it was shrouded with a insulating material around the tails entry and up to the main switch terminals .
That is why the older metal Wylex boards have the extra insulation inside of them , around the main switch end ? Or so i thought !

 

[JD6400]

Des , you beat me !

 

[Engineer54]

A plastic bushing can be a plastic male bush and plastic lock-nut!! If you want to make it look even better, use a coupling and then heat shrink the tails onto and within the coupling... looks a bit more of a professional job too!! ...lol!!

They are more interested in the tails insulation being compromised than anything else!! Being as tails are generally insulated and sheathed, that penetrating hole into the CU, would have to have been left pretty sharp, to cut into and penetrate through both layers of PVC!!

 

[Deleted member 9648]

Some manufacturers used to produce a bush with a built in clamp to prevent tails movement especially for this purpose...havent seen them for years though.

 

[martinh287]

I had seen one of these clamps last week thort it was a brilliant idea but its the only one ive ever seen

 

[Victor]

Here in France, where almost every installation in a TT system, EDF provide a 500mA main S type RCD and anything after that is the clients resposibility. Metal CU's are acceptable but as has been said previously, glands and gromets should be used, but this is not always the case, depending on the electrician (some just don't give a T$%^)

 

[kingeri]

Thanks for the responses guys. Surely it would still be easier to just fit an insulated DB? Why fit a metalclad one only to then have to consider additional insulating measures? Hmm. Still thinking about this one!