Wylex 3rd Amendment CU - TT recommendation

[Midwest]

I've picked up my first 3rd Amendment CU, a Wylex one. Inside was a sheet containing recommendations for maintaining IP ratings for cable entries and when using a TT supply, bottom right in attachment. Now I thing I'm right in saying that the pictured 100ma RCCB's are manufactured in insulated plastic, of the type to be 'outlawed' by reg. 421.1.201. Would you say that these RCCB's are 'switchgear assemblies', as described by said reg?

 

[Richard Burns]

I would say that they are certainly similar assemblies and also likely to catch fire (well as likely as the CU anyway!) and so they must be in metal boxes, therefore you still need another RCD to protect those tails..... until you can get the RCD outside the property and do a risk assessment that says a fire in this will not affect the building structure.!!
Well thought out IET idiots.

 

[Midwest]

Chicken & egg thing, perhaps someone can come up with a double insulated RCCB enclosure, with a surrounding metal casing! Have Wylex dropped a boo boo?

 

[Dave OCD]

It seems to be getting a bit ridiculous, even on a TT install if the tails come in through a proper gland correctly fitted surely they'll be fine ? I bet a time delayed 100ma RCD won't leave you much change from £60+ on top of the more expensive board and extra work.

 

[drzsta]

Yes they did mess up there. I would probably put a 100mA type S in place of the main switch and use apprpriate glanding with double insulated tails going directly into the terminals of the RCD.

 

[Midwest]

Yes they did mess up there. I would probably put a 100mA type S in place of the main switch and use apprpriate glanding with double insulated tails going directly into the terminals of the RCD.

I can see that using an RCD as a main switch will prevent the metal CU becoming live, should the bus bar somehow become shorted out. There is still a possibility of a short between metal case and tails after they have entered the CU, even if you use a stuffing gland. There's a good video by John Ward (someone else posted before) which highlights the issue. Can't cut & paste here for some reason, just google 'TT supply to metal consumer units.

Like the idea of the consumer unit pattrass, dunno how much they cost though?

 

[drzsta]

It would be very unlikely of the tails being damaged and touching the metal case of the DB if you use the appropriately sized gland and carrying the double insulation of the tails directly upto the termination point. There is little possibility of severe damage to the tails once inside the C.U.

 

[Midwest]

It would be very unlikely of the tails being damaged and touching the metal case of the DB if you use the appropriately sized gland and carrying the double insulation of the tails directly upto the termination point. There is little possibility of severe damage to the tails once inside the C.U.

Just playing devils advocate, although unlikey, it is possible, depending on how the cables are routed inside the CU. That's why prior to the 3rd Amendment, insulated CU's or external 100ma RCD (with metal CU's) were recommended for TT supplies. Just using a plastic bush was not enough. So now because we will have to fit metal CU's, we can suddenly forget about the prior recommendations for TT supplies, that does not make sense to me. It is possible to have a highly dangerous situation to exist, when using a metal CU with TT supply. The onus is on us to resolve this. Seems no thought has been given by IET/BSI to this conundrum.

 

[sammyshooter]

Bit of a side step, but have you noticed how heavy that wylex unit is? I was shocked!

 

[Midwest]

That's cos it's made of metal

 

[Midwest]

Rang Wylex TechSupport. They hadn't seen recommendation sheet supplied with their CU. They gave me the mobile number of their sales operations manager (or something like that). So I rang him. He was bit surprised. He suggested that a ‘common sense approach should be made, and that using a plastic RCCB before a metal CU is a lessor of the two evils. If the RCCB was to catch fire as opposed to the metal CU becoming live, that would be less hazardous. But we are the manufacturer and I should seek advice from whom I’m registered’.

Waiting for a response from my scheme,......but its been a while!

 

[NDG Elecs]

Waiting for a response from my scheme,......but its been a while!

It probably will be a good while longer yet, as the schemes don't seem to know any better either! As RB says: Well thought out IET idiots!

 

[Midwest]

Well had my reply. Apparently the answer is on page 32-34, in the new yellow On-Site-Guide. Various diagrams with different CU arrangements on TT system, 'all of which are acceptable'.

Now I haven't got round to buying one of them yet (Xmas present possibly!). Anybody got one and can enlighten us with the basics?

 

[Richard Burns]

Thems the pages, a lot of good it will do you!

 

[Midwest]

Thanks Richard. Having had a quick look, seems the only viable option suggested by the new yellow OSG, is figure 3.6.3 (ii) with a 100ma time delayed RCD as main switch. There's nothing to protect the incoming tails supplying this RCD as it enters the metal CU. Does not suggest the option recommended by Wylex. I understand Wylex offer a plastic stuffing gland and Hager a tails clamp.
Prior to Amendment 3, and I quote 'The enclosures of RCD's or Consumer Units incorating RCD's in TT installations should have an all-insulated or Class II construction. Otherwise additional precautions recommended by the manufacturer need to be taken to prevent faults to earth on the supply side of the 100ma RCD' unquote.
Seems that this paragraph has been deleted from the yellow OSG.