dual rcd consumer unit on tt earth.

[sinewove]

Am I right in saying a duel rcd(plastic at present)cu can be used on a tt system without an upfront type S rcd incommer?
Thanks for any replies,sw.

 

[7029 dave]

I believe you can, but not 100%, I am old fashioned up front s type 100ma is what I would install. The guys will put you straight.

 

[Leesparkykent]

IMO it's prudent to change the main switch to a 100ma S type RCD on a TT system incase one of the 30ma RCD fails.

 

[Richard Burns]

You are right you can install a standard split load twin RCD board on a TT system without an upfront type S.
This would meet the requirements of BS7671 that specifies the minimum standard to be achieved.

 

[Trickyd66]

IMO 100ma Time delay was post 17th edition when you didn't need 30ma on most circuits to cover cables buried in walls, bathroom circuits etc. It provided additional protection if ZE and therefore ZS were insufficient to ensure disconnection in the prescribed time. With a Dual RCD board and all final circuits protected by 30mA I cant see a problem. The main switch will only be protecting the tails to the RCD's. What protects the DNO's tails to the DB??
The issue will come in December when we have to fit Metal DBs... What protects the metal DB... Class 2 metal DB? or do I go into business producing asbestolux enclosures for DB's :winkiss:

 

[NDG Elecs]

IMO 100ma Time delay was post 17th edition when you didn't need 30ma on most circuits to cover cables buried in walls, bathroom circuits etc. It provided additional protection if ZE and therefore ZS were insufficient to ensure disconnection in the prescribed time. With a Dual RCD board and all final circuits protected by 30mA I cant see a problem. The main switch will only be protecting the tails to the RCD's. What protects the DNO's tails to the DB??
The issue will come in December when we have to fit Metal DBs... What protects the metal DB... Class 2 metal DB? or do I go into business producing asbestolux enclosures for DB's :winkiss:

I was recently at NAPIT Expo. The expert panel stated that come January a TT install need only have entry into the board via a stuffing gland or similar. One of the panel even said if you can get plastic trunking, with the tails in, into the board through a knockout then no upfront 100mA required. Sloppy in my view.

 

[Deleted member 9648]

I was recently at NAPIT Expo. The expert panel stated that come January a TT install need only have entry into the board via a stuffing gland or similar. One of the panel even said if you can get plastic trunking, with the tails in, into the board through a knockout then no upfront 100mA required. Sloppy in my view.

It used to be that way years ago. Wylex used to make metal CU's with a special clamped entry bush and clamped connections into the main switch RCD,which meant there was no possibility at all of the tails shorting onto the case. I cant see a problem with metal CU's on a TT today (and no upfront RCD) as long as the same precautions are taken.
Problems will arise though because this approach demands a level of skill and understanding that many of todays electricians just dont possess.

 

[NDG Elecs]

It used to be that way years ago. Wylex used to make metal CU's with a special clamped entry bush and clamped connections into the main switch RCD,which meant there was no possibility at all of the tails shorting onto the case. I cant see a problem with metal CU's on a TT today (and no upfront RCD) as long as the same precautions are taken.
Problems will arise though because this approach demands a level of skill and understanding that many of todays electricians just dont possess.

Hi WP,

I agree with a proper gland and tails being clamped adjacent to main switch as OK. The sloppy bit I meant was a bit of MT2 being used to get the tails into the CU, with no kind of cable clamp. Amateurish at best IMO.

 

[Guest77]

Am I right in saying a duel rcd(plastic at present)cu can be used on a tt system without an upfront type S rcd incommer?
Thanks for any replies,sw.

Yes of course it can, what worries me is you need to ask, I am aware negative posting is not allowed and fair enough so please take this as constructive, get yourself a regs book.

 

[davesparks]

IMO 100ma Time delay was post 17th edition when you didn't need 30ma on most circuits

Can you tell me next weeks lottery numbers too, you must have a good crystal ball if you can see what they are going to put in the 18th edition!

 

[Midwest]

Concur with insulated CU for TT suppy, prior to next year.

Apparently page 33 of the new yellow OSG, gives an example for new 3'A with TT supply.Whilst using an dedicated cable gland designed for such purpose, and perhaps a cable clamp as provided by Hager, there is still the possibility (however small) of the tails touching and making live the metal CU. Should also point out, that most manufacturers use single insulated cables to feed RCD's in dual CU and have unprotected copper busbars, that could loosen & make the metal case live. Hence OSG recommending 100ma RCD s-type as main switch.

 

[Midwest]

Doesn't resolve the issue of the unprotected tails before RCD s-type, advice prior to 3'A was insulated CU or upfront RCD. Suddenly, because we have to use metal CU, we can forget all that!