T T consumer unit upgrade

[Ade1309]

Hi I am installing my first tt system consumer unit I understand that I need a 100 amp time delayed rcd main switch but can I use rcbos in line next to them or Mcbs and does any one know of a lockable board as I need to install it vertically thx

 

[brianmoooore]

Isolation on TT should isolate all poles, circuit protection only needs to disconnect the line conductor/s.

N-E fault would trip the RCD if RCBOs didn't break both poles. this would contravene the one out, all out reg.

 

[davesparks]

I think you are missing the point.

It doesn't make any difference not knowing about the installation, it doesn't change the fact that:

The use of a timed delayed RCD is NOT a requirement.

Maybe I amissing your point, please do explain what your point is??

 

[mainline]

Maybe I amissing your point, please do explain what your point is??

Ade1309​

“ I understand that I NEED a 100 amp time delayed RCD main switch”


The point is that you don't “NEED” a time delayed RCD it's one of the options, it's NOT a requirement.

 

[mainline]

N-E fault would trip the RCD if RCBOs didn't break both poles. this would contravene the one out, all out reg.

Are you saying that every consumer unit that's fitted with SP rcbos aren't compliant ?

 

[timhoward]

I think the intention of both myself and @mainline (we posted at same second)
was to encourage the OP to look into this more. Saying that an S-type up front RCD is needed implied that they hadn’t fully looked at the regs concerned or the OSG’s interpretation of them. I said “it’s not a given”but I’m sure we both meant the same thing.

I could have just answered the question but when someone’s first ever post on here is about installing a TT consumer unit I didn’t want to assume they should be doing the job in the first place.
I have to say that the regs are far and scattered concerning this and it’s little wonder that someone new would find it hard to understand them.
I normally don’t rate the OSG but on this topic it’s very helpful.

 

[brianmoooore]

Are you saying that every consumer unit that's fitted with SP rcbos aren't compliant ?

IF it is fitted with an upfront RCD.

 

[mainline]

Are you saying that every consumer unit that's fitted with SP rcbos aren't compliant ?

IF it is fitted with an upfront RCD.

Ahh the IF word makes a difference.
But yes agreed, and it’s another reason to try and avoid using an upfront rcd.

 

[happysteve]

I wonder if I might throw an SPD-shaped spanner in the works here?

If the guidance in the On-Site Guide (section 3.6.4, page 41 of the little brown book) is that dual RCD metal boards are not suitable on TT systems due to the risk of the single-insulated L feeds to the RCCBs making contact with the metal box, presumably the same argument could be made for situations where the L feed to an SPD inside the metal DB is in the form of a single-insulated cable? In which case a time-delayed RCCB acting as a main switch might be a good idea. 534.4.6 (concerning fault protection of SPDs), and figures 16A2 and 16A3 of Appendix 16 refer. Of course, your SPD will not protect your upstream RCCB in this case.

If your SPD is fed directly from the bus bar - such as the single module Wylex SPDs - then this is equivalent to having all RCBOs, a situation that the OSG says an upfront RCD would not be required (though of course it may be needed, or desired, for other reasons).

 

[happysteve]

... and reading a bit further on in the big brown book, 534.4.7, SPD installation in conjunction with RCDs:

Where SPDs are installed in accordance with Regulation 534.4.1 and are on the load side of an RCD, and RCD having an immunity to surge current of at least 3kA 8/20 shall be used.
NOTE 1: Type S RCDs in accordance with BS EN 61008-1 and BS EN 61009-1 satisfy this requirement.
Installation of Type 1 SPDs downstream of an RCD is not recommended.
NOTE 2: In the case of a surge current higher than 3kA 8/20, the RCD may trip causing interruption of the supply.