TT System Consumer unit/RCD Arrangments

[stefanli69]

HI All,

Wondering what the best configuration is for a Consumer unit replacement on a TT system.

is a 30ma Split load board with a 100ma time delayed RCD as the main switch suitable in todays standards? do you completely remove the red double pole main switch and replace with a double pole 100ma RCD.


any help would be great, this is my first TT board replacement.

Stefan

 

[Taylortwocities]

Having a 100mA RCD upstream of 30mA RCDs is pointless. An earth fault of more than 100mA could/would trip one or both the 30 and the 100.
You don’t discriminate RCDs on the basis of current. You have to use time.
So replacing the main switch with a 100mA RCD not a plan.

You have a problem though. For a TT you need to protect the tails. So a time delay 100mA RCD at the origin might be used. But now you have another problem - what will you put that RCD in. You can’t use a metal box due to shock risk on this type of enclosure with a TT. But the regs say You shouldn’t use a plastic box!!
IET didn’t think about that!

Personally, I’d go for safety. 100mA type S RCD in plastic enclosure at the source and 30mA RCDs or DP RCBOs in the main CU.

 

[stefanli69]

Thanks for the reply,

I did mention a "time delayed" RCD - so they discriminate properly.... This is going to be for My NIC inspection so wanted to get some more info...

so that's what I'll do then, I'll put a 100A 100ma time delayed RCD in a plastic enclosure on the tails between the meter and the CU, and then install a standard metal split load consumer unit with 2 x 30mA RCD's protecting all the outgoing circuits.


Thanks for the help

 

[Taylortwocities]

There's an article here, produced by NICEIC that may help

Installing metal consumer units in installations forming part of a TT earthing system

The technical team at NICEIC take us step-by-step through the procedures involved when installing metal consumer units in installations forming part of a TT earthing system.

www.voltimum.co.uk

it relates particularly to TT systems. You may need to register to access the article.

There is a whole load of discussion on this on the IET forum.

 

[happysteve]

Worth looking at: On-Site Guide section 3.6.3 (pages 36 and 37 currently), for a way to do this without an upfront S-type RCD. The argument there is that on a TT system, if the board has a main switch and all-RCBOs, then "the risk of the busbar... becoming loose and making contact with the ferrous enclosure is minimal." Whereas with split boards, "this is not suitable for an installation forming part of a TT system as there is insufficient fault protection of the single insulated conductors [linking main switch to RCDs]."

If you go down the RCBO route, then you might want to consider those that are double pole (or, more likely, 1 pole with switched neutral) so that you don't need to power down the whole installation at the main switch if you need to isolate one circuit (462.2 and 461.2 refer). Wylex, Crabtree and SBS Trade offer such RCBOs.

See also OSG section 2.2.6 (pages 21-22).

 

[Risteard]

But the regs say You shouldn’t use a plastic box!!

That would only be true if you considered a modular enclosure with just an RCCB in it to be a similar switchgear assembly to a Distribution Board for use by ordinary persons - which is not a view that I hold.

 

[Taylortwocities]

That would only be true if you considered a modular enclosure with just an RCCB in it to be a similar switchgear assembly to a Distribution Board for use by ordinary persons - which is not a view that I hold.

Me neither, I was pushing the envelope n Unfortunately the UK regs use terms like “switchgear” etc without defining what it actually means.

 

[stefanli69]

So I thought I'd put the issue to bed - called the NIC yesterday...

What the guy on the other end of the phone said it to have an upfront time delayed 100mA RCD installed on the tails between the meter and the CU, the tails if possible should be enclosed in trunking for additional mechanical protection and clipped to the back of the trunking so they cant move freely, he said the enclosure is a piece of switchgear and must be metal clad, this enclosure needs earthing to the MET and then have either a split-load 30mA board or 30mA RCBO's.

Job done.

 

[Risteard]

So I thought I'd put the issue to bed - called the NIC yesterday...

What the guy on the other end of the phone said it to have an upfront time delayed 100mA RCD installed on the tails between the meter and the CU, the tails if possible should be enclosed in trunking for additional mechanical protection and clipped to the back of the trunking so they cant move freely, he said the enclosure is a piece of switchgear and must be metal clad, this enclosure needs earthing to the MET and then have either a split-load 30mA board or 30mA RCBO's.

Job done.

No-one suggested it's not switchgear. The Regulation stipulates a similar switchgear assembly to a DBO to BS EN 61439-3 which I absolutely cannot agree that a standalone RCCB is. My Area Engineer has had no issue with an S-type RCCB in an insulating enclosure during surveillance visits.

 

[Midwest]

No-one suggested it's not switchgear. The Regulation stipulates a similar switchgear assembly to a DBO to BS EN 61439-3 which I absolutely cannot agree that a standalone RCCB is. My Area Engineer has had no issue with an S-type RCCB in an insulating enclosure during surveillance visits.

See I’ve seen opinion/guidance stating the opposite. It’s about time this matter was made clear, but as they haven’t seen fit to amend that reg, or redefine the said definition, can’t see that ever happening.