How do I comply TT system

[stevo]

Hi guys wondering what's the best way to do this got a board change on a TT system 3 bed terrace. So we need to offer an SPD however can't have one in a metal C.U without RCD protection on the Tails because of the single insulation feeding the SPD. So to do that that we need an s type RCD upfront. But we can only use a metal clad enclosure there so the price is rising quite a bit.. Or do I forgo the SPD due to cost and take the Tails directly to the main switch obviously correctly supported and properly glanded this will be for my 1st niceic inspection so want to get it right. Cheers

 

[Strima]

I use a unit with SPDs already fitted, main switch and RCBOs. Use a tails gland to support the tails coming into the board.

SPD board and main switch

Double check RCBOs are suitable for use on TT systems, this will be in the manufacturers information.

Tails gland

 

[brianmoooore]

At Strima: Your first link doesn't work.

 

[stevo]

So we are using rcbos to limit the risk of a single insulated cable making the c.u live. But we are ok with the single insulation cable feeding the SPD?

 

[pc1966]

Use the 19-strand "flexible tails" to reduce forces on the wires and a proper cable gland for entry. It also keeps the wires in place as well as avoiding the risk of insulation damage. Of course you also tighten the cable fixing screws correctly ,ideally using a torque screwdriver.

If you worry about the RCBO being a single point of failure then use a 100mA delay RCD up-front. A properly installed CU has far less risk of a L-E fault before the RCD than a RCD failing in the 10+ years it is likely to be used.

More so considering the test button probably only gets exercised during EICR...

 

[Dave OCD]

The 'single insulated' live conductor supplying the SPD from an MCB is generally a very short loop and well away from any part of the metal enclosure, so any risk is tiny if it's tight in the terminals. Where do you draw the line....

 

[westward10]

I think the point the OP is making is that some factory fitted SPDs are not connected through a protective device.

 

[Dave OCD]

Ah sorry I didn't realise that.

 

[stevo]

The 'single insulated' live conductor supplying the SPD from an MCB is generally a very short loop and well away from any part of the metal enclosure, so any risk is tiny if it's tight in the terminals. Where do you draw the line....

I completely agree there. Very unlikely that it would ever make contact. just don't want to be pulled on the fact its not double insulated or rcd protected

 

[Dave OCD]

I suppose you could oversleeve it as an additional precaution ?

 

[pc1966]

I think the point the OP is making is that some factory fitted SPDs are not connected through a protective device.

In that case a 100mA up-front RCD would do the job, as well as avoiding a single point of failure in and RCBO/RCD after the SPD.

Normal 30mA instant RCD are not generally specified to survive high current surges anyway, and having a MCB fails for TT as well as degrading the SPD efficacy due to the additional impedance.

Also the delay ought to prevent false tripping on all but the biggest of surges, at which point other problems might prevail...

 

[pc1966]

Reading the OP again it really does look like the concern is the metal CU and the risk of a tails-to-earth short before and RCD protection.

SPD should be reliable enough in TT case, though it might be worth asking the the manufacturer about that. Certainly some types are actually recommended for TT configurations (MOV L-N and GDT N-E), while others for TN sometimes have MOV/GDT from all L&N to E.

 

[stevethesparks]

I would just add if using and upfront rcd I would also make sure the rcbo's are double pole, otherwise a N-E fault would also trip the upfront RCD

 

[pc1966]

As @stevethesparks says, if you cascade RCD you really need the down-stream ones to switch neutral as well, otherwise on N-E faults you still get the incomer RCD tripping.

I have used the Wylex compact RCBO that do this, but also I have heard good things about the Crabtree mini-RCBO and the latest Fusebox compact RCBO (which are also fairly inexpensive).

 

[brianmoooore]

As @stevethesparks says, if you cascade RCD you really need the down-stream ones to switch neutral as well, otherwise on N-E faults you still get the incomer RCD tripping.

All RCDs are double pole, so neutral-earth faults will trip the instantaneous ones without tripping the delayed incomer.

If you use a board with main switch and RCBOs, then these need to be DP.