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]

The circuit supplying the SPD within the DB I wouldn't really class as a general use circuit such as a final or distribution. It is contained within the enclosure with very limited risk of damage or failure causing a fault that will allow the enclosure to become live. If a homeowner does manage to drill through the cable putting up a shelf then they really need to step away from the tools.

Double check with the manufacturers but a lot of the tails connecting to the SPD will be double insulated. Some manufacturers are happy for the DNO fuse to provide protection if below a certain rating, IIRC Wylex is 100 amps but it's been a couple of years since I looked at their kit.

SPDs do not use current so terminations should remain as installed over time, no movement due to thermic cycling etc. If installed correctly should be fine to provide many years of service. They're designed to work once and the installation should have a full I&T after surge event to identify any issues that have arisen.

If a full RCBO board is installed and the manufacturers state their RCBOs are suitable for use on a TT system if backed up by a double pole main switch and the incoming tails are supported by a suitable means then why the need for a time delayed main switch?

 

[brianmoooore]

If a full RCBO board is installed and the manufacturers state their RCBOs are suitable for use on a TT system if backed up by a double pole main switch and the incoming tails are supported by a suitable means then why the need for a time delayed main switch?

The obvious one is that there's just a single layer of protection, relying on circuitry crammed into a very small package.

 

[pc1966]

As above, there is no need for an up-front RCD if the board is all RCBO, but having it avoids the single point of failure in the RCBO electronics, as you are very unlikely to disconnect on the thermal-magnetic trip for a L-E fault on TT.

If folk would regularly test the RCBO and get it fixed if if failed the self-test that risk obviously would be a lot less.

 

[stevethesparks]

Interesting references 534.4.6 - 8 although not mentioned probably already observed

 

[AJshep]

I'm really late to the thread here, sorry to bump this again..

On TT systems if is required that a Type 1 or 1/2 combined SPD is needed then these are generally connected in parallel to the consumer unit, usually in a separate enclosure using the same size tails that you supply the CU with.
Best practice to put Henley blocks in after your main isolator and split 1 set of tails to SPD and the other to the CU.

Becomes expensive though as a Type 1 SPD is around £180!! And most will just put in a Type 2 and be done with it.
I doubt the NICEIC inspector even noticed this last year.

 

[Oldgimmer]

I'm really late to the thread here, sorry to bump this again..

On TT systems if is required that a Type 1 or 1/2 combined SPD is needed then these are generally connected in parallel to the consumer unit, usually in a separate enclosure using the same size tails that you supply the CU with.
Best practice to put Henley blocks in after your main isolator and split 1 set of tails to SPD and the other to the CU.

Becomes expensive though as a Type 1 SPD is around £180!! And most will just put in a Type 2 and be done with it.
I doubt the NICEIC inspector even noticed this last year.

Hi
Could you explain why you would install the spd this way rather than in the C/U. Such as seen in the pre populated boards.
Also are you saying that if you did use a type 2 Or a type 1/2 It would be in the c/u ?
Thank. you

 

[Oldgimmer]

I am very old retired gimmer , need to make sure my daughters get a job done right, can you help me before I die , I am still good at math and science , might need me one day young. Sir

 

[pc1966]

I'm not sure why you would put the SPD in a separate box unless the CU was old and plastic so maybe not going to do well with any heat from absorbing a big surge.

In terms of TT systems specifically there is the issue that any fault to earth is not likely to every disconnect on over-current protection as the earth rod, etc, will be tens of ohms or higher Ze. So you need to ensure a very low risk of live-chassis short! Which you should anyway, as even on a TN system a L-E fault on the DNO fuse is going to ruin everyone's day.

For example using proper gland for the tails entering the CU and clipping them externally so they are very unlikely to move and flex free of the incomer's terminals, etc. Checking and properly torquing the terminal's screws, also using 19-strand easy-fit tails also helps. In the past a plastic CU would have been the choice but not now for domestic due to the fire containment aspect.

Once you have a SPD involved then you might worry that if it were to fail internally shorted then you would have the same issue of the CPC being permanently live in that situation. One way to avoid that is to have the incomer "isolation switch" as a 100mA delay RCD so anything that faults downstream will switch off everything. The SPD then comes off that busbar and these days you would normally prefer RCBO for all final circuits rather than dual 30mA RCDs & MCBs. Again, for proper selectivity they should be N-switching so a N-E fault won't trip the incomer as the RCBO disconnects both L & N of the faulted final circuit. Wylex, Crabtree and Fusebox all make N-switching compact RCBO and they are not that expensive.

 

[happysteve]

Really great summary by pc1966.

If I may add... single-module SPDs by Wylex have the L input connected directly on the busbar: it sits on the bus bar (to the left of the main switch) in the first "way", just like a circuit breaker or RCBO. There are no internal line connections protected only by the service fuse, apart from the busbar itself. I strongly suspect the Crabtree Starbreaker DBs work the same way (with arguably a safer busbar). This might negate the need for a time-delayed RCD acting as a main switch, but this is something the designer will need to weigh up.

 

[AJshep]

Hi
Could you explain why you would install the spd this way rather than in the C/U. Such as seen in the pre populated boards.
Also are you saying that if you did use a type 2 Or a type 1/2 It would be in the c/u ?
Thank. you

I really only mentioned specifically putting the SPD in a separate enclosure if a Type 1 SPD is required, which it may not be.

1) it's much easier to terminate, type 1 spds require a min conductor size of at least 16mm as they are expected to take a hit from direct lighting strikes, trying to put this in a populated consumer unit is tricky.
2) I'd rather have the SPD away from all my 6KA rated RCBOs as if it did take a hit then it would hopefully minimise damage.

 

[pc1966]

I really only mentioned specifically putting the SPD in a separate enclosure if a Type 1 SPD is required, which it may not be.

1) it's much easier to terminate, type 1 spds require a min conductor size of at least 16mm as they are expected to take a hit from direct lighting strikes, trying to put this in a populated consumer unit is tricky.
2) I'd rather have the SPD away from all my 6KA rated RCBOs as if it did take a hit then it would hopefully minimise damage.

If Type 1(+2) and you really need it then it is not a bad idea. In such cases you would want to use the Kelvin style wiring (V-wiring) where the L and N go "trough" the SPD where it has dual screw terminals, and as you say for that the E should be 16mm and as short and direct to the MET as possible.

Also if you really do have cause to fear diviner retribution (as I do) then having some LPS or at least a rod short & direct from the MET is also a good idea.

 

[arifpatelprestondubai]

The T-T system uses an earthed neutral at the supply transformer and an earth electrode at the user's installation.

 

[t01474]

Fuse box say that their SPD is suitable for TN, TNC-S, TNC and TT systems?

FuseBox SPDCUT2 SPD with 32A MCB Kit

Fusebox SPDCUT2 Surge Protection Device is designed to give suitable protection for TNC-S, TNC and TT systems. Supplied with a 32amp mcb.

www.fusebox.shop

 

[pc1966]

Fuse box say that their SPD is suitable for TN, TNC-S, TNC and TT systems?

Generally the 1+1 layout (so MOV L-N and GDT or MOV N-E) are fine for any of the arrangements.