What category observation?

[yellowvanman]

House is TNCS, remote garage has a 60A supply that is TT'd and contains supplies for an adjacent (defunct) swimming pool, and a gas boiler.

SWA enters a plastic box, earth is left in the box whilst L-N enter metalclad CU.

Zdb is 0.2 ohms - probably due to the gas bonding in the house.

In theory a supply for TT shouldn't enter a metalclad enclosure unless its RCD protected, but because the Zs is so low the garage supply fuse will blow if there was a fault to the enclosure.

Should I include this in an EICR and if so what category?

 

[Welchyboy]

Mmmm tricky one that, I would probably go for a c2 as you shouldn't really rely on bonding to ensure a low enough Zs reading, is there any way to put a 100mA rcd on the house end of the sWA or between the PVC box and metal cu?

 

[Guest55]

C2 - tt should have upfront rcd.
as for the Zdb at 0.2 ??? is this taken from the earth rod or the swa from the mains ??
i'd be using the earth rod reading as the basis of Zdb for the garage as its supposed to be seperated from the house equipotential zone.

 

[telectrix]

connect the TNC-S to the garage CU. just need to ensure you have sufficient csa to allow for extraneous components, if any.

 

[yellowvanman]

Thanks guys.

Answers to some questions raised

Zdb of 0.2 is with the rod, as mentioned the gas pipe from the house must pick up the TNCS from the house.

16mm 3c SWA so it is possible to include this in the equipotential zone.

I know there is nothing in the regs to say don't use TNCS for a swimming pool but a lot of people don't like it, and I can see reasons why. JustI noticed the regs do suggest using an earth rod or mat with TNCS, so if the swimming pool is reserected then it should all be OK with the existing rod, subject to testing being done on that swimming pool building (omitted for this EICR as in disrepair at the moment)

 

[malcolmsanford]

YV you would need your earth mat/rod though to be of as value less that 20ohms though.

What size is that metal clad CU, maybe worth while to advise changing it for a insulated one

Like most things in the regs there isn't exactly a regulation against not protecting a metal enclosure in a TT system with an RCD, just that it says the preferred protective device is the RCD, it does not rule out all other devices.

Also as earlier posted there is nothing in the Regs precluding you for allowing a low Zs via bonding, in fact supplementary bonding is a way of reducing a Zs value, but not the greatest of practices.

I would disconnect your boning at source and then take a Zdb, which will still benefit from all the house CPC, and also maybe supplementary bonding as well, and I would bet that your 0.2 would not be sufficiently changed with bonding removed

I assume that there is RCD protection within the TT system

 

[yellowvanman]

CU has 10 ways, as it happens the RCD main switch (30mA) fails on disconnection times (for x5), as its an old MK board can't just swap out the RCD.

As customer doesn't want to spend too much money on this at the moment, I'm going to intercept the feed cable by replacing the plastic adaptable box the SWA is glanded to for a enclosure that will take a DP RCD.

Regarding entering TT supply into metalclad box, reg 531.4.1 applies

 

[malcolmsanford]

Aren't the talis/SWA protected by the TNC-S at origin

 

[yellowvanman]

Aren't the talis/SWA protected by the TNC-S at origin

Yes, 60A BS3036 rewireable fuse.

 

[Engineer54]

Maybe you need to do a little more investigation here. If your getting a 0.2 ohm Zdb, without bringing any earthing with the cable, it may just be that the rod has also been connected to the Swimming pool reinforcement (Ufer earthing). If you have a 3 core 16mm SWA cable I'd use that cable to to extend the PME equipotential zone to the outbuilding (gas boiler). I'd also disconnect the earth rod from the remote DB/CU if it has been connected to the Swimming pools reinforcement and leave the pool itself on that rod. You can always drive in another rod outside of the existing rods area of influence and connect that rod to the MET of the remote DB/CU (for additional PME N-E link). What sort of distances are we talking about to this remote building by the way??

 

[yellowvanman]

Garage is about 25m from the main building, swimming pool is housed in another building a couple of meters away from the garage.

Don't believe the rod is connected to the swimming pool reinforcement as it is about 5m away from the swimming pool building.

 

[Engineer54]

5m is nothing when you think about it, it may even just be one of many rods, just the only one accessible for maintenance/testing purposes. You should be able to see if there is more than one cable connection to the rod...

Have you tested the Ra value of the rod?? Can't see you getting 0.2 ohms on a single rod position, unless your on old salt mashland!! Can't see as the the gas pipework will be giving you much on it's own, i would imagine the pipework would have been wrapped against corrosion etc...

 

[yellowvanman]

Only one connection to rod!

Haven't tested Ra but as mentioned before suspect getting low Zdb because gas pipe comes from house and obviously bonded there!

Didn't mention this before but obviously TNCS supply but there is also a rod connected at the house, left that as is when I changed the CUs in the house.

Think I'll extend equipotential area to garage when I fit a new RCD before the metal clad CU.

 

[Engineer54]

Only one connection to rod!

Haven't tested Ra but as mentioned before suspect getting low Zdb because gas pipe comes from house and obviously bonded there!

Didn't mention this before but obviously TNCS supply but there is also a rod connected at the house, left that as is when I changed the CUs in the house.

Think I'll extend equipotential area to garage when I fit a new RCD before the metal clad CU.

My bad, completely ignored the bonding at the main house and the remote building. I really should read posts/threads more thourghly before making comment!! lol!!