Advise on whether outbuilding should be TT’d

[Gavin Price]

Hi all first post so please go easy on me. Just to give a bit of background on myself I have been qualified for 12 years and have recently completed my 18th edition and 2391.

I’ve recently been carrying out an EICR on a MOD industrial waste water treatment site.

The question I have is about a supply to a Nissen hut style building. The main site supply is TNCS with a feed coming from the MCC to some overhead lines (TPN&E) which then feeds (SP&E) the Nissen hut. It then goes to an enclosure on the post with an 100mA rccb then to a 40A MCB. It then goes from there in singles in galvanised conduit to the hut which has an externally mounted DB with a 30mA main rccb on it. The building is a good 50M from the MCC.

I’ve given a code 2 on the fact that there is no earth electrode at the hut due to the fact it has a corrugated metal roof which can be touched with a possible difference in potential to earth. Do you think this is correct?

I should add that all Zs and RCD results were acceptable.

Thanks

 

[Strima]

Did you test the roof to see if it's extraneous?

 

[Gavin Price]

Did you test the roof to see if it's extraneous?

I haven’t tested it yet but it i assume extraneous as it is a metallic structure which makes up the majority of the building.
There is an earth running from the DB to the roof so it currently forms part of the installation.

 

[Wilko]

Hi - the metal roof is bonded to the outbuilding’s EMT, which in turn has a bonding conductor back to the installation’s MET. My thought is - this isn’t a prohibited design, but the conductors do need to be sized correctly. Sometimes that sizing can mean it’s cheaper and easier to TT the outbuilding.

 

[ipf]

Hi - the metal roof is bonded to the outbuilding’s EMT, which in turn has a bonding conductor back to the installation’s MET. My thought is - this isn’t a prohibited design, but the conductors do need to be sized correctly. Sometimes that sizing can mean it’s cheaper and easier to TT the outbuilding.

Agree, but it's probably going to result in far inferior earth test results all round.

 

[Gavin Price]

Just a bit more info on it. The earth coming from the overhead lines is a 10mm2 with 25mm2 tails, then 2.5mm2 supply and earth from the enclosure on the post to the DB through the conduit in the ground. I got the MCB size wrong yesterday as it is in fact a B20.
Am I right in thinking that because the incoming supply to the site is over 150mm2 then if the supply is not TT’d then it should have the minimum requirement of a 50mm2 bonding conductor to the roof?

 

[Wilko]

Hi - yes, if it’s PME supply to site and supply is bigger than 150mm, then Table 54.8 says the bonding conductor is 50mm. And that includes the run back to MET. If you’ve GN8 to hand they give a nice example in Figure 5.15.

 

[Gavin Price]

Hi - yes, if it’s PME supply to site and supply is bigger than 150mm, then Table 54.8 says the bonding conductor is 50mm. And that includes the run back to MET. If you’ve GN8 to hand they give a nice example in Figure 5.15.

Thanks I’ve just purchased it.