Commercial premises - most common earthing arrangement?

[cornburn]

Hi everyone

I've just had to do a quick emergency repair in a commercial premises as I noticed earth wires hanging out of a junction box while I was in the roof on another job! It was a right bodge so I crimped all connections properly and tested in full. I want to write them a minor works cert out but have a problem, as I couldn't get to where the supply comes in and there's no one currently on site to ask who the supplier is to make a quick call about the supply.

It's a 3 phase supply with a main room going off to about 20 different 3 phase boards and a few single phase boards dotted about. Can't find any documentation anywhere.

So, as I'd like to get the cert and other invoice in what do you think are the chances of it being a TN-S or TN-C-S supply? All readings were obviously good and well within limits for either, but I don't really want to wait until next week to muck about trying to find out which box to tick. Any other suggestions on how to find out what supply it is?

Cheers

 

[stretchyboy]

You'll have to write a limitation on the certificate, that you couldn't gain access to the supply and therefore have no supply characteristics. You cant guess them.

 

[cornburn]

Cheers, if I can't think of anything else then I'll try that. As you say, probably better than guessing if figures are good enough for either arrangement.

 

[stretchyboy]

Thats right, the only down side is that you aren't verifying the over-current protection of the supply or the size of earthing / bonding conductors, but as it was a call-out, that is all you caan do. You could comment that you would recommend a full inspection is carried out if one hasn't been completed within the last five years.

 

[cornburn]

PIR plans will be asked about, especially after some of the bits I found hidden away. I dunno, you go in to add a spur and look what happens! Could bring a bit more work though so can't complain eh!

 

[stretchyboy]

Thats the way to do it , they can see when it's being done professionally

 

[cornburn]

OK, as an update - the DNO haven't a clue what earthing arrangement they have, which means I'll have to go back and have a look on site now they're open.

The work we did was from a submain so nice and normal but the supply is going to be a 6 inch wide SWA going into a room that will have 20+ isolators feeding the various boards around the place.

Anyone willing to give any tips once I've identified the meter? I'm not expecting to see anything that resembles a main fuse with main earth coming out of the block, will most likely be hidden away behind lots of covers and look more like an industrial setup at the supply end so I'm not sure it'll be obvious to me whether it's a PME or TNS.

Any help appreciated, avoiding a third trip out there would be a good thing!

Cheers

 

[bugsy]

Check the main bond at the MCCB panel board or main isolator, trace back the main earth bond , could be taken into the C.T. / meter chamber but if it's pme it should state the fact on the metering eqpt.

 

[Notsosmart]

Check the main bond at the MCCB panel board or main isolator, trace back the main earth bond , could be taken into the C.T. / meter chamber but if it's pme it should state the fact on the metering eqpt.

As Bugsy said an example of which;

Ps. Note the lack of protection to the incommer cable & the dent already on the meter cabinet

 

[cornburn]

Thanks chaps, very helpful. I'll give it a gander in the week and hope it's accessible enough!

Cheers