TT Installation Earthing arrangements

[bryn]

Hi Guys

Went to look at a job today customer just moved into an old cottage wants new 40amp supply for cooking range.

Cottage has had some work done previous has MEM 2000 CU with 100A RCCB.

Could do with benefeit of your advice on this don't do many TT's, as I said it is very old cottage wiring looks ok RCCB ok and trips within requirements my question is cottage appears not to have a consumers earth ZS at sockets etc 21 ohm which i am taking as the incoming TT supply.

I am intending to reccomend to customer that if they require new circuit then Earthing arrangement needs updating as well i.e. put a rod in customer side.

anyone got any thoughts on this one please.

 

[Lurch]

I can;t really understand the question fully.

Have you tested Ze? Are you concerned about the Zs of 21 ohms? The Zs seems pretty good for a TT, Ze you would be looking at up to around 200 ohms as a decent reading.

 

[sparkytim]

It must have a Consumers earth of some sought, the Zs is very good for a TT

DNO does not provide earth for TT

Have a good look What is leaving the Earth bar in the CU or MET?

You may find an old single green conductor 2.5mm or the like, trace it and see where it goes if you can.

Take a walk around the outside and look for it.

New circuits will need 30 m A RCD protection and current standard bonding as you recommend

 

[lilman]

If you can't find an earth, the low reading is probably through the gas/water pipework. I just finished a job where there was no rod or RCD on a TT - only gas and water bonded; 17ohms. Stuck a rod in and got a reading of 15.5 ohms. Ordinarily I find that the bonds create a better path to earth - so was quite surprised to get a lower reading today.

 

[Andy™]

If you can't find an earth, the low reading is probably through the gas/water pipework. I just finished a job where there was no rod or RCD on a TT - only gas and water bonded; 17ohms. Stuck a rod in and got a reading of 15.5 ohms. Ordinarily I find that the bonds create a better path to earth - so was quite surprised to get a lower reading today.

of course youd get a lower reading by adding an earth rod - resistance in parallel. even if it was 1000 ohms, your Zs would still be lower (but probably not enough to be detected)

 

[bluetof]

If you can't find an earth, the low reading is probably through the gas/water pipework. I just finished a job where there was no rod or RCD on a TT - only gas and water bonded; 17ohms. Stuck a rod in and got a reading of 15.5 ohms. Ordinarily I find that the bonds create a better path to earth - so was quite surprised to get a lower reading today.

Yes, same happened to me a few weeks ago. No earth rod but a reading in the low teens coming from the main water bonding. I stuck a rod in and got a reading of 87 ohms. Agree with you that often the bonds give a better reading.

In answer to the original question.

" As required by Regulation 131.8, no alteration or addition should be made to the existing electrical installation until the earthing and bonding arrangements are adequate ".

Earthing from a water installation pipe is no longer considered adequate and does not meet the requirements of BS 7671.

Sorry to ramble a bit, hope this helps. Guinness

 

[bryn]

Thanks for your replies I am going back next week as place bit overgrown and owner not aware he had a rod so i will have more time to look next week.

As I only got half the story originally as we all get can you just pop around and quote for this.

Thanks again

 

[PBC_1966]

You may find an old single green conductor 2.5mm or the like

Or in older installations it may be black or bare.

Ordinarily I find that the bonds create a better path to earth

To be expected really: Multiple tens of feet or even hundreds of feet of metallic pipework buried in the ground versus a little modern 4 ft. earth rod.

 

[muffin]

for a 30 ma trip you need a reading of 1666ohms or less.

50volts(maximum touch voltage)/0.03amp(30ma)=1666

you may see your local supply authority state tt=21ohms. however this is actully the reading they are gauranteeing if they put an earth rod down right next to the transformer.

however you should aim for 200 ohms or less ,as a higher reading than this is deemed un stable.

you need to remove the wire going to the rod and test this only to prevent you getting an incorrect reading obtained through paralell paths e.g water,gas pipes.

you can not simple convert the supply to pme or tncs, this has to be assessed.

max ze should be no more than .35 and there should be an earth wire going down at every third pole(from memory correct me by all means).

hth

andy crosse