Earthing arrangement

[billhopkins]

Hi

Firstly apologies in advance. I am recently qualified ( a bit later in life) and well aware I require years of experience. I was doing work in a 2 bed second floor flat. I have always done a bit of very light electrical work and had my regs over the years so I know a little. I'm now further on and by the book I am fully qualified.
I'm testing what I can at the minute for practice.

The building is very old and the incoming 3 phase supply splits to service a few flats and ground floor flats. Above the 3 phase cables there were a series of 60A main fuses. and one of them ran to the meter and then the cut of switch for my 2nd floor flat. The earthing cable from the cut off ran to an MET within this 3-phase metal box. Assume the black cable clamped to the inside of the box the neutral cable. The earth is clamped outside of the box on the right (second photo) I got a Ze of over 2 Ohms so I either tested it incorrectly (hope not) or maybe the earthing is TT ( which I doubt). I have this as a TN-S system however I realise I am inexperienced so would really appreciate feedback.
I have to go back and do some remedial so would be good to know.
Worth noting I did not disconnect the MET from this box and disconnected from the consumer unit in the flat.
Thank you

 

[UNG]

So what readings did you get when you tested

 

[billhopkins]

So what readings did you get when you tested

2.2 Ohms

 

[pc1966]

The black ought to be neutral by the old colour codes and general appearance.

Is it really connected to the box inside (i.e. that is the N-E link in a TN-C-S system)? From the photo it is hard to see if that is a neutral busbar or something that is linked to the enclosure.

Certainly 2.2 ohms is bad for TN, but in that region is would be very low for TT (though bonded service pipes can appear as very good earths, until they get replaced by plastic of course...).

 

[billhopkins]

Yes I should have worded it differently. It must be a neutral busbar. Regarding the readings. Is there a way to know/check and verify the earthing arrangement here. The earth cable clamped to the box is 25mm I think. The MET inside this box runs to each individual cut off. The ground floors is offices and not flats. No way to disconnect the power and the MET here.

 

[westward10]

Not sure you should be accessing that enclosure that it the main DNO service cable. Normally you have link from neutral to earth to form a TN-C-S system.

 

[pc1966]

Would be interesting to know what that incoming cable is. Normally it would have some outer earthed layer (lead sheath, steel armour, or copper "waveform" conductor for N+E) and you would expect the supplier to have it connected in some way to the MET. The lack of support/gland is a bit worrying.

As @westward10 pointed out that might be the DNO's property, in which case you ought to contact them if there is not an adequate earth provision from the supply.

 

[westward10]

Picture isn't clear but that could be a sectoral aluminium cable which probably makes it DNO. The lugs look like aluminium.

 

[pc1966]

Picture isn't clear but that could be a sectoral aluminium cable which probably makes it DNO. The lugs look like aluminium.

Good point, I had forgotten about aluminium!

 

[westward10]

The edges to the conductor look quite sharp and solid indicating aluminium.

 

[billhopkins]

This is a picture from above the 3 phase enclosure. There were no labels on the enclosure and I was trying to find the main cut out fuses to determine what type and size they were. They were inside the enclosure. In the photo here the top left cut of switch is for the flat I was working on. The earth cable goes back to the MET on the 3 phase enclosure. I am still no wiser as to what the earthing arrangement is here. And were should I take my Ze from? Thanks

 

[westward10]

You need to contact the supplier I suspect the installation has no effective earthing system.

 

[billhopkins]

You need to contact the supplier I suspect the installation has no effective earthing system.

Ok. Thank you.

 

[UNG]

From the picture it is not clear what that earth cable connects to below the box

I agree with westwood that your best option is to contact the DNO and get them to verify the earthing arrangement

 

[richy3333]

TNCS at 2+ ohms - somethings wrong! Where are you testing and getting that result?