Measuring Ze - what to disconnect?

[ban-all-sheds]

I can't imagine nobody has encountered this problem before.

This is my cut-out:

Sorry it's not very clear. The key issue is that it has 2 cables going into it. TN-S supply, and there's a cable sweated onto each sheath running to the MET.

I believe what I'm looking at works like this:

I can't believe our street has the only example of this in the country, so...

Anybody encountered this before? How do you go about measuring Ze?

 

[Spark123]

Disconnect the PEBs and the CPCs from the MET and measure L-MET (Main switch off first etc - I'm sure you know the score).
The wires you speak of are on the suppliers side and are not your responsibility and definately not advisable to interfere with.

 

[ban-all-sheds]

Disconnect the PEBs and the CPCs from the MET.

Why?

 

[Spark123]

Why not?
That is how Ze is measured i.e. in isolation from your own parallel paths back to earth, you are in effect trying to get a reading under your worst case scenario.
Parallel paths in other persons properties are out of your control.

 

[ban-all-sheds]

1) The parallel paths in another persons property are indeed out of my control, which means that if they are not also disconnected then not only is my Ze reading misleading it does not represent my worst case scenario.

2) What will happen to the exposed and extraneous conductive parts in the other property when I press the button on my tester?

 

[Spark123]

Would you look upon it differently if the two earthing conductors were sealed inside a new plastic head?
1) Unfortunately there is nothing you can safely do about it.
You could argue that there are parallel paths at no 1, no 2, no 3, no 4 etc etc all along your street and other streets leading to your local substation too. They are also out of your control and they may be providing a lower Ze.

2) If the earthing conductors remain connected then nothing dangerous ought to happen. If you disconnect them then you are creating a danger to next door (and yourself).

 

[JohnW2]

What's going on at the bottom of the cutout? It looks as if there are some sort of metal glands. Are you sure they are not in continuity with the cable sheaths, and joining them together within the cutout? If that were the case, then your setup would seemingly be essentially normal for TN-S, the 'dual' conductors from the sheaths to MET being unimportant (albeit unnecessary) if the sheaths were joined together within the cutout.

Kind Regards, John.

 

[JohnW2]

2) What will happen to the exposed and extraneous conductive parts in the other property when I press the button on my tester?

Wouldn't the answer to that question be the same (in fact, for many 'other' properties) with any TN-S installation?

Kind Regards, John.

 

[westie101]

At the bottom of the cutout is where the armouring is terminated by sweating it to the lead sleeve and incorporating the earth wire connection. The lead sheath then enters the cutout through a wooden grommet.

I have seen these where the earth bridges between the sweated terminations and just a single wire to the MET, others designs used a copper strip similar to a piece of Meccano, but again sweated

This design of cutout will be bitumen filled to the bottom of the fuse carrier.

It looks like a Lucy Oxford 60A design

 

[ban-all-sheds]

Would you look upon it differently if the two earthing conductors were sealed inside a new plastic head?

Ignorance is bliss? They'd still be there.

1) Unfortunately there is nothing you can safely do about it.

I could wait until they're all out.

You could argue that there are parallel paths at no 1, no 2, no 3, no 4 etc etc all along your street and other streets leading to your local substation too. They are also out of your control and they may be providing a lower Ze.

But I take your point - that's always going to be the case unless I've got a dedicated cable back to the substation Tx.

2) If the earthing conductors remain connected then nothing dangerous ought to happen. If you disconnect them then you are creating a danger to next door (and yourself).

Well I'd not be disconnecting theirs.

 

[ban-all-sheds]

What's going on at the bottom of the cutout? It looks as if there are some sort of metal glands. Are you sure they are not in continuity with the cable sheaths, and joining them together within the cutout?

Dunno - I'll have a closer look at some time.

Wouldn't the answer to that question be the same (in fact, for many 'other' properties) with any TN-S installation?

Yes - it seemed rather direct and personal with what is in reality a MET shared by my neighbour - I didn't think it through properly.

 

[ban-all-sheds]

At the bottom of the cutout is where the armouring is terminated by sweating it to the lead sleeve and incorporating the earth wire connection. The lead sheath then enters the cutout through a wooden grommet.

So would there be a connection between the sheaths in there, or does my neighbour's earth really depend on me not disconnecting the conductors from my MET?

 

[westie101]

Looking at the layout you have, the neighbours earth is, as you suspect, via the MET (which was probably supplied by the DNO)

 

[ban-all-sheds]

Blimey.

And of course he will have absolutely no idea about that, nor would any electrician who wasn't familiar with these houses - all anyone will be able to see in his house is a cutout with a single cable, and a sweated conductor running to a MET.


He'd better hope he never falls out with me






(joke)

 

[westie101]

We often come across VIR loops. such that the looped property has no earth and never had one.
Interestingly electrical contractors usually discover this on Friday afternoons as they carry out tests following a re-wire.
They often get a bit unhappy when we point out that it is really their problem not ours. (it's amazing the number that do not know we have no obligation to provide an earth in these cases - but often do)