Testing Ze when you cannot disconnect the bonding?

[davepusey]

I was recently watching John Ward's video series on Electrical Installation Testing. In the part where he discusses measuring Ze, he states that you only test the earth connection to the cut-out and need to disconnect it from the bonding and the CU to do this correctly. In his example diagram he showed a seperate MET which made this simple to do.

The attached photo is my own house's supply cut-out, installed in the early 80s when the house was build. As you can see, the CU, the Water Bond, and the Gas Bond, are all connected together at the cut-out and there are no accessible terminal screws. There is also no seperate MET.

I have no need to actually test anything, but I am curious to know how a professional electrician would perform a Ze measurement if he encoutered this arrangement?

 

[marcuswareham]

Discconect cables at the water and gas pipe end, or maybe they would take a cover off the service head

I don't know for sure, but discconecting the other ends of the cable seems simple to me

 

[westward10]

You can disconnect the bonds where they connect to the services however you would have to be sure that is the only means of connection.

 

[telectrix]

You can disconnect the bonds where they connect to the services however you would have to be sure that is the only means of connection.

to do that you need to remove the service fuse as the N cover is secured with a screw , the head od which is under the fuse holder,unless he drills throgh the little circles over the eath connections.

 

[westward10]

to do that you need to remove the service fuse as the N cover is secured with a screw , the head od which is under the fuse holder,unless he drills throgh the little circles over the eath connections.

I mean disconnect at the water and gas services.

 

[telectrix]

personally, i'd drill the access holes. saves disturbing the fuse and/or crawling under the kitchen sink.

 

[David Prosser]

You could cut them and put the six cut ends into a M.E.T. Then when the meter gets replaced or the seal fairies visit it could get tidied up.

 

[davepusey]

You could cut them and put the six cut ends into a M.E.T. Then when the meter gets replaced or the seal fairies visit it could get tidied up.

That's what I would have expected to be done. Although would it really be necessary to connect all three of the original wires that now just go back to the cut-out? Surely one would be enough and the other two, now redundant, wires could be cut back flush.

 

[David Prosser]

That's what I would have expected to be done. Although would it really be necessary to connect all three of the original wires that now just go back to the cut-out? Surely one would be enough and the other two, now redundant, wires could be cut back flush.

Most likely not necessary but much safer than trying to cut them of flush. Also impossible to see what size they are from here.

 

[Strima]

The other option, although not ideal, is to test and record that Ze was taken with parallel paths in place. This happens more so in commercial and industrial settings where the supply cannot be shutdown for financial reasons.

Unless you have life maintaining equipment in a domestic setting then, as above, I'd terminate into a MET. Whoever installed the bonds in the first place didn't take into consideration the need for testing and this should have been picked up on the initial verification for the installation when it was brought into service.

 

[buzzlightyear]

I have no need to actually test anything, but I am curious to know how a professional electrician would perform a Ze measurement if he encoutered this arrangement

either by measurement or calculations.

 

[ferg]

I think if you look at it again there is an earth terminal block next to the cutout and the seals have fallen off.

That photo must be an optical illusion.

 

[davepusey]

I think if you look at it again there is an earth terminal block next to the cutout and the seals have fallen off.

That photo must be an optical illusion.

Here is the whole box if that helps?

 

[ferg]

Here is the whole box if that helps?

I see my sense of humour has fallen flat on it's arse agin.

 

[buzzlightyear]

see my sense of humour has fallen flat on it's arse agin.

But which one, its a bit cheeky saying that. any nobody like a smart arse.lol.

 

[davesparks]

The other option, although not ideal, is to test and record that Ze was taken with parallel paths in place. This happens more so in commercial and industrial settings where the supply cannot be shutdown for financial reasons.

That is not Ze, that is a Zs measurement.
[automerge]1592940600[/automerge]

That's what I would have expected to be done. Although would it really be necessary to connect all three of the original wires that now just go back to the cut-out? Surely one would be enough and the other two, now redundant, wires could be cut back flush.

Every time I have come across this it has dated back to the days when 6mm main bonding was installed so all 3 conductors are 6mm.
Now the conductor from the service cutout to the MET should be 16mm for a PME supply (which this is as per the label)
So if they are 6mm then all 3 would need to be connected to achieve the ideal 16mm CSA.

 

[Taylortwocities]

either by measurement or calculations.

If I remember from my college course many moons ago, you can also request the Ze figure from your local, friendly and ever helpful electricity supplier.
I've never tried it though. Anybody else had success with this?

 

[davesparks]

If I remember from my college course many moons ago, you can also request the Ze figure from your local, friendly and ever helpful electricity supplier.
I've never tried it though. Anybody else had success with this?

They will give you their nominal design limits, which are usually also available from their websites (buried somewhere, if you look hard enough)
For example 100A TNS will normally be quoted as 0.8ohm
100A TNCS will normally be quoted as 0.35ohm for existing supplies or 0.25ohm for new supplies (this is true for UKPN at least, not sure if the others work to 0.25 now/yet)

 

[Taylortwocities]

Yes. Those (IIRC ) are the standard figures from DNO Engineering Recommendation P23/1 - which, I believe, is the origin of the 0.35 and 0.8 figures.

 

[7029 dave]

T bar straight through them knock outs.