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Hi guys,

I've been a sparky for around 15 years and currently decided to go alone. My 1st question is, ive added a circuit on a warehouse thats supplying an outside cabin, now ive tested the cable ive installed but i have no access to the incoming supply and no test sheets are available for me to get the ZE reading and can't see what the earthing arrangement is. What options do I have?

Thanks
 
Hi guys,

I've been a sparky for around 15 years and currently decided to go alone. My 1st question is, ive added a circuit on a warehouse thats supplying an outside cabin, now ive tested the cable ive installed but i have no access to the incoming supply and no test sheets are available for me to get the ZE reading and can't see what the earthing arrangement is. What options do I have?

Thanks
Would have though doing a ZS and R1+R2 would be part of your tests? Then you work the formula Ze = Zs - R1+R2, there's your Ze, from there should be able to gauge earthing arrangement, failing that contact DNO
 
Would have though doing a ZS and R1+R2 would be part of your tests? Then you work the formula Ze = Zs - R1+R2, there's your Ze, from there should be able to gauge earthing arrangement, failing that contact DNO
may not be a true Ze though as any bonding would still be connected when measuring Zs.
 
Would have though doing a ZS and R1+R2 would be part of your tests? Then you work the formula Ze = Zs - R1+R2, there's your Ze, from there should be able to gauge earthing arrangement, failing that contact DNO
I don’t have GN3 handy right now, but I’m pretty sure it tells you not to calculate Ze this way because of the parallel paths issue.
 
I meant to say its not off the main board, its a sub board and he doesn't know where the board is that feeds it.
 
There must be a source of supply somewhere. Being a warehouse unless it is of considerable size it is likely to have a DNO supply somewhere as opposed to a private transformer. Ze testing is likely to be difficult even if you have a standard DNO service head, depending on the scale of operation isolating the supply to verify Ze may prove to be inconvenient to the client.
 
I understand that, ive asked for existing test certificates but he says he doesn't know where they will be. I'm gonna try the DNO but not hopeful. Don't really know my options as he doesn't really know where any DB's are.
 
There must be a source of supply somewhere. Being a warehouse unless it is of considerable size it is likely to have a DNO supply somewhere as opposed to a private transformer. Ze testing is likely to be difficult even if you have a standard DNO service head, depending on the scale of operation isolating the supply to verify Ze may prove to be inconvenient to the client.
I understand that, ive asked for existing test certificates but he says he doesn't know where they will be. I'm gonna try the DNO but not hopeful. Don't really know my options as he doesn't really know where any DB's are.
 
How big is the site there must be several dist boards dotted about, try and follow their supply cables or follow cable trays etc...
Asking for old test certs is often a fruitless search.
Have you asked the obvious as to where their electric meter is.
 
How big is the site there must be several dist boards dotted about, try and follow their supply cables or follow cable trays etc...
Asking for old test certs is often a fruitless search.
Have you asked the obvious as to where their electric meter is.
To disconnect one circuit we were looking in big drive thru freezers, we eventually found a DB outside. There are plenty of DB,s dotted around and can't really follow cable trays as some are quite high up, through walls, underground etc.

If I do find it, how can I do ZE as they can't shut down for me to do the test.

Thanks for the help
 
You can't do a prescribed Ze in that situation.
 
If I do find it, how can I do ZE as they can't shut down for me to do the test.
The only way I can think of safely doing Ze without shutting down to isolate the supply earth from any parallel bonding would be to use a clamp-style of earth tester (e.g. Megger DET14C or similar).

You measure Zs = Ze || Ra

Where Ra is the bonding resistance to the Earth in parallel with the suppliers earth Ze

Then you clamp the supply earth line and measure Ze + Ra then compute Ze from the two numbers. I leave that as an exercise for the reader.
 
This happens quite often, if it's a minor change/addition on a large site, it frequently isn't possible to shut the whole site down.

When I review the certificates etc that get passed to me, I compare the Zs along with R1+R2.

So for example, the allowed Zs is 1.1 ohm based on the mcb/rcbo and the measured is 0.5 ohm, with R1+R2 of 0.4 .

In this case the Ze is around 0.1 ohm, even if this was to double/treble/quadruple by removing the parallel earth paths, its still well within the 1.1 ohm - so the whole situation is clearly safe, even if we don't have actual values for Ze.

However, if this same circuit had measured Zs of 1.0 ohm, R1+R2 of 0.2 ohm this would give a Ze of 0.8 ohm, any change in this due to the removal of parallel paths could easily result in Zs going above the allowed value.

In this case, I would look to get further tests as I couldn’t guarantee that its safe as it is. This would be finding the parallel paths, for example the board feeding this board, may have no parallel earth paths, and may itself have a R1+R2 of say 0.6 ohm, so the likely Ze is more in the order of 0.2 ohm this would still be an issue in this case as only a 50% increase in value due to removing parallels would hit the limit.

It is normal therefore to use Zs at the board to be regarded as the Ze on the form (in fact ammendment 2 on the 18th suggests this clarity on the form), and only really require further investigation if the values are close to the limits
 
I don’t have GN3 handy right now, but I’m pretty sure it tells you not to calculate Ze this way because of the parallel paths issue.
I've just got back and had a read, GN3 doesn't specifically say you can't make this calculation but I'd still be wary of it for the reasons mentioned by @telectrix and @Julie. I think I was getting confused with calculating R1+R2 by subtracting a measured Ze from a measured Zs which is definitely frowned upon if not verboten.

GN3 does note the confusion over the term Ze for DB's not at the origin of the installation. It suggests using Zs at Db as Zdb and using that on the schedule of tests as per the model forms and as explained by @Julie.

I did find this nugget in the Unite Electricians Guide to Inspection and Testing on page 74, Ze on existing installations; "On periodic inspections it is not necessary to disconnect the main earthing conductor whilst this test is performed". Presumably because you're mainly looking to confirm the presence of a reasonably low earth connection at the origin of the installation, be it TN or TT.
 
I did find this nugget in the Unite Electricians Guide to Inspection and Testing on page 74, Ze on existing installations; "On periodic inspections it is not necessary to disconnect the main earthing conductor whilst this test is performed". Presumably because you're mainly looking to confirm the presence of a reasonably low earth connection at the origin of the installation, be it TN or TT.
I do not agree with this. Perhaps Unite could have worded it more like "it's not always practicable to disconnect the main earthing conductor".
Wherever it IS practicable to do so, it should be disconnected for the Ze test. I have come across several installations that have somehow lost the connection to the suppliers earth, and the Ze reading was perfectly fine with the earthing conductor connected (due to parallel bonding connections). In fact I had one only yesterday. An upstairs flat that takes it's supply from a common service head in the downstairs flat. An electrician had done some work in the downstairs flat and had disconnected the earthing conductor for the upstairs flat for some reason. I had to reconnect it.
Of course I realise domestic is usually much simpler, and switching off the supply to conduct a proper Ze test is usually straightforward. But my point is, that the Unite statement is implying that it is ok on every periodic inspection to leave it connected. Hmm...
 

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