Earthing arrangement

[BLBelfast]

A thing that has made me doubt what ive always done. When testing various domestic premises I have been doing a PEFC test and a PSCC test to determine if the installation is TN-S OR TN-C-S.

I was always told this is fine as the ka value will be the exact same on a TN-C-S. However today I got a Value of Pefc 617 and PSCC 586. That would make me think it was TN- S but I highly suspect this premises was TNCS. Is their an allowance for that much of a difference when testing to find out the earthing arrangement. Got a Zdb of .4 and a Ze of .21

Any help appreciated

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[telectrix]

if it's PME then the N and E are shorted at the cut-out, so the pefc and pssc will be the same.if not, it;s not PME. or there could be a bit of hogh resistance on the N-E connection, as your figures are fairly close.

 

[tomspark93]

Could you not see what the connection method is at the cut out?

 

[Andy78]

Could you not see what the connection method is at the cut out?

You can't always tell by looking. Many TNC-S connections are not marked up as such, and many heads that have an earthing conductor into the side of them do not necessarily have a N-E link inside the head.

 

[BLBelfast]

if it's PME then the N and E are shorted at the cut-out, so the pefc and pssc will be the same.if not, it;s not PME. or there could be a bit of hogh resistance on the N-E connection, as your figures are fairly close.

Yeah I'm 99% sure it's a TNCS system. Never thought about the high resistance in the N and E link. The cable is small enough that it looks to be TNCS aswell. I was always taught to test or ring the DNO. You can't always tell by simply looking.

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[Deleted member 26818]

The Ze alone suggests TN-C-S.
The difference between the Ze and Zdb though, does suggest a high resistance somewhere.
How long are the meter tails?

 

[davesparks]

Did you test the line impedance as well as the earth impedance?


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[Risteard]

The Ze alone suggests TN-C-S.

I wouldn't agree with that. Just because a typical value quoted for a TN-S system is 0.8 Ohms that doesn't mean that it might not be much lower.

How far is it from the substation? We don't know.

 

[Deleted member 26818]

You wouldn't agree with it, because I said it.

 

[davesparks]

The Ze alone suggests TN-C-S.

For a do,estimated supply I does seem to suggest that, but could equally be a sign that the DNO has been in the area bodging old lead distribution and service cables with randomly installed N-E connections all over the place.

 

[Midwest]

For a do,estimated supply I does seem to suggest that, but could equally be a sign that the DNO has been in the area bodging old lead distribution and service cables with randomly installed N-E connections all over the place.

Concur with davesparks. I recently asked the DNO about converting a TN-S system to TNC-S, mainly cos the installation had an earth rod as well and I wonder if the lead sheath was unstable or something. DNO came and had a look, and said to convert to a TNC-S myself. He said the lead sheathed supply cable was probably connected to a new PME supply somewhere out in the road anyway!