High Zs for circuit with metal conduit earth

[endecotp]

Dear Experts,

My flat is wired predominantly with metal conduit, with the conduit itself as the earth. For one of the socket ring circuits I have the following numbers in an EICR:

r1 = 0.50
rn = 0.51
r2 = 0.68
(R1+R2) = 0.68
Zs = 1.31

Two questions:

Firstly, this I believe is "test method 1", but I don't think that is suitable for a steel conduit installation, is it? Or have I missed something.

Secondly, the measured Zs is high. How much do I need to worry about this, and how can I try to diagnose where the problem might be?

 

[securespark]

Is the Zs similar on all outlets on that circuit?

What is the Ze?

 

[endecotp]

Ze - I think this is shown as "Zs at this board", which is 0.20.

I believe the measured Zs values were higher further from the CU.

 

[Spark123]

I'm not sure on how they came about with the r2 figure given it is conduit, unless the conduit is in a ring?

 

[EFLImpudence]

Same thing, I suppose but:

How can R1+R2 be the same as r2 and
Zs be more than r1+r2 ?

 

[flameport]

None of those figures make any sense.

To obtain r2 on a conduit installation, it would be necessary to disconnect both ends of the conduit at some point, and measure the resistance between the two separate conduits. For obvious reasons, this is never done.

If the figures are to be believed, then (r1+r2)/4 should be the same as R1+R2. They are totally different (0.295 and 0.68)
Assuming the worst case with R1+R2 as 0.68, and the Ze or "Zs at this board" is 0.2, then Zs for the circuit is the two added together, or 0.88. Not 1.31
When measuring Zs directly, it is often less than R1+R2+Ze would suggest, due to the presence of bonding conductors, parallel paths via other earthed equipment, etc.

 

[endecotp]

unless the conduit is in a ring?

Much of it is of course a ring; the question is what happens at the CU. My guess is that there is some sort of metal box where the conduit terminates behind the wooden board on which the CU and meter are mounted. All that's visible is a lot of singles coming into the back of the CU. I didn't notice how many CPCs there are.

 

[endecotp]

For comparison, here are the numbers for the other socket ring circuit:

r1 = 0.54
rn = 0.54
r2 = 0.72
(R1+R2) = 0.28
Zs = 0.40

 

[Spark123]

I take it there are spurs on the first ring final for R1+R2 to be greater than (r1+r2)/4.

 

[securespark]

Ze - I think this is shown as "Zs at this board", which is 0.20.

I believe the measured Zs values were higher further from the CU.

Have you got any end-to-end readings for that circuit?

"Zs at this board" is not what you normally call the Ze.

Zdb, perhaps, if it is remote from the incoming supply.

 

[endecotp]

I take it there are spurs on the first ring final for R1+R2 to be greater than (r1+r2)/4.

I'm aware of one short (2m) spur, which is in T&E. I've not looked inside every socket but I doubt there are any other spurs.
The highest Zs value was not on that spur.

 

[endecotp]

Have you got any end-to-end readings for that circuit?

Isn't that what r1, rn and r2 are supposed to be?

"Zs at this board" is not what you normally call the Ze.

Zdb, perhaps, if it is remote from the incoming supply.

The EICR has in section I "nature of supply parameters" "external loop impedence" which is also 0.20.

 

[securespark]

Isn't that what r1, rn and r2 are supposed to be?

I'll go back to bed...

 

[endecotp]

How much do I need to worry about Zs = 1.31, assuming that is the reliable number?

Should I be trying to work out if there is a flakey conduit joint somewhere, or can I ignore it?

 

[EFLImpudence]

It can't be a reliable number, can it?

As I said, it's more than r1+r2 - yes, I mean lower case r.