Zs help

[studentspark]

Socket radial

R2 (long lead nulled) 0.99ohms


Zs 0.44

Max Zs (OSG) 1.85

Now that R2 is high I think

If I presume the R1 will be about the same

R1+R2 would be 1.98

Ze 0.11

So 2.09 calculated Zs

measured Zs 0.44

what are our thoughts on this

 

[riveralt]

Why would assume R1 and R2 would be the same?

If you are using 2.5mm T&E Then CPC should be 1.5mm which gives a ratio to R1 of 1.67.

What was your tested (R1+R2) using continuity test method 1?
Add this to Ze.
The result normally is higher than Zs because of parallel paths.

 

[studentspark]

Yes true
I understand the 1.67 ratio which would put the R1 at about 0.6 (in theory)

so 1.7 quite high but inside Max.

I guess I was thinking roughly the same as the earth is often lower due to parallel paths.

 

[riveralt]

If this is not a new installation installed by yourself - you should check at each point on the circuit to see where the rise in R2 occurs. For Radial circuits the values should rise the further you are out from the consumer unit. It is probable that you have a loose CPC somewhere.

You can get an idea of the expected value of R1+R2 by using the tables in the OSG (red book page 166) together with the length of the cable.

As a general rule, I do not test R2 by itself but rather R1+R2 together. Only if this figure is unusually high against what I expect would I start investigating whether it is R1 or R2 that is the problem.

 

[JohnW2]

Socket radial R2 (long lead nulled) 0.99ohms Zs 0.44 R1+R2 would be 1.98 ...

That surely would only be possible if there were some parallel path to earth connected to the socket radial's CPC, fairly close to the point of testing (which would be pretty unusual)?

Even when one corrects the R1+R2 to ~1.7Ω, it is still much higher than the measured Zs - which is (as far as I can see) only possible in the situation described above. Otherwise, measured Zs cannot possibly be lower than R1+R2 (let alone less than R2).

As you suggested, what surely is wrong is your R2 measurement - whether 2.5mm² or 4mm² cable (both 1.5mm² cpc), 0.98Ω would correspond to a length of about 67.5 metres - which I suspect is not what you're dealing with! Are you sure that the zeroing of your long-lead R2 measurement worked correctly?

Kind Regards, John.

 

[JohnW2]

If this is not a new installation installed by yourself - you should check at each point on the circuit to see where the rise in R2 occurs. For Radial circuits the values should rise the further you are out from the consumer unit. It is probable that you have a loose CPC somewhere.

... but a genuinely high R2 (e.g. due to a loose CPC connection) would increase measured Zs as well as R2 - it still would be impossible for Zs to be lower than R2 unless, as I said, there is a parallel path to earth from the CPC of the radial circuit - which seems pretty unlikely.

I still think that incorrect measurement of R2 is the most likely explanation.

Kind Regards, John.

 

[chivers67]

You could always test the live and neutrals resistence by joining them together at the last socket R1 should be virtually half this, you could then join the CPC and subtract the "R1" figure.

These values would give you a very close figure to what you're looking for.

 

[JohnW2]

You could always test the live and neutrals resistence by joining them together at the last socket R1 should be virtually half this, you could then join the CPC and subtract the "R1" figure. These values would give you a very close figure to what you're looking for.

That sounds rather contorted! Why not just join the CPC to the L conductor and then measure R1+R2 directly?

Kind Regards, John.

 

[JohnW2]

Yes true ... I understand the 1.67 ratio which would put the R1 at about 0.6 (in theory)
so 1.7 quite high but inside Max.

That's the sort of statement which worries me no end. A Zs of 1.7 Ω might just about tick the box in terms of the required disconnection times, but the R1+R2 figures were talking about are not credible (for any domestic installation) and are indicative of either a measurement error (which is what I suspect) or a fault that needs to be located and rectified.

Apart from anything else, even if the disconnection time requirements are just about satisfied, with an R1 of about 0.6Ω, the voltage drop requirements would not be satisfied.

Kind Regards, John.

 

[EFLImpudence]

You could always test the live and neutrals resistence by joining them together at the last socket R1 should be virtually half this, you could then join the CPC and subtract the "R1" figure. These values would give you a very close figure to what you're looking for.

That sounds rather contorted! Why not just join the CPC to the L conductor and then measure R1+R2 directly?

Because you can workout if all conductors are as they should be. Three values from two measurements.

As in this thread, If R2 (or R1) were high you would not know from the single measurement.


Studentspark -

It's not a good idea to 'work out' an R1 by calculating from a suspect R2.
Why haven't you measured R1 & Rn to determine what R2 should be or employ chivers67's method?.

From the other readings supplied (Ze & Zs) R2 should be around 0.21Ω.

 

[JohnW2]

That sounds rather contorted! Why not just join the CPC to the L conductor and then measure R1+R2 directly?

Because you can workout if all conductors are as they should be. Three values from two measurements. As in this thread, If R2 (or R1) were high you would not know from the single measurement.

As a generalisation, I would agree. However, I was talking in the context of our having been told of a direct (I suspect incorrect) measurement of R2. As I've been saying all along, in the absence of a direct parallel path from the circuit's CPC to earth (very unlikely for a socket circuit), it simply is not possible to have an R2 value greater than the measured Zs.

Kind Regards, John.

 

[Spark123]

A single 1.5mm² wire has a resistance of 12.1mΩ /m at 20ºC.
If your R2 measurement is correct your radial circuit is somewhere in the region of 0.99/0.0121=81.8m long. Does this sound correct?

 

[EFLImpudence]

A single 1.5mm² wire has a resistance of 12.1mΩ /m at 20ºC.
If your R2 measurement is correct your radial circuit is somewhere in the region of 0.99/0.0121=81.8m long. Does this sound correct?

OR - If R2 WERE 0.21 it would be 17metres.

Is that more like it?

 

[JohnW2]

A single 1.5mm² wire has a resistance of 12.1mΩ /m at 20ºC. If your R2 measurement is correct your radial circuit is somewhere in the region of 0.99/0.0121=81.8m long. Does this sound correct?

Exactly - or, as I said at the start of the thread, about 67 metres if one is generous and uses the resistivity at 70ºC.

.. and,as I've already mentioned, if the assumed R1 ('calculated' from R2) really were 0.6 (per conductor), we'd be talking about a VD of 24V at 20A!

It surely is obvious that the R2 measurement is incorrect. With the one (very improbable) caveat I've been mentioning, Zs simply can't be lower than R1!

Kind Regards, John.

 

[studentspark]

Thanks for all the comments

More testing needed