No trip Zs tests

[cydodimead]

I have only recently read that the No Trip Zs function of my meter shouldn't be relied upon. Instead the calculated value should be used in my test results. I have always gone with the understanding that the result can be "calculated or measured"

Current job has thrown up some interesting results.

Ze=0.13

1. 32A Type B Ring Final circuit. r1=0.34, rn=0.33, r2=0.59 Measured Zs is 0.36 yet calculated would be 1.06

2. 32A Type B Ring Final circuit. r1=0.55, rn=0.55, r2=0.94 Measured Zs is 0.50 yet calculated would be 1.62

As you can see circuit 1 would satisfy disconnection time (max for 32A type B = 1.16) with either calculated or measured. Circuit 2 however would be ok for measured but not calculated.

Parallel paths obviously play some part in the difference between the calculated and measured results but I didn't think the difference would be so great.

So I guess the question/advice required here is should I rely on the measured Zs value or not? If not then I need to look at circuit 2.

 

[ericmark]

Not to have some parallel earth paths is near impossible hence it is common for measured to be below calculated. However we can't rely on those parallel paths always existing so normally we would use the calculated figure.

However today the earth loop impedance is not really the limiting factor it is the line - neutral impedance which will effect the volt drop and likely with RCD fitted it is the volt drop which limits how much cable.

There do seem to be some odd conventions as far as volt drop and final rings go as it is taken that 20A will be draw from the centre and the remaining 12A will be even distributed so for calculations 26A not 32A is used. This gives around 106 meters limit of 2.5 mm sq in a final ring circuit.

 

[cydodimead]

So do you think Circuit 2 warrants investigation or should I rely on my measurements which of course take into consideration parallel paths? Only gas in bonded as water is plastic.

 

[JohnW2]

Ze=0.13
1. 32A Type B Ring Final circuit. r1=0.34, rn=0.33, r2=0.59 Measured Zs is 0.36 yet calculated would be 1.06
2. 32A Type B Ring Final circuit. r1=0.55, rn=0.55, r2=0.94 Measured Zs is 0.50 yet calculated would be 1.62
As you can see circuit 1 would satisfy disconnection time (max for 32A type B = 1.16) with either calculated or measured. Circuit 2 however would be ok for measured but not calculated.
Parallel paths obviously play some part in the difference between the calculated and measured results but I didn't think the difference would be so great.

Parallel paths can certainly have a marked effect - copper pipe has a dramatically lower resistance than the CPC of 2.5mm² T+E. However, your figures are a bit surprising. Assuming we are talking about 2.5mm² cable, and that you measured r1, rn and r2 at the centre of the ring, unless I've got my arithmetic wrong, your resistance measurements appear to indicate a total ring length for Circuit 2 of about 244 metres - which doesn't sound right! Are you sure that it's not the the accuracy of your resistance measurements, rather than Zs ones, that you should be questioning?

Kind Regards, John

 

[EFLImpudence]

Ze=0.13

1. 32A Type B Ring Final circuit. r1=0.34, rn=0.33, r2=0.59 Measured Zs is 0.36 yet calculated would be 1.06

2. 32A Type B Ring Final circuit. r1=0.55, rn=0.55, r2=0.94 Measured Zs is 0.50 yet calculated would be 1.62

You have not calculated it correctly.

Assuming (???) r1, rn & r2 are the end to end measurements (as they should be - that's what 'r' (lower case) is)
then R1 + R2 (upper case) = (r1 + r2)/4.

So, for circuit 2, r1 + r2 = 0.55 + 0.94 = 1.49.
Therefore R1 + R2 = 1.49 / 4 = 0.3725
Therefore Zs = R1 + R2 + Ze = 0.37 + 0.13 = 0.50 = all's well

It is unusual to have the measured and calculated exactly the same but there it is.
It could even indicate a problem with the main bonding.

Do you have a Fluke? I find the no-trip measurement quite accurate.

Radial circuits would be as you have done it.

 

[JohnW2]

Assuming (???) r1, rn & r2 are the end to end measurements (as they should be - that's what 'r' (lower case) is) then R1 + R2 (upper case) = (r1 + r2)/4.

Ah - one learns something every day. Thanks. As will be apparent from my reply, I thought he was presenting 'uppercase' figures and had simply forgotten to press the shify key!

Kind Regards, John

 

[cydodimead]

Spot on EFLimpudence I didn't do my sums right. Had been a long day and just did a load of radials before the ring tests.

Many thanks for your help