Discuss Repeatability of Zs measurements in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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pc1966

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Recently I got a DL9118 multi-function tester and one of its capabilities is, of course, measuring Zs. But I wondered how it worked in terms of compensating for test lead impedance (no mention in the manual) and after inquiring it was confirmed it has factory cal for the supplied leads.

Next thing was I repeated a test at a CU and got results that differed by over 10% which bothered me, so later I ran some tests using a 13A socket (safer and easier to do) but measuring the prospective short circuit current so I could use both the high-current test without tripping the RCD, and the low-current test that works for Zs on RCD circuits. Results are below for 10 or 11 tests triggered by switching the socket off/on using its own DP switch:

Run​
High current Zs​
Low current Zs​
High current Zs​
1​
0.21​
0.22​
0.23​
2​
0.22​
0.24​
0.23​
3​
0.24​
0.21​
0.23​
4​
0.24​
0.24​
0.23​
5​
0.23​
0.24​
0.23​
6​
0.20​
0.24​
0.23​
7​
0.21​
0.24​
0.23​
8​
0.25​
0.24​
0.24​
9​
0.21​
0.23​
0.23​
10​
0.22​
0.23​
0.23​
11​
0.24​
Ave​
0.223​
0.234​
0.231​
RMS​
0.016​
0.010​
0.003​
Min​
0.20​
0.21​
0.23​
Max​
0.25​
0.24​
0.24​
Max-Min​
0.05​
0.03​
0.01​

First run uncertainty was +/-11% using high current mode, 2nd run using low current was +/-6%, and then trying high current mode again it was +/-2%.

So what I get from this is the instrument can be fairly stable, but cycling this (little used) socket showed a reduction in variability. What is odd is it did not go towards the low peak values. Any ideas?

But what it says to me is the contact resistance when probing might well be the biggest factor in accuracy for low values of Zs. Sure it is small compared to what is needed to satisfy the installation is correct, but interesting (to me, at least).

Anyone else seen variability that concerned them?
 
a) I do wonder if the readings will be a little more stable ,in a
remote Scottish / deep welsh valley location,
Away from Taxi ,local radio ,Wifi ,Phone masts (and your own mobile).
b) (hoping RMS is a statistical thing)
c) MAX-min delta- is a good stability clue !
d) ..Doing similar I found my low current more drift prone.. (Different brand)
 
Last edited:
I find testing at a socket sometimes gives quite variable results, due to (a) the contact resistance of the socket receptacles, and (b) the contact resistance of the switch (if fitted).

With an unexpectedly high results, I will plug/unplug the tester a few times, and switch the socket off/on a few times, to see if that changes the results.

Apart from loose connections in the terminals, another issue that can give poor and variable results is where the E connections have been put into the metal box terminals, and there is no fly lead from the box to the socket, relying on the socket retaining screws for the earth.
 
b) (hoping RMS is a statistical thing)

RMS = root mean square

More correctly it is "standard deviation" and a measure of how far the points deviate from the mean (average) value. It gets a bit overused though, as folk tend to assume random Gaussian errors but real life is frequently more complicated!
 
If you examine your manual it will tell you the variance on the figures you have obtained are + or - 6 places. Therefore your readings are an approximation at best in any set of circumstances where you would normally use your MFT. To put it another way your reading average of 0.22 ohms may in fact be 0.28 down to 0.16. (0.2 reduced to significant number) The point really being that fractional parts of an Ohm cannot be considered as significant at such levels. But put within the requirements of BS7671 and you are testing sockets the tables of acceptable Zs are broad enough to accommodate the minimal variance you are noting. Just the way you put the clips on to the conductor can account for this variance. i.e. do you use the bottom of the jaw and then press the outside of the clip onto the conductor before testing or just clip on and test?
 
If you examine your manual it will tell you the variance on the figures you have obtained are + or - 6 places. Therefore your readings are an approximation at best in any set of circumstances where you would normally use your MFT.

You are right that this is within the accuracy digits stated. But it also points to a basic accuracy issue for the PFC readings that is not clearly covered in the manual (such as no note on the probe lead effect in my MFT's case).

For the above socket I measured and using 245V as the mains voltage in these parts, it gives a PFC of 0.98kA to 1.2kA. Take another case where the underlying Zs is, say, 0.05 ohm instead and now we might see values from, say, 0.08 to 0.02 ohms leading to PFC ranging from 3kA to 12kA.

But why do we worry about PFC? One reason is to establish sufficiently rapid fault clearing, the other is to make sure MCBs can safely break it. Here 3kA is safe for domestic MCB, as they are usually 6kA rated, but 12kA is not (even a few TPN boards are only 10kA)!

The MFT spec says it can measure to 26kA which is 0.01 ohm impedance. Really?

I'm pretty sure other models also claim much the same, so this is not a complaint specifically about this manufacturer, but they are not clear if they can usefully measure anything like that sort of prospective current.

I take your point about the method of probe attachment and that is significant, but in a lot of cases you can't really do it easily so its the spiky probe on the live cable terminal screw, etc. So there is not really any simple answer (without 4-wire Kelvin probes, etc) but I was just a bit surprised by the uncertainty in PFC you get in practice.
 
The MFT spec says it can measure to 26kA which is 0.01 ohm impedance. Really?

I'm pretty sure other models also claim much the same, so this is not a complaint specifically about this manufacturer, but they are not clear if they can usefully measure anything like that sort of prospective current.

The meter does not measure PFC, it calculates it from the measured loop impedance and voltage.
 

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