Zs wanders around

[martygturner]

What are the reasons why a Zs reading from a farthest socket gives inconsistent results.

The circuit is a radial on a 20amp breaker with a RCB type A, Zs goes from 1.8 to 1.41 and at one test reached 2 ohms on L-PE, remains consistent at 0.88 on L-N. Tested on No trip on the MFT - DL9118

There are no parallel paths to earth, all services come in on plastic pipes

ZE is 0.37 on a TN-S

Tested the plug three lead adaptor, no changes to its resistance, removed all the loads on that circuit.

Any help as ever.

 

[pete01]

Could it be a loose/worn fitting socket face....maybe

 

[JohnW2]

The circuit is a radial on a 20amp breaker with a RCB type A, Zs goes from 1.8 to 1.41 and at one test reached 2 ohms on L-PE, remains consistent at 0.88 on L-N. Tested on No trip on the MFT - DL9118 ....
ZE is 0.37 on a TN-S'

Have you confirmed that the Ze is always about 0.37 when you see these variations in Zs ? If it is, then, as has been said, there is presumably a loose connection 'somewhere' - or, I suppose, possibly a malfunctioning meter.

Have you tested from other sockets on the same circuit and, if so, do you see the same variation in Zs at them? In particular, so you see appreciable variation in Zs at sockets close to the origin of the circuit?

 

[Deleted member 142255]

Is the voltage at the socket varying?

 

[Murdochcat]

I would remove the last socket and test on the actual cables.

Old sockets can often give odd results

 

[Jurassicspark]

The readings that you are getting could be due to poor contact through the plug (oxidisation or weak connction)


Did you null the leads? Basic things first.

 

[flameport]

RCD uplift, explained in this document: https://s3-eu-west-1.amazonaws.com/..._-_RCD_impedance_uplift_elimination_21496.pdf

 

[JohnW2]

RCD uplift, explained in this document: https://s3-eu-west-1.amazonaws.com/..._-_RCD_impedance_uplift_elimination_21496.pdf

Interesting - but whilst they are describing something that could result in 'consistently incorrect' loop impedance measurements, I don't see how could it explain the varying measurements that the OP has observed. What am I missing?

However, there are aspects of that article which I find difficult to understand. They start by saying ....

Higher currents reduce coil impedance ....

... and they their diagram goes on to indicate:

0.5 Ω @ 15 mA
0 Ω at 100 mA

..... can you help me understand this 'dramatically current-sensitive impedance' ? The resistance of that coil, in the main path (and also the one in the neutral path) obviously mst be 'close to zero' for significant currents,otherwise it could cause a massive VD and get very hot when carrying high currents.

They then go on to say ...

Higher currents reduce coil impedance but will cause the RCD to trip, preventing the test from completing.The impedance of the coil (A) at 15 mA is 0.5 Ω. This value is added to the basic circuit impedance and the total is displayed on the MFT. The MFT cannot distinguish between the impedance of the circuit and that of the coil.

... which seems to imply that the MFT will always display a loop impedance which is 0.5 Ω (or whatever) too high when the circuit is not loaded. Is that what they are saying?

... and then the go on further to say ....

It is irrelevant how much current is flowing in the L-N circuit. The example in Figure 3 shows the test process in a more realistic circuit, with a motor load and a loop impedance test.

... it is true that the device should not then trip (2.015 A through L and 2.000 A through N, hence residual current of just 15 mA). However, since 2.015 A is a lot higher than 100 mA, they appear to be suggesting that the impedance of the coil would then be 'zero', in which case the meter presumably ought to give an accurate measurement of L-PE loop impedance if the load on the circuit is resulting in at least 100 mA of current flowing, shouldn't it ?

As you can see, there's a fair bit there that I don't really understand, so I wonder if you can help me at all?

Kind Regards, John

 

[martygturner]

Many thanks for advice, redone the basics, had all the sockets off and checked the terminations all ok, two individual sockets on a double showed much higher readings until a plug was worked in and out of them.

Not to sure about testing at the cable...I want the sockets to be in spec when anything is plugged in.

Checked the insulation resistance between the socket legs starting at the consumer unit and then again for the complete circuit, cable is fine.

Swapped out the last in line socket to a bog standard new one.

Retested the Ze, at 0.35 this morning so a slight variation from yesterday , distance to the transformer is approx 150m so think the Ze is probably about right.

Checked three other radials, whilst the unused sockets showed a higher Zs than their twin a few in and outs got the pair to the same level. Readings static, so meter function looks pretty good, swapped out the batteries on the tester - it was not showing any indications of low power but what the heck.

Tested all the sockets on the problem circuit, all displayed fluctuations between tests, as expected the closer to the CU showed a lower value but still with a fluctuation albeit with a lower magnitude of the last socket.

The only differences I can find is at the CU, the problem circuit is grouped with a lighting circuit, Boiler, single outside socket and a emergency light. All the other socket radials are fed from the other RCD. The problem circuit also has 7 sockets on it with 4 being combined USB/ doubles.

So will link out the usb ones and retest, but are these really an issue on L-PE ? All I can think is to link out all the sockets with waggos and check the r1/R2 end to end and add sockets back in one by one, any better ideas.

Can a slightly iffy RCB be an issue on Zs ? or is it always a dodgy connection, connection at CU tight on MCB and on RCB - checked both had a good clamp on the busbar.

The only thing I can think of to test is to move the circuit off to an mcb without the RCB in the way unless there is a better idea.

Again thanks for the help.

 

[martygturner]

Well that turned out well, the rcb was doing something to the Zs values

Used the SPD Mcb which is off the RCB and the values remained constant at 1.38 for the furthest socket.

CU is being swapped out for a RCBO one next week,the last one was poorly done...the spark left a sheet with figures that could have been the work of dickens.

Again many thanks for the help.

 

[JohnW2]

Well that turned out well, the rcb was doing something to the Zs values

Glad you've found the reason. Maybe due to something such as flameport suggested - although as I said about that, I couldn't really see why it should/could result in varying Zs readings.

 

[martygturner]

Glad you've found the reason. Maybe due to something such as flameport suggested - although as I said about that, I couldn't really see why it should/could result in varying Zs readings.

Thanks john, all I can assume is the RCB is a bit duff as now having done the lighting and boiler they too show some variable readings, not large but there around 1.5 ohms. Its a BG RCD.... can these things fail without the test button or indeed the rcb test on the mft not picking it up ?

 

[Taylortwocities]

Old sockets can often give odd results

And I have found that new sockets can sometimes give odd results. Usually because the switch contracts need a few off/on cycles to properly clean the contact. Zs meter measurements are at low current which exposes this little feature.

 

[JohnW2]

Thanks john, all I can assume is the RCB is a bit duff as now having done the lighting and boiler they too show some variable readings, not large but there around 1.5 ohms. Its a BG RCD.... can these things fail without the test button or indeed the rcb test on the mft not picking it up ?

As I've been discussing elsewhere, I think the test button is a relative crude test, and will only 'fail' if the device is way out of spec (or doesn't work at all). A MFT ought to give pretty good figures - but, of course, if there is an 'inconsistent/varying problem' (e.g. due to iffy contacts), then any measuring device will give results which reflect that (actual) inconsistency.

 

[flameport]

can you help me understand this 'dramatically current-sensitive impedance' ?

It affected by the magnetic core - higher current increases the magnetic field in the core and as it approaches magnetic saturation the impedance decreases.
An inductor with a magnetically saturated core isn't an inductor any more.

Not affected by current in both L&N as there is no magnetic field, it's just the difference in current which creates the magnetic field, the same field that is used to trip the RCD.
If nothing else is connected to the installation then the additional impedance should be consistent, however in real installations there is likely to be some residual current from connected equipment, and as that varies the additional impedance seen by the test equipment will also vary.

Then there is the related item of having DC through one of the conductors which will saturate the core, not trip the RCD and prevent it from ever being tripped even if AC is applied. Some older loop impedance test equipment used that as a 'no-trip' feature.