A new fault never experienced before.

[Isetech]

Busy completing al the inspection report repairs on a site and found a strange event while doing the loop impedance testing.

At the time of the inspection report, we had a high loop impedance test result, we contacted the council and they sent someone out within 48 hours, the problem was sorted out, apparently there was a loose connection.

I went back to site to check the loop impedance reading, it was 0.69 ohms, happy with that result. I do a loop impedance test at every single socket outlet to make sure everything is ok, on Monday I noticed the reading displayed higher than 2000 ohms, thought it might just be a bad test lead connection.

Checked all the earthing around the building, 0.00 ohms you might say impossible, actually not. I did zero the leads on the fluke multi tester and the readings were zero. Considering I have just spent 3 days sorting out the earthing, I am not surprised.

I tested the voltage L-N, L-E and N-E and all seemed to be within spec.

Then today as I was finalising the tests I noticed the 2000 ohm reading on the display, so I opened the meter and tested again at the meter, the reading was 0.69 ohms. I disconnected the earth and did the test again, 0.69 ohms. Maybe the 2000 ohms was just a glitch.

Was busy checking the last socket and again, 2000 ohms again.

So I setup the multi meter at a socket outlet reading voltage N-E, this is where it got interesting. The neutral to earth voltage was 0.9 VAC, nothing wrong with that, then it happened the neutral to earth reading started climbing to over 10 VAC.

I checked the loop impedance when the N-E voltage was at 0.9 VAC , the loop impedance reading 0.69 ohms. We switched on all the elements on the stove, and then it happened again, as the N-E voltage increased from 0.9 VAC to over 10 volts, so the loop impedance test went from 0.69 ohms to 2000 ohms.


This people is why it is so important to do a full load voltage test L-N, L-E and N-E.

[GCE]

Then today as I was finalising the tests I noticed the 2000 ohm reading on the display, so I opened the meter and tested again at the meter, the reading was 0.69 ohms. I disconnected the earth and did the test again, 0.69 ohms. Maybe the 2000 ohms was just a glitch..

Not sure if I am misunderstanding something - If you disconnected the earth you should have got a high reading 2000ohmm . I would have thought.

We switched on all the elements on the stove, and then it happened again, as the N-E voltage increased from 0.9 VAC to over 10 volts, so the loop impedance test went from 0.69 ohms to 2000 ohms.

I reread your narrative a couple of times to make sure I was getting the right picture and it is possible that I have missed something - To me it would appear that the stove is earthing the neutral through the elements , when the elements warm up and get rid of the moisture you are loosing the earth neutral bond and the reading is climbing .

I would hazard a guess and say that you still do not have a good incoming neutral earth as the stove is creating a false reading - or have I missed something

[Isetech]

Not sure if I am misunderstanding something - If you disconnected the earth you should have got a high reading 2000ohmm . I would have thought.



I reread your narrative a couple of times to make sure I was getting the right picture and it is possible that I have missed something - To me it would appear that the stove is earthing the neutral through the elements , when the elements warm up and get rid of the moisture you are loosing the earth neutral bond and the reading is climbing .

I would hazard a guess and say that you still do not have a good incoming neutral earth as the stove is creating a false reading - or have I missed something

I am testing at the meter box. I disconnected/separated the earth at the meter so that the loop impedance test is completing the loop to the transformer and back.

I was using the stove to create the highest load possible, the geyser also causes a similar effect.

I think there is a loose connection on the pole.

[Dylboy]

Hello

So at the meterbox you disconnected the Earth then switched on the stove and then measured the loop and when doing so the reading would get to 2000ohm... I always test with the loads off i.e the main switch as that is what the Sans book says and then I know I am not testing into the installation and only the transformer.

I feel by having the Earth disconnected and leaving the main switch on then the loop test is using the Neautral from where you testing and perhaps going into the house and finding alternative paths thus no consistent readings.


I may be properly misunderstanding but keen to see what else people say

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[Dylboy]

Also who would the load effect the loop readings ? I would have a guess the volts would drop and then the ohm's law calc would create a different ohm reading but not major.

Keen to know more please on why the load switched on hopefully will learn something new today

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[Isetech]

Just a few notes to add:

With all the earth wires connected, testing at the meter, as the wheel on the meter spins faster, so the neutral to earth voltage increases, from 0.9 VAC to over 10 volts.

I would normally only do an earth loop impedance test with no load on the system. I was using the multi function tester, which is why I did a loop impedance test.

The geyser is connected to the ELU, the stove is not.

All the earth leakage units are new. They all trip at 21- 25 mA.

When I started the repairs, I could not get an earth continuity reading, because the tester would indicate there was power on the earth. I have stripped all the lights and sorted out all the earthing and some. I have earthed the copper water pipes at the entrance to the property and at both the geysers. All the bonding is done at the geysers and I have personally checked they are tight.

I have fitted bonding strap around the electrical steel pipes and linked the the straps and the earth wires and fitted an earth bar above the DB to link all the earth wires. If you take a lead and walk around the site, everything that is metal has a 0.00 ohm reading.

I think the council electrician has linked the neutral and earth on the pole outside, that could explain the low earth earth loop impedance test result. There is a loose connection on the pole, as the load increases, the wires get hot which increases the resistance and the potential difference between the neutral and earth.

I didnt have the thermal camera to day because of the increase in crime over the past few days, its getting ridiculous. I dont have a fibre glass ladder anymore, so I cant just slap the ladder against the pole and go up to see what is happening with the connections. ( Yes I have the correct qualifications and safety equipment because I worked on overhead lines for 10 years).

[GCE]

I am testing at the meter box. I disconnected/separated the earth at the meter so that the loop impedance test is completing the loop to the transformer and back.

I was using the stove to create the highest load possible, the geyser also causes a similar effect.

I think there is a loose connection on the pole.

Now I am on the same page - Have to agree with you on the loose connection on the pole

[Isetech]

Once again, I learnt something new today.

The council arrived on site to attend to the complaint, pretty good response time eventually the customer was in a panic because they indicated that all the new work that I had just completed must be the problem. They indicated to the customer that everything on their side was 100% correct and all the readings were within spec. The problem was inside the property, so I got a call.

What did I learn, you can do a loop impedance test with no load and it can be as low as 0.4 ohms and the neutral to earth voltage 0.9 VAC. You can disconnect the earth and try prevent parallel readings, etc, etc. and everything will seem normal.

Increase the load to 20 amps and everything changes, My fluke multi function tester just keeps proving why it pays to spend the money on good equipment. Dont worry about disconnecting and isolating and no load and all that stuff. It you want to identify a loose connection on the supply side, load the installations with as much power as the peak load would be, then do 2 tests, one check the neutral to earth reading, if it climbs from under 1 VAC to over 10 VAC and at the same time do a loop impedance test. If the reading climbs form 0.4 ohms to 2000 ohms plus, you have a loose connection on the supply side not inside the property.

Eventually the council electrician identified the loose connection in the box on the pole, it was a difficult one because the wire was tight when he pulled on it, but did notice some discolouring on the insulation, so he cut it back and reconnected, problem solved. Did all the same tests, all clear

[Derlyn]

Ok, so yesterday I also found a fault never experienced before and am still scratching my head.

Loop impedance test on house with TNCS supply ( 2 wires overhead )

Main breaker off.
Test done supply side of main breaker.

Loop impedance 36 ohms fail.

Neutral loop impedance ( both black and green leads on neutral ) 0.2 ohms pass

Test done with both black and green leads on earth 0.2 ohms. pass

Voltages are all correct L-N 235V L-E 235V N-E 0V

I haven't opened to check, but I'm 99% sure the neutral and earth are bridged in the prepaid meter.

Any ideas or suggestions ?

I suspect a high impedance on the incoming live. Am I on the right track ?

[Dylboy]

Weird this as I also had a failing earth loop, then I did the same with the black lead and the green lead on earth and I got a pass, same as you and I don't know how or why yet. Not sure if it the machine and the way it tests or what but so strange.



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[Derlyn]

Going back there later in the week.

What I'm scratching my head about is that on a 2 wire TNCS supply, when doing a line earth impedance test and a neutral impedance test, the readings should be the same as you are testing the same cable. Earth and neutral are common on the same conductor.

The question is. .... when doing a loop impedance test, is the impedance of the live wire in the loop also measured by the meter and included in the result ?

I'm thinking that there may be a high impedance on the live conductor somewhere.

[Dylboy]

Yes ya the line wire is included. The way I see it with the TNCS is that the earth where it now becomes earth cable as in seperated is that at the seperation point there is a high resistance.


In my brain a loop test and Neautral test should just be 2 leads as you are testing 2 cables as such, so having 3 is weird in my brain.

Having 3 works in my brain if the machine has both the earth loop and the Neautral loop in one push of the button.


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