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Posted By Topic: Earth Continuity test

Aug 25 2013 17:50

I was upgrading switchboard at Mt. Eden. I ordered inspection. When inspector turned up and did Earth continuity test we found out that resistance is 2000 Ohm. His test was actually to place his testing rod in a ground approx 200mm in( can not go deeper because of rocks) and then test continuity between wire attached to his rod and earth wire in switchboard Earth rod and earth wire in a house were existing. I replaced earth wire. Test showed the same reading. He tested continuity between his testing rod and house existing rod. Still the same reading. We put some salty water next to house earth rod. Test showed 1250 Ohm He said that i need to get reading 0.5 Ohm to pass. Was it correct test and how I can get reading 0.5 ohm. please advise

Aug 25 2013 18:00

good luck with getting that to 0.5ohm, you need to find a different inspector, start with one that knows how and what to test !!!!!!!!! for starters.

Aug 25 2013 18:01

pass that test you would have to turn the dirt into a superconductor. The 0.5ohm is the measurement between the rod and the MEN point only!

Aug 25 2013 18:36

He clearly does not know what he's doing.
So don't pay him.

Aug 25 2013 19:08

If that story really is true, I would go further and say report them. An inspector lacking such basic knowledge is a menace.

Aug 25 2013 19:39

The test described is just stupid... did it really happen?

Aug 26 2013 08:43

The test described is not part of testing-for-compliance, nor testing-for-safety; but may be an "extra" he's decided ought to be done.

Or maybe he thinks this is how to do earth continuity of MEC.

In which case since their job is supposed to be ensuring competence of electrical workers, they clearly failed in this case.

Not enough info to judge whether this is a case of ignorance (dangerous!) or officiousness.
Not sure that even ignorance is an offence, so doubt the Board would do anything. Though reporting may result in some auditing.

But regardless invoking breach of contract might wake his ideas up and make him realise he's not a policemen, just a specialised subby being paid to do a particular and well-defined job of work.

just little old me
Aug 26 2013 10:26

Like previous have said, not enough info on what happened, but sounds to me like a call to the EWRB is needed here.

Aug 26 2013 22:20

Like many of the questions asked in this forum, if you are a licenced sparky then you should know the answer.

Check your 3000 for the answer. Of course an earth loop tester is essential too.

Aug 27 2013 08:46

For the life of me I cannot see why you would need an earth loop tester to test the continuity of the main earth.

Almost any megger will have a low ohms range that will allow you to test the continuity of the main earth conductor from the Earth bar to the Earthing electrode. (Not STAKE)

The test the Inspector appeared to be carrying out is to determine the resistance of the Electrode to Earth and there is no minimum value in AS/NZS3000 or the Regs.

Works that are Designed will usually have a minimum reqiurement for the earth bank but this is up to the designer.

Jan 17 2018 18:59

Sorry for digging up an old post. If the inspector was looking for the earth rod resistance, then he should use the 61.9 method or using the fall of potential method. People always tell me the ground is not a good conductor because they use multimeter to probe the earth in their garden and found the resistance is very high. Yes, they are right but they are also wrong. If the measuring distance is short the resistance should be high.
R=p/(2 x pi x a). Assume that soil resistivity p is constant. The resistance (R) reduced as the measuring distance (a) increased. The last time I measured the earthing system of the HVDC link (single pole operation), the earth resistance between the shore in Wellington and Benmore power station (middle of south island) was less than 0.1 Ohm.

Jan 18 2018 09:15

Actually R=pl/A where l is the length of the conductor and A is its cross-sectional area. This means resistance increases with increased distance for the same conductor. Pi only comes into it for circular conductors, and the soil is not a circular conductor (but the surface area of the probe might be, so there's some merit there since resistance measured by a probe depends more on the surface area of the probe than the cross sectional area of the ground). Soil resistivity is dependent on moisture content so it's not constant for all soils, or even for the same piece of soil with different probe depths.

I can assure you that you weren't measuring the resistance of the ground if you got 0.1 Ohms between Wellington and Benmore. More likely you were measuring the resistance of the HVDC cable.