Testing for presence of a main earth when one can't be found

[WhyEr]

Just wondering what methods everyone is using for testing for presence of a main earth should one be unable to be sighted.

From my understanding the fact that the main earth & the neutral run in parallel - that should mean that there should be some sort of current flowing through the main earth if one is present - will just need to clamp the main earth with a current tester capable of a very low setting.

Also if you cant find the an earth stake to test resistance are people just running their own new one - triggering inspection?

I haven't had this problem yet but would like peoples thoughts as i'm sure i'll come across it at some stage!

[AlecK]

finding some current in an (unlabelled) conductor that looks like it could be the MEC doesn't prove it is the MEC, or that there's any form of acceptable electrode on the other end of it. For example; could be an EBC rather than an MEC.

You're right that the main earthing system operates in parallel with the mains N.
But have to remember that it's also in parallel with all the other earthing systems and mains Ns of all installations fed from same transformer.
Finding a current even in a labelled MEC doesn't tell you which way the current is flowing .
In fact most installations will have some current, on both MEC and EBCs, even with main switch off;
and the readings can be in A rather than mA.
That's because current doesn't just take "quickest way home", ie the line of least resistance.
It follows all available paths, in proportion to impedance.
The shortest way home for currents going down another installation's MEC is not through mass of earth all the way to transformer; it's through least possible amount of earth and up your installation's MEC.

Separate electrodes, rather than using water pipe, have been required since at least 1976.
The connection of MEC to electrode must be accessible for testing.
And labelling has been required for all that time ( with a few Exceptions); so you'll know when you find it.
Until 2010, the MEC also needed a label at switchboard end.

So if you can't find it; choices are either look harder, or assume it doesn't exist.

[DougP]

But if the MEC is obvious at the MSB, then disconnecting it a doing a loop test to it will prove that it does go somewhere.
I would expect somewhere between about 20-60ohms on average. Sometime less, sometimes more - up to about 150ohms possibly.
If you get less than 5 ohms, I would suspect it's connected to the MEN system somewhere some how (and not just a stake - unless you're right next to the ocean).
If it's over 150-200 ohms roughly, then it's not very effective. But without driving a new one and testing that, you won't know if it can be any better.

But as Alec says, look harder. It will either be around the house, or up in the roof connected to the water pipe on the header tank. If it's anything other than a simple single dwelling, then it can get more complicated.

[WhyEr]

Main earths in older buildings can be difficult to find, a couple of examples - under a deck that was added at a later date only found it we I looked a little harder. another one, concreted poured over and around it - only found it as we were doing a OHUG and had to cut the concrete for the new mains, had trouble pulling the cut piece of concrete out, the corroded earth wire and pin were embedded in the concrete - we were replacing the main earth anyway because at the time we couldn't find it.

[WhyEr]

Thanks. Yeah like Doug said I was more meaning if it was fairly obvious which Wire was the main earth at switchboard & say you had already traced the EBC.

I thought that if you could clamp it with an earth leakage tester & get a reading then it would at least show something was connected to earth not necessarily a stake.

Fault loop test on the earth would do the trick too. Good feedback.

Yeah I’ve had a few where difficult to find but found them all so far, just got me thinking - I’ve never found one on a header tank but good to know to check for that also if the obvious places don’t suffice.

When replacing the main switchboard I test for resistance - but if i was ever unable to find I guess I’d just have to run a new one.

[Mazdaman]

I found a main earth attached to the steel pipe feeding water to the concrete header tank. Long before I spotted it the house had been concerted to another type of water supply and the steel pipe had been cut on the 'ground' side of the earth clamp. Thus the main earth was clamped to about 1m of steel pipe that was still attached to the header tank.

So the main earth was temporarily extended and clamped to the ground side of the cut pipe while an earth electrode was organised. Is it possible to upload photos to this site or only links to a photo sharing site?

[AlecK]

Using the pipe feeding the header used to be very common, 1940s to 1960s, as it was always accessible and typically just a short run from MSB.
Always? a bare conductor.
Earlier ones and places with tank water supply (no water mains) tend to have an actual electrode; generally a but of galv pipe.

Water mains get replaced with plastic pipe; and / or header tank gets removed with conversion to either mains pressure or a PRV setup for the w/h - leaving the labelled earth clamp lying hidden under the insulfluff / batts.
Or tank left in place - but no longer connected to water system - so even after a lot of searching; just sighting a labelled connection proves nothing. So some sort of test is required.

Using leakage clamp would provide an indication, so as long as you can be sure that your testing the MEC and not something else I suppose that could be called "main earthing system exists". Thinking about it, I suppose the direction of the current doesn't matter as much as the fact of there being a current shows some sort of circuit is operating.
Certainly easier than using a trailing lead, though proving not > 0.5 ohms would be further evidence.
DougP's idea of an EFLI test would be even better, as it's actually passing a reasonable current - again as long as you can be sure of the conductor ID .
But not sure I'd accept it by itself, without sighting the other end.

Personally I think removing the requirement to label the MEC at swbd end was a backward step.

[Mazdaman]

.... removing the requirement to label the MEC at swbd end was a backward step

I didn't know it had been changed and have always put a main earth tag at the DB. The inspector I worked with also did this and I can see no reason, given the trivial cost, of not doing it. Thanks as always for your help on this forum!

[AlecK]

It happened when we moved from NZS 3000 to AS/NZS. Until then we had to use 2 labels - but my understanding is Oz didn't use any.
Compromise was to require only the electrode end to be labelled, and only where a removable connection was used.
Eg a cadweld or similar can't be disconnected, nor can a continuous run of 25 mm2 cable with bare end used as electrode and insulated end used as MEC. Whereas any sort of terminal or clamp can be disconnected.

I agree the cost is trivial; and it certainly makes identifying the MEC much easier.
And i use them even when not needed at the electrode end as well; again because it provides a great ID
(along with the label at swbd that - for modern installations - tells us where to look for the electrode)

[WhyEr]

Yeah it’s A strange one to get rid of considering it’s such a simple task & adding no cost to the job - I was always taught both ends & will continue to do so : )

[WhyEr]

Slightly off topic - but couple of years ago I did a switchboard & POE upgrade - went looking for the main earth & found that a rod had just been stuck in the ground & the new 6mm main earth had been run out but never connected - just hanging around a few metres away - apparently a licensed sparky had done this a year earlier - took off the old POE and the neutral was hanging on by a single core - had i not come when did, could’ve been a big bloodbath.....

[AlecK]

I've had similar, a mains-&-swbd upgrade with new electrode & MEC.
But despite having happened in days when all work had to be inspected by the Supply Authority, the end of the MEC had never been connected to electrode.
Otherwise had been a good job, so i suspect the Inspector arrived early and just trusted the sparky to finish things - but this bit got missed - and obviously never tested.
Which reinforces that we can't afford to trust even good sparkies; and work can't be validly inspected before it's completed.
The point of inspection is to ensure this sort of silly error doesn't get missed.

[Craig W]

Meggar have a video on youtube about test the MEC with the installation isolated at the main switch and the MEC disconnected. i found it quite interesting