Earth testing - how?

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22 Aug 2009
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Having (finally) finished wiring my workshop I guess it'd be nice to test the earthing before I hook it up to power. Over here the earth goes back to the service provider.

I know in principle what's needed, but am not sure where to source a tester. (megger?) Can these normally be hired? The other caution is the question of whether or not there is a risk of damaging equipment. Thoughts anybody?

There's four 32A ring mains to groups of 13A sockets, and four dedicated 16A spurs to machines - all through an RCD.

The inverter for the dust system fan and the rotary converter for the three phase band saw are fed separately from the RCD to avoid nusiance trips - both are hooked up to their respective supply, the inverter output cable screen and panel are grounded back to the consumer unit too.

The new consumer unit is wired in parallel with an existing consumer unit via an isolating switch from the incoming cable, the original unit now handles just the lighting and the power to a laundry room next door.

My thanks by the way to Bob and Roy who have been of great assistance - it took a while to get my head around the thinking...
(Traditional) Meggers are for insulation testing ("Megohms"). You want a low resistance tester, not a high one :)

The earth impedance ("resistance" for practical purposes in this case) needs to be as low as possible, the standard used to be at least equal to or better than 4 Ohms, IIRC. That's actually rather high IMHO.

The idea is that, in the event of a fault, the fuse or breaker always blows fast enough so as to prevent you getting a fatal dose of electricity through the heart's nerves (and the RCD trips, too). Or so I've been told...

Anyway, the closer the earth is to neutral potential (and this is determined by the resistance between the two), the safer it all is. So you want good insulation live-to-earth (and live-to-neutral), and low resistance earth-to-neutral.

There are specialist testers, but old fashioned, moving-coil AVOs are good for earth resistance testing. It's one case where yer typical digital multimeter isn't wonderful, because it's not as accurate on low resistance ranges. Unless it's built for that specific job, of course.

Someone wot really knows 'll be along in a moment, I expect... :)
Pardon the boob on the meg, thought from hearing the word bandied about that it was the same thing Eric. I have a fairly decent digital multi meter, but am not sure how low it goes on resistance..
To be honest, I agree with Roger.

In any case, it's not whether your DMM has a low resistance scale (both mine do), but in part what it's source impedance is on the lower ranges. DMMs typically tend to be in the megohm source impedance range, and the current flowing is tiny. Whilst this makes them safe, it makes measurement fairly inaccurate. In contrast, an old fashioned moving coil meter passes a much larger current (still in the mA range, mind), which is easier to measure accurately.

The above used to be the case with the first DMMs that came out. My main one is getting on for 25 years old now, but still working fine. I don't regard it as reliable for anything other than continuity testing on its low resistance range. Modern DMMs may have a different type of circuit, but my most recent purchase is a digital clamp meter (it has probes too!), and that's not too good on low resistance either.
Addressing your original question, you can hire an electrical installation tester from companies like this: Megger MFT1552 weekend hire £25 plus delivery and return charges Ex VAT.

These people are in Scotland so delivery may be pricey. There must be someone similar in Ireland?

A very useful complement to Whitfield’s “The Electrician's Guide” is “The Part P Doctor” by Alan Lynch. It is a step by step illustrated guide to making and testing a domestic installation. Definitely worth £17.

If you are only testing the earthing ( the continuity of protective conductors in your workshop tested with the circuits dead) there should be no possibility of damage to equipment. I haven’t been following your workshop wiring (is there a thread?) but would say earth testing is essential rather than nice. I believe you don’t have Part P in Ireland but do you use the IEE 17th edition or equivalent?

You say “Over here the earth goes back to the service provider.” which, if we are not at cross purposes, is what we call a TN-S system. The earth goes back to the supply company’s sub-station where it is connected to the company’s earth electrode and the neutral cable.

I may be reading too much into that comment but do you intend to test the Ze (External earth loop impedance test)? As you know that test is done live with any supplementary earthing temporarily disconnected. Or do you intend to accept that Ze is within limits, IIRC 0.8 ohms or less for TN-S over here.

Thanks for all that guys. The complicating factor behind the post is that I got mixed feedback from the few electricians who looked at it as to whether or not (since it's an extension of an existing installation in the workshop) certification is necessary. Our rules are similar to UK except that we don't involve the local authority.

Since posting I've veered towards biting the bullet and getting it certified - although the justification seems more bureaucratic than practical. Having seen some of the messes so called electricians have left behind them on other jobs it's hard to know what (well... not that hard!) to make of most of the self interested bleating and mess of bureaucracy that the industry goes on with.

I did line up an electrician to if needed check it out when finished before starting - hopefully he'll do as he promised since it's fairly clear that many do everything they can to block DIY.

My concern in the end is to verify that it's safe. It's pretty standard (13A ring mains, 16A spurs/radials and a couple of other radials on the inverter and rotary converter) - so barring a major oversight it's OK on basics like circuit design, breaker and wire sizing. I tested each circuit for continuity and short circuits before hooking it up to the CU too.

My thought was to verify that polarities are correct, that everything is properly earthed, and also that the earth from the original CU is good. (it's the type you described Graham) There's probably other testing too that could/should be done too.

I'll make a call in the morning...
To test the earthing it is normal to do your 'dead' tests first. With a ring main this involves connecting the earth on one of the two ring cables to the other cables live conductor and the same with the remaining earth and live. So at the consumer unit you will have two separate connections containing a live and earth from each cable. You then connect an Ohm meter and and record the reading which is known as the value of R1 + R2.

With a radial you connect together the live and earth at the consumer unit and go to the end point on the circuit and measure across the live and earth with the Ohm meter to get R1 + R2.

This test tells you you have a complete circuit as if there is a break in either the live or earth you will get an error signal and no Ohm value. Once complete reconnect the circuits.

You also need to test the supply earth using an Earth loop tester which is a Live test. Isolate the supply and disconnect the incoming earth from the consumer unit terminal bar. Connect the loop tester to the live and disconnected earth and record the reading. This gives you a value of Ze.

Ze + (R1+R2) =Zs ... or near enough

You then need the Earth Loop Tester again. This is a Live test and this measures the value of Zs which is the complete circuit of Live and Earth from the sub-main to the property and the completed circuit you are testing.

The reason to test the Earth loop is to see if you have a low enough Ohm reading to trip the circuit breaker or fuse on whatever circuit. Each type of protective device has a Ohm rating at which it will 'trip' at the required time (normally 0.4 seconds).
If a circuit has a higher Ohm reading than the device fitted will operate correctly at, then measures need to be taken to rectify the situation by a number of means.

I see you have all the circuits connected through an RCD? This effectively will disconnect the circuits within the 0.4 second limit no matter the Ohm value of the circuits or even if the earth wire is broken. If there is a fault to earth either through the earth wire or to ground, the RCD will trip.

In England we can't just rely on RCD's to provide protection on a TN system and have to get the circuit breakers or fuses working correctly first. RCD's are just a supplementary form of protection but now required in most electrical situations.


Thanks for that Steve, I'd seen references to R1+R2 before but wasn't sure what it was about. As it happens I bought a manual yesterday ('Practical Guide to Inspection, Testing and Certification of Electrical Installations by Kitcher - updated to 17th Ed) but have yet to sit down with it - I like to understand the principles behind stuff properly if I can.

The electrician is due in mid next week to do the testing, so that should be educational too.

The power to the MCBs for the inverter and rotary converter doesn't go through the RCD (rec by the Omron inverter manual - likelihood of nuisance tripping), but everything else does. (the 13A socket ring circuits and the 16A spurs to the machines) The rings are on 32A and the spurs on the 16A type C MCBs recommended by Hammer in the machine manuals.
Hi guys. Just to say that the electrician has been in, and that it all checked out just fine. So my thanks again to Bob, Roy and all those that helped.

All my machines (Hammer K3, F3 and A3 410) are running now, as is the Aggazani NRA 600 on a 5.5Kw Transwave rotary converter. (I've only had a quick play with the band saw, but boy is it smooth and easy compared to its predecessor)

The VFD for the dust system fan is wired, but not run yet - that's the last step on the electrics. Next up is the Pentz cyclone dust system install..