High Loop Impedance reading

[asalcedo]

A washing machine engineer has refused to work on my faulty washing machine because he carried out an earth fault loop impedance test and found a high reading, I believe it was 3 ohms.

He requires the circuit to be made safe before doing any work on the washing machine.

A PAT test has been done and confirms that the appliance earthing insulation and earth leakage were ok

I have been told by the PAT engineer that I need a niceic electrical contractor to test the wiring

Is this the only avenue to address this problem?

How do I correct a high loop impedance reading?

Thanks,

Antonio

[Bob Minchin]

You need to find out where the earth problem lies by tracing where the earth in your property comes from and then onto the ring mains. It is just possible the earth supplied by your supply is faulty but that is pretty unlikely.

3 ohms is very high and needs sorting out pronto

[A.Lee]

Is it protected by a RCD? If so, it is likely to be 'safe'.

I'd be asking what the supply characteristics are. If it is a TT supply, 3 Ohms is very low, and quite acceptable. To the OP, if you have that circuit protected by a RCD, then you can probably have an earth fault loop impedance of up to 1667 Ohms, and still be compliant. Of course, it isnt ideal, as you want it as low as possible, presumably this is a 32A ring final circuit, so ideally you want it below 1.4 Ohms, but it all depends on the supply.

More information required before any good advice can be given.

[meow2222]

Common TT installs normally have > 3ohm earth impedannce. If it has an RCD or ELCB thats ok. If it has neither, 3ohm's too high.

If its the ring cct thats >3ohms, as it sounds like, best check your socket earth screws are done up tight.

I dont know why a wm repair guy would be testing your earth impedance, or why he didnt plug it in elsewhere.

NT

[Dave Plowman (News)]

I'd first ask for proof he was qualified to carry out such a test - and also ask to see the certification for the instrument he used. Before telling him to f*** off, and getting in someone else.

[John Rumm]

First you need to establish if it is a problem. That will depend on the type of earthing you have.

There are three common ways that the earth in a property is supplied. These are explained in detail here, along with pictures etc so you can figure out which you have:

In summary, if it turns out your earthing is TN-S or TN-C-S (common in towns) then, yes it sounds like there is a problem.

If you have TT, then in fact your earth is remarkably good.

FYI:

Assuming it is a fault, and assuming the earth impedance is ok at the main earth terminal, then it would indicate a fault in circuit feeding the socket where he carried out the test. If could be damage to the cables, or a lose connection at any socket between that point and the consumer unit.

Finding it is not too difficult if you have some basic test equipment like a multimeter, and are confident enough to carry out some tests. Post back here for more information.

[Martin Crossley]

Plug it into an RCD adaptor and throw a couple of (not anti-static) rubber mats on the floor for him to work on.. That gives him a working environment safer than normal. I'm surprised he doesn't carry these with him anyway.

[Andy Burns]

About 6 years ago a newly purchased washer-dryer developed a fault and the manufacturer sent a bod round to fix it, the first thing he did was whip out a PAT tester and check the machine and the socket, presumably it's their standard "Health and safety + Human Rights Act guv" approach.

[asalcedo]

Thank you for the links and explanations.

Based on the pictures, I have a TN-S supply.

I do not have RCDs in the house, only MCBs

That particular circuit has a 32A MCB.

The washing machine is connected to that circuit via a spur socket.

The PAT technician has told me that if I install an RCD, or RCBO the circuit would be made safe. But based on the information provided here it seems that even with the RCD and given the high impedance the circuit might not be safe still. Is the PAT technician's advice wrong?

Thanks,

Antonio

[Brian Gaff]

Yes that is the case but from the mere working on principal, I'd suggest that as its unplugged there is no reason why parts cannot be changed if you know the fault. Chances are the socket its plugged into was an afterthought so might be worth checking the resistance elsewhere on the system and hopefully its ok and its a problem of dodgy connections into the system used in the kitchen or wherever the machine is. it could be as simple as doing gup some screws.

Brian

[Brian Gaff]

With respect I think he is protecting himself from being sued at leaving the device in a dangerous condition. You know what folk are like these days.

Brian

[Fredxx]

| Internode Ltd - 'Internode Limited - Computer Consultancy

| John Rumm - john(at)internode(dot)co(dot)uk |

How old is your installation?

Are you sure it's TN-S? If so the earth fault loop impedance should be a fraction of 3 ohms! Yet 3 ohms seems too low for a fault in your installation.

Generally if you added an RCD or RCBO that would make your installation safe. If your consumer unit can accept RCBOs, I would add them to your ring circuits. But first I would get down to the bottom of understanding the type of earthing you have.

[meow2222]

Then 3ohms is not safe. Time to check all your earth screws are tight.

true, if the 3ohms is stable.

NT

[Tim Watts]

On Friday 24 May 2013 11:11 [e-mail] wrote in uk.d-i-y:

TT Earth?

[John Rumm]

No, in principle he is correct. In situations where the earth loop can't be got low enough (like TT installations) an RCD is relied on the open the circuit in the event of an earth fault (i.e. a short circuit to earth).

While adding an RCD to the circuit would be advisable anyway, one ought to find the actual fault as well.

There is a good method for easily finding where the fault is, and its described here:

You should be able to do it with a reasonable multimeter. However it does involve taking the cover off the consumer unit, and disconnecting the circuit wires from the MCB and neutral and earth bus bars. If you turn off at the main switch, then *most* of the CU will be safe. However note that the wires into the main switch itself will still be live, so don't go poking around near its input terminals!

[asalcedo]

Please see picture attached, is this TN-S?

+-------------------------------------------------------------------+ |Filename: VG5V0290es.jpg | |Download: |+-------------------------------------------------------------------+

[John Rumm]

Looks very much like it yes[1]. I take it there is a terminal block just out of site in the photo to the right of the meter where all those earth wires join?

[1] The metal boxed CU is another clue its not TT - you would typically use a plastic one for those installs.

[Fredxx]

[Tim Watts]

On Friday 24 May 2013 16:38 Fredxx wrote in uk.d-i-y:

If it is TN-S, agreed - 3 ohms is well out of the safety zone (0.8 Ohms is the max permitted).

There's a bad connection somewhere. Or a very loose terminal.

[Andy Wade]

It's incomplete and misguided, rather than wrong. An RCD on the final circuit(s) in question would restore effective automatic disconnection in the event of an earth fault (i.e. an L-E short). But protecting all circuits would require multiple RCDs, since fitting a whole-house RCD is no longer considered acceptable. In practical terms you'd end up having to fit a new consumer unit, and to certificate that the underlying earthing problem would still need to be sorted!

You need to get a qualified electrician, not a PAT-tester. Explain that a high Zs reading has been found on one circuit and ask him/her to confirm and investigate. They should start by checking continuity of the main earthing conductor and taking a Ze reading - that part of the earth-fault-loop-impedance external to your house wiring. If significantly higher than 0.8 ohm the problem lies in the DNO's supply network and the local DNO will need to be called out urgently.

If the Ze is OK the electrician will then check individual circuit R1+R2 (line+CPC resistance) readings with a low-ohms meter, or the overall Zs with a loop tester. The location of any high readings can then be found using normal fault-finding techniques, and fixed. It might be something as simple as a loose earth terminal screw somewhere, or it might not...