Differential vs Earth Leakage Detector

[skippy]

Hi everyone,
I do my own electrics and just finished wiring my own house including the switchboard so in general I am quite comfortable with electrics but I am not an electrician. I do however, have a doubt regarding the operating principles of electrical safety devices.
(a) In Australia we used to use (and imagine they still do) a device called an Earth Leakage Detector. If I understand correctly, when a device has a fault there can be but not always, electricity flowing back along the earth wire which is detected by the earth leakage detector and this trips the system telling you there is a fault. Is this how it functions?
(b) Here in Europe they use a device called a Differential. If I understand correctly, AC current comes along the active wire enters into the appliance and completes the circuit via the neutral wire and no matter what happens to the appliance (no load or heavy load), there is always an equal relationship between the active and neutral wires. If there is a problem such as current going to ground (returning back along the earth wire) then this will create a difference between the active and the neutral wires which then gets detected by the Differential which trips the circuit. Is my understanding of this one correct?
I am assuming that a Differential and an Earth Leakage Detector are two different things. Can someone please put me straight on these points?
Thanks

[Al_The_Man]

Originally Posted by skippy (b) Here in Europe they use a device called a Differential. If I understand correctly, AC current comes along the active wire enters into the appliance and completes the circuit via the neutral wire and no matter what happens to the appliance (no load or heavy load), there is always an equal relationship between the active and neutral wires. If there is a problem such as current going to ground (returning back along the earth wire) then this will create a difference between the active and the neutral wires which then gets detected by the Differential which trips the circuit. Is my understanding of this one correct? I am assuming that a Differential and an Earth Leakage Detector are two different things.

The Differential works the way you surmised, Although you could also make an earth leakage detector that did not operate on the differential method, I don't think it would be as practical as you would probabally have to monitor ground current which may difficult to do, my guess is they are the same thing.
Al.

[skippy]

Thanks Al !

[epineh]

The Earth leakage devices were the older style of protection, as you mentioned they measured the current in the earth conductor and once it was over a certain level (generally 30 mAmps) it would trip. This is fine but in most installations the earth conductors are bonded at various points to things like water pipes, steel frames whatever, which can have harmless circulating currents causing nuisance trips, which is what used to happen (yes I am an electrician).

The newer style is what we (in Australia at least) call an RCD or residual current device, or what you are calling a differential device, as you mentioned the neutral and active of a circuit are monitored and any difference between the two over the trip point (still usually 30 mAmps) and the device trips. These are much more reliable and in fact I think you would be (un)lucky to get one of the original style earth leakage devices, though a lot of people, including electricians still refer to them as such.

If you are doing your own wiring, (not that I condone this in any way...end disclaimer), please be familiar with your earthing system, I don't know what system is used where you are but there are a few different types in use around the world, we use the MEN system here, if that is the system used there please make sure you know what you are doing, if it isn't setup correctly and with certain fault conditions you can get situations where everything metallic in your house is at mains potential !!! This can also happen with other earthing systems, even with RCD's installed.

Cheers.

Russell.

[skippy]

Hey Russell,
thanks for putting me straight on this on. With regards to Earthing systems, here I have all the earths collecting in one point above the switchboard using a bridge type contraption (don't know the name in English) so that each earth wire can be easily disconnected to see where a problem is (measure current going to ground) and from there the main earthing cable is connected to: (a) a 1.5m copper spike in the garden (yard) and (b) the reinforcing bar within the concrete walls. Sounds very good but in saying that we got a new washing machine some weeks ago and when I installed it I was sitting on the floor with my sweaty legs touching the tile floor, I plugged it in and each time I touched the back panel of the washing machine (galvanised steel panel) I would get a decent tingle. Touching the earth pin of any of the electrical sockets in the same circuit with the washing machine plugged in and a moist finger would also give me a small boot. Unplug the washing machine and no shock could be felt. I then put an extra cable between the washing machine cable and the wall socket and disconnected the earth wire and ran it through my multimeter (amperage) but didn’t discover anything of substance. When standing (instead of sitting on floor) no shock could be felt either. I know my earth system is working and has continuity and the right size cable, etc. and I have a 30mA Differential and that doesn’t trip even though it’s wired correctly and tests correctly. Conclusion: Don’t know but just don’t sit on the floor with sweaty legs and touch the washing machine body when it’s plugged in.

“If you are doing your own wiring, (not that I condone this in any way...end disclaimer),.....”

It was funny when I first came to Europe (Belgium) from Australia and we rented an unfurnished apartment but unfurnished means no curtains, no light fittings (just a screw connector at the end of the two wires coming out of the ceiling). In fact in Germany it generally means no kitchen either, I kid you not! So I turn off the light switch, climb up the ladder and go to fit a socket and bulb and end up getting a decent boot. Ah, the switch is the other direction so I change it, BAM, another boot. What the hell? Turns out that instead of having an active and a neutral, the majority of the houses there (anything more than 20 years old) use a wire from each of two phases (out of the three phases in the street) and difference between them is 220v therefore when you turn off the switch, one wire of the wires in any appliance is still live. For bathrooms they use double pole switches but everything else is single pole.

I was also amazed to go into any hardware shop and see non-electricians buying three phase circuit breakers. I guess with mostly concrete and brick buildings the worst that can go wrong (apart from electrocution) is a burnt cable.... This isn’t a criticism against Belgium (or Europe) as it’s a great place. Just wanted to point out the difference in attitudes towards working with electricity. I know in Australia most people just call an electrician and are fearful of electricity whereas here most (or at least lots) would have a go themselves. In my case I’ve learned what I need to know in order to do what I have to do and I respect electricity, but by the same token I couldn’t tell you if a motor had star or delta windings.

I've been following your router build, keep up the good work!
cheers
Phil

[Al_The_Man]

Originally Posted by skippy when I installed it I was sitting on the floor with my sweaty legs touching the tile floor, I plugged it in and each time I touched the back panel of the washing machine (galvanised steel panel) I would get a decent tingle. Touching the earth pin of any of the electrical sockets in the same circuit with the washing machine plugged in and a moist finger would also give me a small boot.

Regardless of how much current you can measure, what this means is there is a potential between the Ground conductor feeding the W/M. and Earth ground.
Essentially it means that the power feeding the machine has some kind of leakage to the frame. if the frame had exactly the same potential as Earth ground, you should not experience any sensation as you are sitting on earth ground.
I would say that, either your W/M ground path has a higher than acceptable resistance back to the trip or the Earth leakage trip is not that sensitive or the ground path back to the grounded neutral has higher resistance than it should.
Does the service company provide a ground? I suspect not, in that case the ground path would be through ground back to the grounded star point of the 3 phase supply transformer, where ever that is located.
Hypothetically, think of this, if the supply transformer grounded star point was open or high resistance, sinking any amount of ground rods would be of no avail.
Al.

[epineh]

Hey Phil, as Al mentioned there must be some kind of potential difference from the floor to the WM, also most people don't realise, you can still get a sizeable boot from a protected circuit - without tripping (you don't need to ask me how I know this). It is these sort of problems that can be tricky to diagnose. Also I am not familiar with the type of grounding system used there and in any case there are too many variables to help via forum...things that are unrelated may be to blame, broken neutrals can play havoc with earthing systems, and the fault may not even be with your wiring, possibly next door may have a fault, it can happen.

As a rule electricians will use a "Megger" which does two things, measure continuity (low ohms) and insulation. It does a better job than a multimeter for earth cont. and for insulation will test using up to 1000Volts (typically, they do go higher), this makes finding faults a lot quicker/accurately.

Cheers.

Russell.

[Al_The_Man]

Many years ago in the UK, using a low resistance Megger we had to measure the complete ground loop resistance using the neutral back to the star point back through ground to the ground conductor, whether Ground rod or metalic water supply pipe.
With the common use of non-metalic water supplies, E.L. Trips became more common.
The system here in N.America is slightly different, as to ground conductor supply.
Al.

[skippy]

Sorry I didn't get back here to reply in the last two weeks and thanks for your answers. Just to recap on what you're saying. I should call an electrician who is equipped with a Megger and he should be able to tell me if I have a problem or not and what exactly it is? I have to be very choosy about who I call here. An electrician came to my neighbour's house last week and had to borrow my step ladder as he didn't own one. The week before that I had to lend my 3 section ladder to a tree lopper as he only had a 2 section one and one month ago I diagnosed why my other neighbour's new electric water heater didn't work properly. (The plumber had fitted a vertical model in horizontal position therefore it never filled up with water properly due to different positioning of the inlet/outlet pipes on horizontal and vertical models). I guess you get the point!

[epineh]

It might be a good idea, I would suspect both a minor fault with the washing machine and a fault with your earthing system. Some people are a little more prone to get little tingles as well, you mentioned a moist finger, small cuts and grazes will also increase your susceptibility to getting a boot, skin acts a little like an insulator, but not a very good one.

Good luck, and you better ask when you ring if the sparky has his own tools, including a megger . I guess an electrician without a ladder is fine if he is REALLY tall ... lol

Russell.

[Al_The_Man]

Originally Posted by skippy; 347458. An electrician came to my neighbour's house last week and had to borrow my step ladder as he didn't own one.

If he can't afford a ladder, what are the chances of him owning a Megger
Ideally he would need both types or the combined high voltage insulation type and the low resistance reading type.
One to check the WM insulation and the other the ground path resistance.
Al.

[epineh]

Just about all meggers we use do both, high voltage insulation testing and low impedance testing. Also set on 500 Volts good for waking up any apprentices sleeping on the job
Russell.