Peculiar electrical problem

Back again, many thanks chaps, plenty to read and ideas to act upon. I've print out the text and will give to my friend, I'll come back some time later to let you all know the result. John

It does not have to be leakage to a conductive (metal) part extraneous or otherwise. An old junnction box screwed to wood and exposed to water will cause leakage current. The RCD senses line / neutral differential current so that is what you need to measure when testing.
A mill bolted to a even slightly damp concrete floor would provide a parallel path to earth.

Always happy to be educated!

In this Fluke diagram showing how to measure AC current, it's explained you can't put the clamp over Live and Neutral:

This I understand to be because the AC current flows one way in the Line wire and in the opposite direction in Neutral. The two generate magnetic fields that cancel out, so the clamp meter reads zero. Highly misleading.

However Live leaking to earth is a different case. The current flowing in Live is not equal to the Neutral current, so the magnetic fields don't quite cancel. A sensitive clamp meter will measure the difference, which is equal to the earth leak current.

Chris has me worried though: if the clamp is put over LN and E and the leak is due to a suppressor, the clamp won't detect it because the 3 magnet fields balance. It will detect a leak due to faulty insulation and a damp floor, because then not all the leak current goes back along the earth wire. Is that right?

Can anyone design a circuit that would reliably show how much current a device was leaking under all conditions?

I don't believe the debate alters the advice given to John.

Dave

Yes, but a really good earth rod in damp soil might manage 20 Ohms and the same at the transformer. By contrast, the earth path through all the green and yellow might be, say 0.5 Ohm. So assuming that any earth leakage is to the bed or case of the machine, the vast majority (99%+) will go down the green and yellow.

The insulation of your junction box should be sound. Around one megaohm is about the minimum acceptable resistance, but a new machine in good order should achieve one gigaohm.

Yes, I think so, but that is exactly what you want to avoid - you do not want the bed of your machine at anything above earth voltage. There is a big difference between faulty insulation and earth leakage. If the bits and bobs in your VFD are "leaking" they are supposed to return to earth via the CPC.

If there were no earth leakage at all, why have RCDs at 30 mA rather than say 10 mA. (Yes 10 MA ones are available.)

Chris P asked

"I am afraid that it is not quite as simple as that.

What type of earthing do you have please: TN-S, TN-C-S, or TT?"

Like most properties of the age of mine in a town or city the earthing is TNS. Obviously TNCS could cause confusion dependent on where you measure the neutral current but ultimately phase current less neutral current should ideally be but rarely is 0A. If it isn't then there's a leakage to earth which is simple enough. Finding the path of that leakage may well be complex, although I would agree with Chris that you would expect the majority to flow in the CPC and main earthing conductor.

Regards

Martin

Edited By Martin Shaw 1 on 26/09/2023 00:02:08

OK we anre now deviating away from the original issue of LEAKAGE currents of around 0.03 amps (30 milliamps mA) into Mains earthing systems that have to carry FAULT currents of hundreads of amps. The leakage currents form VFDs and similar devices are not due to faults. A resistance of 10,000 Ohms (10k) with 240V (UK mains) across it will cause a current of 0.024A to flow (Ohms law I = V/R). This close to the 30mA trip current of a domestic RCD. !0K is thousands of times higher resistance than a mains earth. Lets forget about earthing systems.

SOD. The Fluke picture is not helpful becuase it is addressing using a clamp meter for it's typical application of mesuring load current not leakage. I can design a vey simple means of detecting leakage current. A current transformer with two primary windings one carrying the current in the line the other tha current in the neutral. Oh wait that's a current clamp meter around live and neutral. It's also what is built into most electrical installation meters.

Chris P. I said a junction box screwed to wood and exposed to water (a fault condition) not a new, dry box.
The earthing system in use TT, TNC-S etc is irrelevent to this discussion.

As for finding where the leakage is that is not difficult given a sensitive clamp ammeter. First measure and note the leakage current by placing the live and neutral in the jaws of the clamp meter.
To deterine if the leakage is through the desigined "earth" protective conductor add the earth conductor to the L&N in the jaws. Any remaining current is leakage to a connection outside the designed earth system such as damaged insulation.
This process can be carried out at an individual piece of equipment, a particular section of an installation e.g. a ring and the whole installation. Thia allows the exact source of the leakage to be identified. However the fact you would be dealing with individual conductors means that the first layer of protection against shock e.g. equipment case, outer insulation has been removed exposing you to a higher risck of electric shock. This work should only be carried out by competent persons.

As an extension to the testing, it is also possible to tell how much of the leakage current is due to the filters in a VFD or similar. To do this you need a phase sensitive current sensor. As the filter leakage is due to capacitors the current is ninety degrees out of phase with that caused by "normal" resistive leakage.
Note that this intentional current nomally flows through the metal case of the filter as well as any earth wire. You should never earth a filter or VFD purely by bolting it to an earthed metal panel ther must also be a flexible cable connection. This is because if the filter or VFD was unbolted from the panel the leakage current would become a shock risk to anyone touching it.

I've designed this sort of equipment. An example is protection of the high current feeder between a generator and a circuit breaker panel without using fuses in the feeders. You just run the feeder conductor through a current transformer at the generator end and another at the load end. Connect the transformers secondaries in series anti-phase to a sensitive current relay. Load current flows through both transformers and cancel out. Any current due to a fault between the two transformers (to neutral or "earth" only flows through the generator transformer and is not cancelled causing the sensitive relay to operate and shut down the generator.

Robert.

Then I think that you can quite reasonably conclude that your machines are sound.

However, with TN-C-S current in the earthing conductor may have nothing to do with leakage so if you find it, you cannot conclude that something is amiss.

(And don't forget that even with what looks like TN-S, there may be N-E links nearby.)

My mother’s bungalow had solid brick walls, the earth trip was operating so after unplugging everything the problem remained, dampness in a bedroom 13A socket was the problem, curiously it never happened again after a good drying out with a hair dryer. A high voltage insulation tester commonly referred to as a Meggar is the most useful tool for a fault of this nature.

Mike

Hi Chris,

Can you please explain your reasoning behind the earthing system making a difference to machine leakage current? I think I'm missing something.

Any connection between neutral and earth downstream from an RCD will alost certainly cause it to trip.

Robert.

Robert, A couple of hundred Ohms of resistance in an TT earth path might make a milliamp or two of difference, but that is not my point.

If the supply is TN-C-S and there are any diverted neutral currents, they will swamp any earth leakage so you cannot necessarily measure it by looking at the current in the earthing conductor.

I was referring to the N-E link, which is normally in the service head, but there will almost certainly be other links nearby such as a neighbouring property or street lamps.

Some of my electrons appear to be going from N to E then through the bonding to the gas pipe then to at least one neighbour's gas pipe, along their bonding to the main earthing terminal and then back to the transformer via E or N, or a combination of the two.

Just now, I got a reading of 0.40 A with my clamp meter around L, N, and E together.

N-E connections up-stream of the RCD or consumer unit are immaterial to RCD operation and leakage currents.

The whole point of measuring L-N differential current rather than earth conductor current is to capture all leakage.
400mA of leakage is a bit excessive. I'd want to investigate what is going on. what is the current in the protective conductor?
One thing is if you have metalwork with a low resistance to ground connected to the mains ground your system may be passing some mains return (neutral) current from your neighbors. If theter is a neutral fault in the street you could be badly affected.

Radio amateurs on TNC-S can suffer from this when their RF earth, often extensive and low resistance, is connected to mains earth via their radios. With a neutrla fault 100's of amps can flow.
The latest guidnace on NOT connecting pipework etc to TNC-S systems seems to be a way of avoiding this inhernet problem with TNC-S The only advantage of TNC-S is lower cost to the network providers.

Robert.

Yesterday, I measured 3.0 A. It seems to fluctuate quite a bit. Generally, it seems to lie between 10% and 25% of the current in the Line.