Core imbalance in RCDs

How closely matched are the active and neutral windings in the toroid of a typical RCD? Would it be possible for the imbalance, if any, to be of such magnitude (eg 1mA/A ???) that high inrush currents (eg 30A) could of themselves be enough to trip the RCD?

- Franc Zabkar

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Franc Zabkar
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IIRC here theyre tested to trip between 5 and 15mA for 30mA rated units. If those nrs arent exact, theyre about right.

30A)

yes, hence they must all be tested and some are rejected.

NT

Reply to
bigcat

toroid

electrician?

theyre tested in such a wau as to ensure the faulty ones are caught.

it isnt unclear at all.

balanced,

The question had just been answered. Are you moron or troll?

not really. Unless you made the RCD yourself.

response

m/sec

they answered a different question. Whats new.

NT

Reply to
bigcat

I've done tests on the effect of the geometry of a bar-primary on the accuracy of a toroidal CT, at 400Hz and up to 500Arms.

The secondary current remained within 0.1% for all manner of geometries.... off to one side, at an angle going through the core, even when the bar did an abrupt right-angle as it left the core. Quite surprising.

Filter capacitance in an appliance would be the first port of call in most cases.

However, we had a switch-on nuisance tripping in this house that took me days to find. It was random, but eventually the pattern was that it happened when any high current appliance was turned on, anywhere in the house. All appliances were minutely checked and I started to suspect the RCD also.... but the explanation turned out to be stupidly simple.

We had acquired a Neutral-Earth short, caused by recent work in the kitchen.

If the Neutral voltage is low, an N-E short will not trip the RCD but what happens is that, every time a high current load is switched on, some of it's return current goes back via Earth, not through the Neutral wire in the RCD. This is what unbalances the RCD.

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Tony Williams.
Reply to
Tony Williams

I don't have the A/NZ standards for RCD's but here is a typical RCD tester which has pretty good specs and measures tripping current +/-

0.25mA and tripping time +/-0.1mS.

formatting link

Reply to
Ross Herbert

On 6 May 2005 17:16:07 -0700, snipped-for-privacy@meeow.co.uk put finger to keyboard and composed:

But how are they tested? Are they tested by applying leakage in the presence of full load current, or is the leakage applied in the absence of load current, as is the case when tested by an electrician? If the latter, then any inherent imbalance would not show up.

Just in case my question is unclear, allow me to rephrase it.

My scenario is one where there is zero earth leakage. Under such circumstances the core in a perfect ELCB should be perfectly balanced, ie the net flux should be zero. I'm asking whether it is possible for there to be a non-zero flux in the core in the absence of earth leakage. I could envisage this occurring if the active and neutral windings were not geometrically identical, causing one to contribute more flux than the other.

Let's assume that manufacturing tolerances produced a core with a non-zero flux. For a load current of 1A, this flux may be equivalent to that resulting from a 1mA imbalance, say. In this case, a 30A inrush current would cause a 30mA imbalance which would be enough to trip the ELCB.

Is this a feasible explanation for nuisance tripping at switch-on?

When I asked a local manufacturer my original question, their response was inconclusive:

"There is a possibility of variance in the mA rating of an RCD. Clipsal conducts the following tests 15mA 2 seconds and 30 mA 40 m/sec or Less. In domestic applications this would be suitable but we do offer what we call a G Type. This 'G Type' has a 10mS delay. Tests have shown that most false tripping occurs within this 10mS period."

- Franc Zabkar

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Reply to
Franc Zabkar

If you have more than one primary turn, I would imagine that winding the two primary conductors so that they magnetised different parts of the core could cause imbalance due to slight variation in the core material.

....Just an aside, in case anyone was tempted not to wind them as a bifilar pair.

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Adrian Tuddenham

It would be easier if you see a small magnetic field around any piece of wire which carries AC current.

This magnetic field, this inductance, can be amplified, or multiplied, in different ways.

One is to put magnetic material close to the wire, or around it. Another is to arrange the wire in a loop, each turn will add to the total magnetic field. Use both methods simultaneously and you get a coil with a core.

Every part of a turn counts. For example a wire which is 3/4 wound around a core creates a magnetic field which is 3/4 as strong as one full turn.

When we look at readymade inductors, on toroid cores for example, we see that the wires which leave the coil leave it a a right angle. That is because it makes it easier to calculate the inductance. Wires at right angles to the coil do not influence that field. You can use parts of turns in your calculations if needed, if a wire leaves the coil at another point that the entry point.

Your example pictures;

Number one shows a wire straight through a bead core. That is not a full turn, it is rather 0.3 of a turn.

Your second examples shows a toroid with one loop, in total 1.3 turns.

The third picture, no comment, can't be sure what it shows.

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 Roger J.
Reply to
Roger Johansson

understand

slightly

Well at least youve answered my question above.

NT

Reply to
bigcat

There is no such thing as a fractional turn in a transformer.

There are only integer turns coupling and leakage inductance.

Mark

Reply to
Mark

I think the iron core makes it difficult to measure the fractional turns effect, because the effect of another pass through the core means so much more than shaping the wire around the outside of the transformer.

The effect of fractional turns are a lot easier to measure in a coil without a core with a hole in it.

When there is a core which goes around the wire you get a strong increase in inductance every time the wire passes through the hole.

--
 Roger J.
Reply to
Roger Johansson

On 8 May 2005 05:32:10 -0700, snipped-for-privacy@meeow.co.uk put finger to keyboard and composed:

Neither. I may be ignorant, though. In any case, you didn't understand the question. Let me rephrase it in a way that even you can understand. When an RCD is tested at the factory, is it tested using active and neutral currents of 30mA and 0mA, or is it subjected to full load currents of 10.030A and 10.000A? Is it possible for a

10.000A current in the active conductor to magnetise the core slightly differently than a 10.000A current in the neutral conductor? Judging from the other responses, the answer is no.

What prompted my question was a flurry of letters to Silicon Chip magazine discussing the workings of a current transformer. There were questions as to what constituted a turn or "half turn". This challenged my understanding of the fundamentals.

Maybe someone can set me straight. The three diagrams depict a conductor passing through a toroid, or maybe a current clamp meter. In B and C, the conductor has a single loop, or "turn". In B, the ends of the loop are outside the core, whereas in C they pass through the centre. IIUC, cases A and B produce the same flux, but C produces twice the flux. So a "turn" refers to the number of times the conductor passes through the centre of the core, not to the number of loops. Consequently it makes no sense to talk of partial turns. Have I got this right?

__ / \\ toroid I -> / \\ A ------- | ------------ 1 "turn", 0 loops \\ / \\__/

___________ B __________/___ 1 turn, 1 loop I -> / __ \\ \\/ \\/ /\\__/\\ \\ / \\__/ toroid

____ / \\ toroid / \\ | ____|______ C _________\\/___/ 2 turns, 1 loop I -> /\\__/\\ \\ / \\__/

- Franc Zabkar

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Reply to
Franc Zabkar

If we take a ferrit core shaped like a straight stick, a thousand turns of wire on it. Like the ferrite antenna in a transistor radio.

I take another conductor, which carries an AC signal and strong current, and put it close to the ferrite stick, tangentially.

It will induce a weak current in the fixed coil.

If I wrap the outer conductor closer to the core, and wrap it around most of its circumference the induction will be increased. If I make a full turn, or more turns, the influence/voltage will increase even more. But even if the outer conductor only passes by in a straight line some of the signal will be seen in the fixed coil.

That is an example of a fractional turn transformer.

Fractional turns are much more difficult to measure with a circular core which goes around the wire, but it must be there, or the fundamentals of physics wouldn't work.

By the way, I looked up "CT test kit" and got one single hit, it is a test for chlamydia. Of course you are talking something else, some transformer designer kit, but I am not clear over what you have tested and how.

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 Roger J.
Reply to
Roger Johansson

On 09 May 2005 09:14:59 GMT, "Roger Johansson" put finger to keyboard and composed:

I would have thought so, too, hence my original question. However, elsewhere in this thread, empirical evidence was presented that suggests that it does matter at what angle the conductor enters and exits the core.

See "current transformers can be dangerous":

formatting link

Editor says half turn, reader says full turn.

Also, "disagreement "With regard to the one conductor through the core being considered as a full turn, many students question this but the lecturers were adamant that it was correct and that it definitely could not be treated as a 'half turn'".

Perhaps if I redraw the conductor this way:

Z A _____________ ______________/___ I -> I -> / \\ B \\ Y / \\__/

X

If I attach a current clamp meter between points A&B, X&Y, and Y&Z, I expect to measure currents of I, I, and 2I. This would suggest that the number of turns at these points are 1, 1, and 2, respectively. Unfortunately I don't have access to a meter, so I am unable to test my idea.

- Franc Zabkar

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Reply to
Franc Zabkar

On Tue, 10 May 2005 07:51:39 +1000, Franc Zabkar put finger to keyboard and composed:

Oops, that should read "does not matter".

- Franc Zabkar

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Reply to
Franc Zabkar

I still have some CT test kit still around, and it is a trivial experiment to do, as below.

The CT has a 1000 turn secondary and I still have an old 100-way plug+socket arrangement, with 100 wires, arranged as 50+50 turns. So those 50+50 turns can be the primary, wired either as series- -adding or series-opposing. A primary current of 1A is equivalent to 100A in a bar-primary.

Here are the results.

  1. Series-adding 50+50 turn pri, 1000 turn sec. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 1a. Straight through, return bundle as far away as poss. Ipri= 1.007A, Isec= 99mA.
1b. Return bundle touching the outer of the toroid. Ipri= 1.016A, Isec= 99.9mA. 1c. Bundles wrapped to form a 1-turn close loop. Ipri= 1.02A, Isec= 100mA.
  1. Series-opposing 50-50 turn pri, 1000 turn sec. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 2a. Straight through, return bundle as far away as poss. Ipri= 1.020A, Isec= 7.6uA.... yes, microamps.
2b. Return bundle touching the outer of the toroid. Ipri= 1.030A, Isec= 4.65uA. 2c. Bundles wrapped to form a 1-turn close loop. Ipri= 1.030A, Isec= 0.75uA.

A wire through the middle of the toroidal CT constitutes one full turn, what happens around the outside is second order, and there is no such thing as fractional turns.

--
Tony Williams.
Reply to
Tony Williams

Nice pictures.

The first picture shows a 1/3 turn, as the wire goes straight tangentially on the outside, result 35mV.

Picture 2, the effects on both sides cancel each other, zero result.

Picture 3, bending the wire shapes it better around the outside, stronger signal, we could call it 1/3 turn plus a little more, =0.4 turns.

Picture 4, the wire influences the opposite side of the toroid so it cancels some of the influence on the other side.

We could also put the wire through the core once, to see how much stronger the influence gets, to compare the result with the fractional turns experiments.

--
 Roger J.
Reply to
Roger Johansson

Okay, let's concentrate on toroids. I can imagine an experiment which could solve this problem.

If I had my electronics stuff around I would take a toroid transformer, put an AC voltmeter on the coil with most turns, to make it easier to measure the induced voltage. (maybe an amp before the voltmeter)

Then I would send an AC signal through a wire and hold it close to the core, see its response, start shaping it around the core, 1/4 of a turn, half a turn, 3/4 of a turn, and see if the voltage goes up gradually as the wire is shaped better and better around the toroid core, but without going through it.

I think we will find that the induced voltage goes up as we bring the wire closer to the core, and as the wire is formed into an increasing part of a full turn.

It would be interesting to see if a half turn gives exactly double the induced voltage of a quarter turn.

--
 Roger J.
Reply to
Roger Johansson

If you don't mind I'll drop the full paragraph back in, because it was carefully worded.

A wire through the middle of the toroidal CT constitutes one full turn, what happens around the outside is second order, and there is no such thing as fractional turns.

Afaik this thread is only about toroidal CTs to which my remarks were carefully addressed. I have no opinion about fractional turns for other topologies.

[snip]

As already noted, there is no discernable difference in the secondary current between a straight 100-turn primary bundle, or when the bundle is twisted to try and get a 1-turn tight loop.

In the bench setup here the CT has an internal dia of about 28mm and the 100-wire bundle has an od of about 10mm. So the straight bundle can go through the CT at quite an acute angle. There is little apparent difference in the secondary current between the bundle going through the centre at a right angle, or leaned over at an acute angle. Now that really flies in the face of the tidy diagrams in textbooks. I have no explanation for that.

Careless wording, sorry. There are some bits and pieces still around here from when I did precision CT measurements and selections about 10 years ago.

There are a few actual CTs, 1000-turn secondary, originally used in an aircraft with a bar-primary to measure feeder currents of up to 500A, 400Hz.

And there is a little jig that allowed me to put on 100-turns without actually winding them. It is just a 100-way plug+socket with 100 wires, wired so that the socket can be passed through the hole in the CT and when they are mated the 100 wires are connected in series. It was a lucky accident that the arrangement was separate 50+50 wires, so they can be connected as series-adding or -opposing.

For this experiment the 50+50 turn primary is just being stimulated at 50Hz, off a variac, low voltage transformer and series power resistor. Ammeters to measure the pri and sec current. Bit rough and ready, but enough to demo what the OP asked for.

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Tony Williams.
Reply to
Tony Williams

  1. | | ____________ || |--- 1A || Toroid | 35mV ||____________|--- | \|/
  2. ____________ | |--- | Toroid | 0mV |____________|--- 1A----------------------->

/ 3. / /____________ || |--- 1A || Toroid | 50mV ||____________|--- \\ \\ \\

  1. __________________ | ____________ || |--- 1A || Toroid | 34mV ||____________|--- |__________________

:-)

Bit rough and ready. I'll do it more carefully some time later today, to confirm (or not).

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Tony Williams.
Reply to
Tony Williams

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