It is a 20/200A clamp meter and I made sure it was about 1m from the fittings. I've never had any doubts about its accuracy but having said that I've not tried currents this low before. I'll try and repeat with an ammeter in the next few days.
Inductive cause of leaks to earth
- Thread starter JohnW2
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Whilst not wishing to infer your meter didn't indicate a current (or at least appear to) I think we need to get numbers into a realistic framework
0.2mA on a 20A range is 1 part in 100,000 or 0.001% of FSD
An AC clamp-on meter accuracy is probably no better than +/- 1.5% which on a 20A range is +/- 12.5mA
I would suggest your clamp-on is picking up and showing a magnetic field flux, rather than an actual current flow.
A differential current meter might be a better measurement tool whereby the difference between Land N currents are displayed
0.2mA on a 20A range is 1 part in 100,000 or 0.001% of FSD
An AC clamp-on meter accuracy is probably no better than +/- 1.5% which on a 20A range is +/- 12.5mA
I would suggest your clamp-on is picking up and showing a magnetic field flux, rather than an actual current flow.
A differential current meter might be a better measurement tool whereby the difference between Land N currents are displayed
I don't have a problem with any of that, which is why I wrote;
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Did you not perhaps mean to write that ±1.5% (of FSD) accuracy on a 20A range is ±300 mA?
I certainly agree that a clamp meter designed to measure small mA currents ought to be used. However, I would have thought that (in the situation we're discussing) a single clamp on the earth would probably be preferable to measuring the 'differential currents' (not that many people have the means of doing that, anyway!), since the latter is likely to double the measurement inaccuracies.
Although residual current devices manage to essentially, a measurement/quantity which is 'a very small difference between two very large numbers' is, in accuracy terms, a mathematician's or engineer's nightmare!
Kind Regards, John
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If one discounts the (I personally think daft) suggestions about 'inductive leakages' which caused me to start this thread, if IR-ing (with DC) L&N-E gives very high readings, then if there is any 'residual current' (measured as a current in the CPC ('earth conductor') must, to my mind, be due to capacitance - either a deliberate (e.g. 'filter') L-E capacitor or just 'stray capacitance'.
However, as Jackrae, I don't think you can take seriously attempts to measure currents as low as 1 mA or less with a meter whose accuracy may well be pitifully inadequate at such currents. With my clamp meter on its 20A range, when clamped around any random bit of metal can easily give apparent readings up to 10mA or 20mA.
Kind Regards, John
I believe I've already agreed with this
If you mean in relation to your intention to repaet the measurements with something more appropriate, I think you have.
However, what about the (possibly too well disguised) implicit question in my first paragraph. If there is a true measurable amount of 'earth leakage', do you see any explicit L-E capacitors in any of those fittings, or are we down to just 'stray capacitance'? To get just 1 mA of L-E leakage at 230V 50 Hz would require about 14,000 pF (0.014 μF), which would be a hell of a lot of 'stray capacitance'!
Kind Regards, John
Well spotted, good to see you're awake at the back of the class :
:I particularly liked your current 'measurements' on the lump of metal. I know the earth's magnetic field is changing but not by enough to cause eddy currents
So I've repeated my earth current tests from yesterday, results in red, then wrapped 10 turns onto the clamp - orange and 20 turns- green finally ac current with DMM, 200mA range blue, 200μA range pink
QUOTE="SUNRAY, post: 4878137, member: 124249"Out of curiosity I've just measured the flou fittings which I use for portable work.
These are all very old, all certainly pre 1994 and most are a decent number of years before that. Total of 17 including 6x 40W 'quickstarts' (fat tubes only) containing 2 wound devices, 6x 30W 3ft and selection of 5x 40W . All meggared as >400MΩ (some >2000MΩ) and earth current measured 6 lamps at a time;
0.9mA/6, 0.9, 1.1, 1.3, 0.07, 5.9μA 6X quickstarts
0.2mA/6, 0.2, 0.5, 0.8, 0.03, 2.4μA 6x 30W
0.4mA/5, 0.4, 0.5, 0.6, 0.01, 3.7μA.5x others
1.5mA/17, 3.3, 3.9, 4.2, 0.09, 12.1μA. 1.5mA calculated, the rest measured for all 17
So the 20A clamp is consistant in its original measurements but where I hoped to see 10x or 20x reading it completely failed to prove itself, as it did on checking all 17. THe DMM also didn't help itself either on the 200mA range, being a factor of 10 out compared with the μA range.
The bit I found most interesting 12.1μA for all 17 fittings gives a 'leakage' impedance of 19MΩ and meggared at 52MΩ, my conclusion being 33MΩ capacitive reactance or 0.1μF
However getting back to sunbed post; multiplying my 17 lamps=620W (measured at 2.85A, 2.86A) up to 6.7Kw is still only 130μA so it makes me wonder where thhe problem lies with the sunbeds flou kit. Even if I'm a factor of 40 out it's still only 5mA.
I'm starting to feel I've gone very wrong somewhere, please feel free to correct me.
QUOTE="SUNRAY, post: 4878137, member: 124249"Out of curiosity I've just measured the flou fittings which I use for portable work.
These are all very old, all certainly pre 1994 and most are a decent number of years before that. Total of 17 including 6x 40W 'quickstarts' (fat tubes only) containing 2 wound devices, 6x 30W 3ft and selection of 5x 40W . All meggared as >400MΩ (some >2000MΩ) and earth current measured 6 lamps at a time;
0.9mA/6, 0.9, 1.1, 1.3, 0.07, 5.9μA 6X quickstarts
0.2mA/6, 0.2, 0.5, 0.8, 0.03, 2.4μA 6x 30W
0.4mA/5, 0.4, 0.5, 0.6, 0.01, 3.7μA.5x others
1.5mA/17, 3.3, 3.9, 4.2, 0.09, 12.1μA. 1.5mA calculated, the rest measured for all 17
So the 20A clamp is consistant in its original measurements but where I hoped to see 10x or 20x reading it completely failed to prove itself, as it did on checking all 17. THe DMM also didn't help itself either on the 200mA range, being a factor of 10 out compared with the μA range.
The bit I found most interesting 12.1μA for all 17 fittings gives a 'leakage' impedance of 19MΩ and meggared at 52MΩ, my conclusion being 33MΩ capacitive reactance or 0.1μF
However getting back to sunbed post; multiplying my 17 lamps=620W (measured at 2.85A, 2.86A) up to 6.7Kw is still only 130μA so it makes me wonder where thhe problem lies with the sunbeds flou kit. Even if I'm a factor of 40 out it's still only 5mA.
I'm starting to feel I've gone very wrong somewhere, please feel free to correct me.
Well spotted, good to see you're awake at the back of the class :
However ....Since it appeared that you had intended to calculate "±1.5% (of FSD)" (and got the arithmetic a little wrong!), I 'followed' that and pointed out the arithmetical error.
However, although I agree in principle with your point (that SUNRAY needs to use a more appropriate meter), the situation is nothing like as 'dramatic' as your figures suggest. An accuracy of "% of FSD" is not how meters work - i.e. the potential error does not remain constant across the whole range of measurements.
Accuracies of meters are usually quoted in the form "±X% of reading ±Y digits". A typical clamp meter (say a 20A/200A one) might have a quoted accuracy of, say, "±1% of reading ±5 digits" or "±2% of reading 5 digits".
If (as it typically the case) the resolution of the meter were 0.1A (i.e. "to one decimal place", such as 15.3A), then 'one digit' is 0.1A. Hence, on a 20A range, if one saw a reading of, say, "15.0A", then "±1% of reading ±5 digits" would equate to ±0.15A ±0.5A, namely ±0.65A - in other words the indicated reading of "15.0A" could mean anything from 14.35A to 15.65.
I'm not at all sure what sort of clamp meter SUNRAY has, since a typical 20A/200A one would have a resolution of 0.1 A, whereas his figures indicate that his must have a resolution of 0.0001 A (0.1 mA)! Whatever, at very low currents (close to the limit of resolution), the "±1% of reading" becomes insignificant, and is totally swamped by the (±5 digits) - for example, with a resolution of 0.1 mA, the accuracy of a reading of "1.0mA") would be ±0.01 mA ±0.5 mA - i.e. ±0.51 mA. As a percentage of the displayed reading, that would obviously be an accuracy of ±51%.
I don't think that the 'readings' one gets from 'a lump of metal' are anything to do with any true induced currents in the metal, but are merely 'noise' (and stray pick-up of voltage) within the electronics.I particularly liked your current 'measurements' on the lump of metal. I know the earth's magnetic field is changing but not by enough to cause eddy currents
Kind Regards, John
I think what you have probably demonstrated is how relatively futile it is to attempt to measure very small AC currents with any 'ordinary' equipment (probably any equipment at all!). If I understand correctly, the dramatic differences between results with your DMM on its 200 mA and 200 μA ranges seem to 'say it all'.
As I've just effectively asked in my reply to Jackrae, I'm a bit confused by your "20A/200A" clamp meter. Does it really have a resolution of 0.1 mA (i.e. 0.0001 A)? As I said, I'm more used to seeing 0.1 A.
Kind Regards, John
Ah yes, it's weird.
On 200A range resolution is 0.1 and shows 'A' but on 20A range resolution is still 0.1 but appears to be autoranging and showed 'mA' during these tests. and 'A' when checking the running current of the lights.
On 200A range resolution is 0.1 and shows 'A' but on 20A range resolution is still 0.1 but appears to be autoranging and showed 'mA' during these tests. and 'A' when checking the running current of the lights.
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That's very odd, and totally outside of my experience. I have never personally seen a 20A/200A (or similar) clamp meter with a resolution better than 0.1 A - so would be expected to show 'zero' for anything less than about 50 mA.
What is the make/model of your meter, and do you have any documentation relating to it?
kind Regards, John
Sadly no info, it's badged as 'BT no. 5' (and the printed label on the back is pretty much worn clear) which means nothing as BT would put out a spec and manufacturers would provide samples which became type approved and purchased(or not). We alway hoped the MeterMultiRange no. 1 would be the Wayne Kerr version as it was the best of the selection BT puchased, and Linstrom for cutters, Wilson for 81's etc.
Easily explained by very low voltages and differences in meter resistance.
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