[Gllug] Revisiting clamp ammeters

[James Courtier-Dutton]

On 23 November 2010 13:23, j.roberts <j.roberts at stabilys.com> wrote:
> On 23/11/10 11:05, Simon Wilcox wrote:
>> This was discussed a while ago and the advice at the time was that clamp
>> ammeters could only measure a single wire, clamp them round a standard
>> 3-core cable and they don't work.
>>
>> This device seems to claim that it works round standard cables:
>>
>> http://www.unitetechnologies.com/products/power-monitoring--control/cl-amp.aspx
>>
>> The paper catalogue for Comms Express explicitly says it monitors
>> 'standard 3 core single phase AC power cables' although it doesn't seem
>> to say exactly that online:
>>
>> http://www.comms-express.com/products/unite-cl-amp-single-unit/
>
> I'm a long-term (albeit occasional) user (and owner!) of various high
> current non-invasive ac and dc measurement systems.
>
> The spec (and pic) for the above device shows it being used on the
> *feeder cables* (12/16/32A) rather than a 3-core flex mains lead.
>
> I'm always prepared to be wrong, but my theoretical background, at
> least, (and a quick Google) tells me that there is no simple reliable
> way of non-invasive external current monitoring of a three-core lead
> with return current flowing within it - the magnetic and electric fields
> of a close parallel twisted pair of ac conductors tend to cancel each
> other out (which is part of the point), and I'm not aware of a general
> solution to this (specific bodges are always possible of course as the
> cancellation is inexact).
>
> So my best belief is that all current (sorry!) devices will require the
> separation of conductors and attachment around only one of them.
>
> I'd be delighted to be corrected though.
>

>From my knowledge of electromagnetism, I would say that a clamp unit
around a 3 core wire should be able to give some sort of output.
You can pick up emanations from the cable which is how those "cables
in wall detectors" work.
What I am not so sure about, is that what is picked up is proportional
to the current in the cable.
"cable in wall detectors" pick up the emanations even if there is no
current in the cable. The mains cables just act as a huge aerial as
they are connected to an alternating voltage source.
One way that it might work is if the detector is closer to say the
live wire than the neutral wire.
The currents in the live and neutral would then not exactly cancel
each other out because the induced current in the sensor is
proportional to the distance from the source.
In any case, I think it would be very hard to calibrate accurately,
whereas having the detector round just one wire is much more accurate
and easier to calibrate.
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