[Gllug] Revisiting clamp ammeters

[j.roberts]

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.


MeJ
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