Current measurement novelty

[Poe]

Now I'm familiar with current clamp measurement devices like this one:

Clamping it around both the hot and neutral at the same time and their magnetic fields cancel, right?

So today one of our customers told me they were measuring current with this unmarked box that's tied to a 2+ground power cable.  The box wasn't surrounding one isolated conductor though.  Rather just zip tied to the outside of the entire power cable with both hot/neutral conductors (+ ground).

I put my current clamp meter over the cable and, no surprise, didn't measure anything.  Then I put it around the isolated hot wire and measured 7A, the same 7A reading as this thing!

How the hell does this thing work?

There's no markings IDC pins or outward indicators of operation.  Just a black plastic box zip tied to a cable.  The readings didn't drop when it was pulled away from the wire, just dropped to zero after 3 millimeters or so.  I didn't get a chance to change the orientation or anything else.

It has me baffled.

The tech didn't know where it came from, but if I don't figure it out I'm driving the 6 hours back there to find out.

[Marco]

How's your French?

The abstract is in English and claims to be able to do this. From the pictures I can see it puts magnetometers all around, but at a guess I'd say that's not really necessary and the method would work with a sufficiently large array on one side of the cable as well.

[Someone]

There are some simple (but expensive) clip on probes that measure the fields in tiny detail around the flex cable, they can find each conductor and measure the flows through them rather than summing the total like a traditional clamp does (a useful feature that is often exploited for making differential/common mode analyses).

[Poe]

Interesting.  I've never seen that before.

The technique described in the french paper appears to need the entire sensor array shielded from stray magnetic fields in order to work though.  It also appears to require calibration for each unique physical orientation.  So if the box was moved to a different cable with different size conductors or insulators, the results would be way off.  Even in their test it appears they needed shielding inside the cable and parallel conductors.

Is this correct? 

Someone - any more information on those clip on probes?  My research hasn't come up with anything.

[Rerouter]

For AC, it sounds like its just using lenz's law as a half turn transformer.

[Someone]

Someone - any more information on those clip on probes?  My research hasn't come up with anything.

It was some time ago that I saw these units being offered and cannot remember who it was but there is this group advertising their patent:
http://www.suparule.com/patent-technology/ac-current-measurement-in-multi-core-cable/