Mains Contactless Power Meter

Hi guys. Hey I've been searching for a while for this but can anyone tell me how to build a circuit that reliably measures AC voltage and current running through a power cable? I want to create a power metering device that can be retrofit to existing individual power cables, provide an analogue input into something like an Arduino device which I can program to provide real-time consumption data.

Also, I understand clamp meters require the live wire to be separated. I can't do this so it must monitor power with all 3 cores present.

Any ideas?

Thanks.

A capacitive coupled device will measure voltage, and an inductive one will measure the current. That's the basics. I built up such a device some years back for monitoring three phase power cables. I can't say much more, the device was for a company, and therefore proprietary. Suffice to say that the sensing heads had to be small, and were based on PCB planar sensing technology, with op-amps to do the impedance conversions, and put out a voltage for processing with ADCs. They sat adjacent to the cable, therefore the current sensor did not require placement around the cable, with sufficient clearance that flash over would not be likely (not a problem in your case). The physics and geometries were worked out by someone else, and established distances, size, and the likely linearity and dynamic range. The system had to meet some tight specifications, but in your case they could be relaxed, and look up tables could be used to sort out any non linear dynamics. That should give you some ideas. I am sure there will be plenty of others,
cheers,
Richard

Thanks Richard, some ideas there alright. And I'll do my own investigating but it's ideas is what I'm after for now

Interesting idea. I suggest that you put out a voltage for processing with ADCs. It will capacitive coupled device will measure your voltage.

Re my previous comments, the current measurement was possible because the system was three phase, with no neutral wire adjacent with the return current in it. Obviously, as you say, where the current return is in an adjacent wire in a cable you have a problem, since the outgoing and return current should be the same. Normally you need access to the switchboard to do a current measurement, since this is the only place where the live wire is accessible separately. There are a range of cheap power meters that plug into wall sockets, that sit between the power source and the appliance. If the wiring isn't fixed, then you would have to insert a system between you source and load, easy to do, a few plugs and sockets. There are big safety issues, are you are used to working in this sort of high voltage environment,
cheers,
Richard

When you say all cores, do you mean all three phases? Wouldn't there also likely be a neutral return path included in those bundles of an AC system? I'm trying to better understand what you are actually attempting to do.

Yes, all three phases. I am talking power distribution, and not a domestic setup. In a three phase system there is not necessarily a neutral (delta) The work involved measuring the voltages and currents on three phases. It was complex, since the sensor on one phase could "see" the other two phases to a lesser extent ( of course, the other two sensors were the same) Involved quite a bit of maths to work it all out, but not my problem, I just did the hardware.
Perhaps a clearer picture of what you are attempting to do would help, how accessible are your cables,
cheers,
Richard

Actually I'm trying to measure single phase only and yes, the bundle does include the neutral in my case. So is it simply not possible to measure current if the cable contains the live and neutral cores?

If the return current equals the outgoing current, and they are tightly coupled as in a cable then it is not possible with a magnetic flux sensing system. Earth leakage breakers rely on the imbalance between the two currents to operate, since in a fault condition some or all of the return current will bypass the neutral return,
cheers,
richard