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IAM readings versus CT readings
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Author:  rkc [ Sun Mar 18, 2012 4:10 pm ]
Post subject:  IAM readings versus CT readings

I have a CT clamp sensor on one of my ring mains, and also an IAM on one of the appliances (a freezer) that is plugged into one of the sockets on that ring main.

I am monitoring the readings for both using a perl script and rrdtool

When the freezer compressor cycles on, the reading from the IAM goes up from 0 to 80ish watts (as expected), but there is NO noticeable increase in the reading from the CT clamp. I would expect to be able to see bumps in the 'whole circuit' reading that correspond to the bumps in the IAM reading, but I don't. The reading for the circuit is small enough at the time that an extra 80 watts should be noticeable.

I thought I must have got confused about what ringmain the socket was on, but plugging in a larger load (an iron) confirmed that the socket IS on that circuit (and the consumption was recorded on both sensors).

I wondered if this was a 'power factor' issue - that one reading takes power factor into account and the other doesn't, and that the power consumption is in fact a lot less than the 80 watts shown by the IAM and thus doesn't register. But my understanding is that the CT reading would be the one that would overread in such a case - the IAM _could_ give a true power figure as it can see the voltage too (though I don't know if it does).

Am I cracking up? Have I misunderstood how power factor works in relation to these sensors?

Author:  Automan [ Sun Apr 15, 2012 10:34 am ]
Post subject:  Re: IAM readings versus CT readings

The lack of increase on the reading on the CT may be due to the power factor of other devices plugged in on that ring circuit be they on or in standby.

They only way to confirm would be to unplug or switch off at the wall all devices plugged into that ring circuit.

Some TV set can have quite a leading power factor when turned off but the primitive CT will not know this is apparent power rather than true power.

Then your fridge turns on which will have a poor power factor and it is possible the TV could improve the power factor of your fridge.

Thus the current the CT measures remains much the same.

BTW, the National Grid have Ring Mains, domestic homes have Ring Circuits :)


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