CT Sensor not working on home AC Feed

Hi all,

I’ve been working on getting a CT sensor to work for a couple of weeks now. I’ve been doing my testing with a standard 13A kettle lead and have separated the live, neutral and earth - putting the clamp around the live wire. This has worked fine and I’ve got some accurate, calibrated readings.

However, when I put the clamp around the main incoming live wire to my house (in the consumer unit) I get readings that are 4 times what my testing has lead me to expect. Unfortunately it’s not simply a case of dividing the figure by 4 as this doesn’t work for readings below 250W. The return voltages from the CT sensor seem to have completed changed and the figures are useless. As far as I’m aware, the incoming cable is single phase and is a single cable therefore I shouldn’t be seeing current duplication.

Has anyone had to deal with this before or could point me in the right direction?

Many thanks, James

Welcome, James.

What you’re saying doesn’t make sense at all. I’ve never had this problem reported before. We’ve often had people put the c.t. on a two-core or 3-core mains lead and read nothing, but never had it reading 4 times.

But first, what equipment have you bought? How are you checking the accuracy?
Do you have an emonTx V3? Have you by any chance been testing on inputs 1 - 3 and now you’re using input no.4? Because input c.t.4 is a high sensitivity one, scaled for 18.33 A maximum (or thereabouts) which is close enough to your 4 × to be a prime suspect, though the calibration should have corrected that.

A c.t. is a current source, so the voltage is not meaningful until you measure across the burden resistor. But for input 4 of the emonTx, the burden resistor is different - it’s 120 Ω rather than 22 Ω, which will multiply the voltage by 120/22.

If it’s not that, I could expect a small difference between measuring your kettle - at say 10 A (2.5 kW) and maybe >30 A for the whole house, but a factor of 4 at those currents is ridiculous. I could accept it if you’d calibrated using a LED lamp, because the 100 A current transformer - a “clamp” is something I use when gluing two pieces of wood together - isn’t specified for accuracy below 10 A (though it’s reasonably good down to a few hundred milliamps).

Maybe a photo of where you’ve put the c.t. will help. Our ‘shop’ standard c.t. will pick up current flowing in a wire outside the core but close - however with 100 A in that other wire, it shows a small fraction of an amp, so it’s not that.