OpenEnergyMonitor Community

CT clamp/feed problem

Tags: #<Tag:0x00007f68210dcdd0> #<Tag:0x00007f68210dc6a0>


I have an emonpi that I’ve been using happily for many years. Recently, I had to unplug it to do some work in the understairs cupboard where it lives. When I plugged back in, the CT clamp that is reading the solar now gives a constant output of around 50W when it’s dark (i.e. when there’s nothing coming off the panels), so is messing up my solar feed. It also seems to give a constant reading of around 530W during daylight (i.e. when there’s anything at all coming off the panels). I’m in the UK, so obviously I don’t expect much at this time of year :smiley:

I recently switched to Octopus Agile (incoming and outgoing), and the outgoing has started as of 4th Jan (yesterday), so not sure if that would be affecting things. Obviously a smart meter was fitted to allow this, but after the engineers had been in, there didn’t seem to be any change to the readings being sent to the EmonPi. I’ve confirmed that it’s the clamp itself by plugging it into the other socket. I’ve also confirmed that the feed drops to 0W when the clamp is removed from the wire.

Could it be that there’s something up with the clamp itself? To be clear, the clamp itself was not removed from the cable during the work I did, only from the EmonPi itself.

It would be good to get this fixed, as I’ve got readings going back many years, and now they’re not right!

Thanks in advance

It’s not clear what the timeline is. You unplugged the emonPi to work around it, then some time later had your meter swapped. When was it known good, and when did the problem arise?

Do you have an a.c. adapter, and was the c.t. and a.c. adapter plugged in (and the adapter switched on) when you powered up the emonPi?

Do you have a Grid c.t, and can you put both on the same cable to compare?
Or can you put the Solar c.t. on the meter tail and compare the reading against the new meter? (You might have to get the power by calculation from the rate of meter flashes.)

Did your emonPi update itself when you powered up again? If you’d changed a default setting previously, that might have been overwritten by an automatic update.

If there was a significant current passed while the current transformer was unplugged from its burden, it might well have been damaged by the excess voltage. I’d also suggest unclipping it and looking for damage, especially a broken core (because the core is ferrite, an extremely brittle material).

But both these sound unlikely, if you removed the c.t. from the wire and left it plugged in, and then it reads a very small power (say < 10W), then the input is probably good. You really need to check when there’s a decent current and a known current (or power) present.

Not necessarily. I think you’ve also confirmed that it could be genuine power on that cable you clipped it over.

From what you’ve written, I can’t add much more.

Thanks for the comprehensive reply!

It appeared to be the fact that I had powered the emonpi on without the AC adapter being plugged in. The fix seemed to be to shut the thing down, plug in the AC power, and then re-plug the USB power cable. The readings now look correct.

Thanks very much for your help @Robert.Wall! :grinning_face_with_smiling_eyes::+1:

That’s actually in the FAQ. What happened was it assumed a constant 230 V and didn’t have the phase information to calculate real power, so it multiplied the current by 230 V to give you apparent power. But in getting to that current, the rms calculation also rectifies all the noise that’s picked up and adds it in, giving you a false “power”. That’s what you were seeing.