New Install: 100A CT sensor reading half meter value

HI all,

I’ve just bought and installed an Emon base, emon TX and 2 CT sensors - 100A for my mains incomer and 50A for my solar feed. All bought this week form openenergymonitor. The install has gone fine and all is up and running, and I’m impressed except … the 100A sensor is reading pretty much spot on half the reading I get from my smart meter display.

I’ve tried a few different placements, taken if off totally to check (zero reading) and rebooted everything.

Then I tried the 50A sensor on my mains incomer and that gave a much closer reading - anything from about 8% out (lower consumption eg 400w) to 4% (high consumption 1850w).

I’ve looked in existing posts and didn’t find anything that matched my scenario closely (especially new kit all bought from oem), so:

  • Is there some configuration or calibration setting that I’m missing or hasn’t been set correctly for the 100A sensor?
  • Can I also get the 50A sensor closer to the ‘real’ figure?

Thanks in advance.
Andy D.

Hello Andy and welcome to the forum!

Is the 100A CT plugged into CT channel 1 nearest the RJ11 emonVs voltage sensor input? and the 50A CT plugged into CT channel 2?

Those readings do seem a bit high, I would expect closer to 1% (apart from at very low power’s <100W) though it may be hard to make that comparison accurately with the smart meter display, how do your daily totals compare? Is your emonVs voltage sensor connected and powering the emonTx4? What kind of Vrms voltage readings are you seeing? If the Vrms value appears stuck at 240V then it might be that it’s not getting the voltage signal and calculating apparent power instead of real power…

If you do have the CT channels connected as I described you may want to check the configuration & calibration values applied, see: You should have the default calibration values.

Hi Trystan,

thanks for the reply. The CTs were both plugged in to their correct channels, although when I tested the mains with the 50A CT I did plug that into channel 1. I’ve now plugged the 50A CT back into channel 2 but still monitoring mains incoming and get the same picture as when it was in ch1 e.g. 270w from the meter and 220w from OEM - close but no cigar :slight_smile:

The Vrms is around but not exactly 240V and it is fluctuating slightly - it has been upto 246V last night - so I believe the voltage signal is correct.

Ill check out the configuration as per the link but the setup is in the garage so ill have to set a laptop up down there unless I can read them from emoncms?

Hi again,

so after a bit of faffing - serial monitor connection on a windows laptop just didn’t want to know, whichever I put the connector,so I unplugged everything, brought the Tx upstairs and plugged it onto a rpi which worked fine - I finally checked the values.

They all seem set to the defaults to me pdf screenshot attached and monitor readings below. Any other thoughts??

Band 433 MHz, Group 210, Node 17, 7 dBm
vCal = 807.86
assumedV = 240.00
i1Cal = 300.30
i1Lead = 3.20
i2Cal = 150.15
i2Lead = 3.20
i3Cal = 150.15
i3Lead = 3.20
i4Cal = 60.06
i4Lead = 3.20
i5Cal = 60.06
i5Lead = 3.20
i6Cal = 60.06
i6Lead = 3.20
datalog = 9.80
pulses = 1
pulse period = 100
RF on
temp_enable = 0
JSON Format Off
Serial Monitor.pdf (61.9 KB)

Thanks @shandyd05 that all looks great, i assume the rating as printed on the CT’s match your expectation as well? the 100A CT should have 100A printed on it and the 50A one 50? Did we send you a 200A CT instead?

What is your smart meter display? is it one of these? How to use your In-Home Display | Octopus Energy I assume getting it’s readings from a supplier billing meter rather than a CT sensor add on?

The ratings on the CTs are as expected -100A and 50A. The smart meter display is a Geo IHD (as per octopus page) and it does get its readings form the supplier meter (honeywell) ASAIK (no CT). I do have a CT for a solis diverter (to an immersion heater) also on the mains incomer, but I removed that just to make sure there was nothing going on there.

Ok great, I’m pretty much out of ideas. I assume it’s a single phase supply there? Nothing unusual on that front?

Probably the next thing to try is to send you another unit to test and for us to check the one you have there. Il send you an email when I get back in later with details.

Yes single phase and not aware of anything unusual. Ive had it disconnected form the CTs and in the house while I checked things, set up ssh etc. so Ill plug it all back in and check if anything has miraculously changed after a few reboots!

No Voltage Reference…

Or was that just when you were testing it?

No, that is the SET VALUE that is used WHEN the a.c. reference voltage is unavailable. It has been adjustable for a long time now, via the API (emonTx V3).
He’s reading 244 V so definitely has a voltage reference, and in any case it would have to have been set to 122 V without the a.c. being available for the power/energy to read half.

Yes, I know that. My point was, when that set of settings were extracted, there was no voltage reference.

But you are assuming he is taking that reading from the EmonTX4. there is no definitive statement that, that is the case.

It maybe the reading is and there is a Vs but until that is stated (and I cannot see that it has) it is an assumption.

He’s configuring 6 c.t’s, so it’s not an emonPi and it’s not an emonTx V3. Trystan will know from the Shop records what he’s “just” bought, plus he says “new kit…”. (The V3 was out of stock for a while before the V4 became available.)

I can’t think of a credible error that would give this, I think it must be a hardware fault - the c.t. or the emonTx. If he plugs the c.t. into one of the other sockets, it should read 50/100ths or 20/100ths of the true value (it won’t damage the emonTx - all inputs are 0.333 V) and if he sees half that again, it’s almost certainly the c.t.

[Likewise, the 50 A c.t. should read 100/50ths of the true value when plugged into the No.1 socket - which will prove the input of the emonTx.]

We know that Andy has an emonTx V4 and an emonVs.

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Thanks. That wasn’t clear from these posts though.

Also, doesn’t confirm the Vs was connected first before the USB power (if both are connected) which would mean the Vs was not picked up.

“Assume nothing”

I think this is good diagnostic information :slight_smile:

You can generalise it to:

When a c.t. is plugged into any of the sockets, the reading from the emonTx V4 should be

(the actual current flowing) ÷ (the c.t’s rated current) × (the socket’s configured current)

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Crumbs I’ll have to make sure I don’t look away as long in future - I’ve been slightly chided in the 3rd person a number of times since last on :wink:

To be clear yes it is an emon TX4 - I did say I’d just bought it and its the only model available I believe, but still, mea culpa. I did however state that it is an emonbase not an emonpi and the emonVs is mentioned at least once.

‘No voltage reference’ - that was just when I was extracting the setup information as requested, because I couldn’t get to the serial configuration in situ. I had previously stated the reference voltage data was varying.

I also did connect the 50A sensor to ch1 at one stage for just the test you suggested @Robert.Wall

At that stage it then read close to the meter reading, but still too far out for my liking.

However, the good news is that after plugging everything back in I’m now reading pretty close - certainly to within a few %. The only differences being that I plugged the TX4 in the base via USB and I corrected my schoolboy error of connecting the solar (50A) CT round the whole damn cable, which obviously gave a reading of 0 all the time! The less good news is that I’ve no idea why or if these changes should impact anything or if it was some other user error, but for now Ill continue to monitor to ensure it stays ok.

Finally, thanks to all who contributed.


I wonder if you plugged things in, in the right order. So CT, then Vs. If it isn’t a Mini-VS then you don’t actually need to plug in the USB power - power comes on the VS cable.

If you plugged in the USB (which powers the TX and boots it) then the VS, you might see what you saw as the EmonTX only detects if there is a VS on boot.

I didn’t plug in the USB at all initially. As you say power comes from the Vs and signals were being received by the RF. I plugged the USB from the TX4 into the emonbase latterly to receive the data that way, as Id originally intended tbh, although it did add another variable into the mix, which probably wasn’t the best plan with hindsight!

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