CT Sensor connection validation

Hello all,

Newbie and first time poster to the forum and I’m soo not technical. We’ve had a emonPi connected to a couple of emonTHs for the last couple of months (all out of the box configuration), but I’ve just moved to try and get our newly installed solar to be be monitored via a emonTx, our electric supply is via 3-phase and I’ve had a job trying to find a definitive resource on the correct way to connect it and was hoping someone here to validate that it is correct?

I read a few posts but mainly a post from last year that suggested the following;

First, you must find which phase the a.c. adapter is on.

It does not matter whether that is the brown, black or grey phase at your meter, but the other two ct’s must follow in the correct sequence. To the sketch, these are all the same:
a.c adapter & CT1: brown, CT2: black, CT3: grey
a.c adapter & CT1: black, CT2: grey, CT3: brown
a.c adapter & CT1: grey, CT2: brown, CT3: black.

My setup is;
L1 is Brown : CT3
L2 is Black (A.C Adapter on this one) CT1
L3 is Grey CT2

I’ve created a graph of the feeds;

The inputs are simple and only contain Log to Feed for each.

Have I connected the CT correctly and does the graph look as it should?

Many thanks in advance.


Welcome, Tony.

I take it that you are using the three-phase sketch - is that correct? From the look of the picture, I strongly suspect that isn’t the case. If you’re not using the 3-phase sketch, the emonTx will never read anywhere near the correct powers because of the phase relationship of the three lines.

Now if you’re using the latest version of the 3-phase sketch, the “definitive resource” you are looking for is the pdf file that came with the sketch. You can now allocate the c.t. to any phase. You could not do that with the earlier sketch. But the reference phase remains the line that the a.c. adapter is monitoring. If for simplicity you use the default setting, then your setup is correct.

The 3-phase sketch is here: Update to 3-Phase PLL sketch - #5 by Robert.Wall

2 posts were merged into an existing topic: Update to 3-Phase PLL sketch

Many thanks for the reply Robert, apologies for the delay I’m working away at the moment and won’t be able to anything until I return home, but I shall endeavor to setup the CTs with the details in the 3-phase sketch documentation above.

Essentially when I purchased the emonTx I asked for the latest 3 phase firmware, is this the same as a sketch or is this different software that needs to be loaded? How can I determine what version I have installed?


“Sketch” is Arduino-speak for the user-supplied software that customises the device.

If you have a programmer, on power-up it will print details to the FTDI port which the Arduino IDE can display. If you don’t, the LED on the main board (I think visible inside the case if you know where to look) will give 3 flashes immediately on power-up, then remain on steady for a few seconds.

Hi @Robert.Wall I’ve done as recommended. I’ve purchased a USB (FTDI) connector and connected my laptop to upload the latest sketch (took me a while, learning curve and all).

This is what I am seeing now, it is different, but I’m thinking it’s still not right.

The big dip around 8am, does coincide with a washing machine and hot water being turn on (electric heated).

Hopefully, you can make some sense of it and direct me again.

Many thanks in advance.

Only you know what you were running at the time, but that doesn’t look right to me. Did you extract the .pdf documentation file from your download, and did you follow the instructions in the “Installation” paragraph there for identifying the phases and setting up the c.t’s?
(‘Turn everything else off’ includes your PV, because you need to know the direction of power flow.)

On the face of it, it looks to me as if the red c.t. is facing the wrong way (assuming you follow our convention that import is positive - the hot water should be a positive spike), so is yellow. I’m not sure what the blue c.t is doing - that’s where you need to know what you expect to see.
Previously (post no.1) it looked as if red was definitely facing the wrong way, yellow was correct and blue might have been correct - but I couldn’t tell for definite.

Hi @Robert.Wall, Ok I’ve followed the instructions (previously I took the comment the “You can now allocate the c.t. to any phase” too literally :slight_smile: ).

I disconnected everything and identified spikes in power as I turned a kettle on (seeing positive spikes) for each phase and when the solar was re-introduced I could also see it as negative spikes.

I’ve let it run for a few days and here is what it looks like now. It’s been rather cloudy, so not too many negative spikes, but there are some.

It does seem to reflect our usage, hoping it’s looking correct to you.

Many thanks

That does indeed look a lot more plausible.

Can you explain that? It might well be that I need to change the wording. My problem is I know what I meant - you do too, now, but what I want to know is how you initially interpreted it.


I think I took it literally to mean that once I’d uploaded the new 3-phase sketch, allocating any CT to any phase negated the need for calibration (the isolation of each phase for power spikes).

Many thanks for you help throughout this Robert, I know I asked some really basic questions, but I have learnt so much. I do have a host of other questions (for other threads), but for now, thanks. Next step, converting those values so the Solar app can tell me things :smile:

Thanks again

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OK, thanks. I really don’t think I can make it much clearer. Step 3 says put all the c.t’s onto your test load, then finally step 6 says allocate them to the correct phases, do a final check and then move the c.t’s to their final home.

Ask away, I’m glad you’ve learned something and hopefully found it interesting in the process.