3 Phases AC monitoring

Hello all Openenergymonitor community. Some weeks ago I discovered this amazing project and since there I am trying to learn with each of your topics. I am working in an university project in Portugal at this moment, and it concerns the development of a monitorization device for a 3 phases electrical board. Until now the only similar project that I found was the one developed by Mr. Amin Zayani, presented here. After reading the blog post I had some initial dobts to ask for Mr. Asmin

  1. I didn't understand quite well the theoretical principals that you refered in your post. Thus can you suggest me some kind of bibliografy where you inspired yourself?
  2. Are you thinking to present the total list of things(devices,code, tutorial) that you used for the project?
  3. Have you been using a rectifier on your electrical circuit?

I would be very greatful if you could help me.

Great thanks/Muito obrigado

Amin Zayani's picture

Re: 3 Phases AC monitoring

 Hey xaral, glad to know our work is of interest to you.

So the theory is quite simple: you have to keep in mind that the phase shift between the 3 voltage vectors is ALWAYS 120 degrees. If you calculate power of one current vector and a voltage vector of a different phase, then you will get a wrong value, but it you shift it by 120 degrees you will get the correct value.

The hardware is the same as in AC-non invasive 3.0. You need CT's, and an AC-AC adapter. The code is all linked in the blog post.

We did not use any rectifier in the circuit, it is electronically all AC non-invasive 3.0. Nothing more, nothing less.

Here is a link to help you understand 3 phases power systems.

(please no Mr. just Amin)

xaral's picture

Re: 3 Phases AC monitoring

Sorry Amin :)

Thanks for your info. I will try to read all info.

The question now is, I saw last Friday a professional system of electricity monitorization prepare to register the consumption of a 3 phases electrical board. The problem that I saw was the size of the clamps (to measure the current) and the crocodile clamps (voltage).

You can see here the system that I mean CIRE 3/3 .

I would like to know how do you manage these problems, because I imagine that arduino it's not prepared to deal with such values of voltage and current...

What do you think Amin??


Amin Zayani's picture

Re: 3 Phases AC monitoring

 It has nothing to do with arduino. the CT's can have a capacity to measure a max of one zillion AMP, as long as they output a voltage within the arduino's ADC range: 0-5v

There are CT's that measure thousands of AMP and out current. With the right burden resistor you adapt it to the arduino ADC

xaral's picture

Re: 3 Phases AC monitoring

You are totally right. I forgot that aspect.

Nevertheless can you take a look into the clamps of this CIRE equipment on


Until now I just found small clamps capacle to work with small cables. Altough in a case of a big electrical board we will need something more than this small clamps. What do you thing I can do? Do you have any suggestion??

Sorry for all questions, and mistakes...


glyn.hudson's picture

Re: 3 Phases AC monitoring

Large CT's can be potentially very dangerous. I think its best (and safest) to use optical pulse counting to read the utility meter when monitoring currents above 100A. 

In the case of three phase systems the phase shift between the three phases is not always 120 degrees in the case of un-balanced loads.

xaral's picture

Re: 3 Phases AC monitoring

Great thanks for your answer Glyn :)

I experiment last Friday the danger of such powerful system :P

For example in AC non-invasive 3.0 did you use this optical pulse counting??

This is a very important decision for the future of the project and when I discovered the project of Amin and the University campus energy monitoring in France I couldn't resist to ask you all my dobts, and what kind of devices did you use...

Sory if I'm getting to bore for all of you :/

TrystanLea's picture

Re: 3 Phases AC monitoring

Hello Xaral, Mains ac non-invasive 3 is the CT route, for details on pulse counting see our work

here: http://openenergymonitor.org/emon/buildingblocks/introduction-to-pulse-counting


Hope that helps, both options are available, its just a matter of choosing the best one for your application. For now our advice is to use the CT method for currents less than 100A as this is the maximum rating of the CT that we have tested the system with. For currents above this the pulse counting method is available.


Erikthefish's picture

Re: 3 Phases AC monitoring

Measuring 3 phase loads is not a simple proposition when installations could vary wildly, one installation may be consuming 50A on the Blue phase, and 20A on Yellow, and 30A on Red, another may only be consuming 1 or 2 Amps quiescent then consume 30A on all three for short periods.  So the measuring sensor must be able to " Range" to get suitable accuracy to make the measurements meaningful, and potentially cope with domestic installations that have normal domestic loads on say the Red phase, and Night storage heating on either/OR yellow and Blue that is only present between certain times.

With respect to three phase; single phase loads can be considered "Ideal", however with 3 phase loads, especially with inductive loads like motors, it can be difficult to make accurate measurements. Switch mode power supplies also complicate things as they can introduce variable high frequency interference from the load and back into the generator and possibly into the measuring equipment.

For UK domestic installations I think the maximum generation current is limited to 15A by the local R.E.C for three phase generators however.

If a plugtop transformer were used for voltage reference for all three phases, this would mean all three phases could be connected via the emonTX albeit at approx 10vac, this I think would be Un-advisable, and a wireless solution would be the best way foreward? A single plug-in wireless "Wall-Wart" per phase linked to the emonbase?


Rasmus Fiil's picture

Re: 3 Phases AC monitoring

Hello everyone,

I am new to this forum and this technology but I am keen on getting started!

I am involved in a project in collaboration with Roskilde Festival (one of the biggest music festivals in Northern Europe) where we want to measure and log the energy consumption of some of the vendors and food stalls. The measurements is to take place between a distributor board and the vendor with cables up to 63A (5x16mm^2). 

We want to use this non-invasive technology with three CT's but we also want it to be contained in a relatively small box.

Is there any way to measure the voltage ​without using an external adapter?

And if not; where do you plug this adapter and make sure which phase is measured?

Best Regards Rasmus

Robert Wall's picture

Re: 3 Phases AC monitoring

Hello Rasmus and welcome,

The short answer to your question is no. It's hard for me to give you specific advice, because I don't know how you are distributing the power. But I presume you have a 3-phase supply and you will be distributing 3 phases to key points (the distribution board you mention), and from there individual vendors and stalls will take either 3-phases or a single phase. And you want to monitor and log each individual consumer? What sort of accuracy do you need? How many consumers? Do you need real-time data?

As Erikthefish pointed out, and especially in the situation you mention, Amin Zayani's assertions will be wrong. You are likely to have seriously unbalanced loads and significant neutral currents; and so the assumption that the three phase voltages are the same, and that they are always exactly 120º apart, will not be valid. You can of course measure the 3 phase voltages with a direct metallic connection, but you then have major problems because the whole of the electronics must be considered 'live', so must be in an earthed metal box (which makes reporting by radio problematical) or double-insulated. And of course testing can only be done by people with experience of working on live equipment, and you cannot plug a laptop in to the unit to program it or read values without galvanic isolation in the data connection.

The emonTx has been designed with only one voltage input via a low voltage adapter, so it is not immediately suitable without modification. If you have 3 low voltage adapters, you have the size to consider (or if you are building everything into a robust weatherproof box, you could use 3 open transformers, which would take a little less space). One further point regarding accuracy: what is the voltage drop likely to be between the point where you measure the voltage and where the power is consumed? That will affect the accuracy too.

Atmel's application note (AVR465: Single-Phase Power/Energy Meter with Tamper Detection  ) gives a lot of valuable information.


Rasmus Fiil's picture

Re: 3 Phases AC monitoring

Hi Robert,

Thank you for the quick reply. I will try to be more specific in my description of how we want to do this. 

400 V goes into the distributor board with a 63 A cable and 400 V goes out with a similar cable or probably something smaller like 32 A. That output is then used by one vendor only and it is this cable that we want to monitor. The vendor may use both 3 phase and single phase. We are planning to use an SD card to log the energy consumption and we do not need to have real-time data. 

At the moment we are actually considering the possibility to measure the voltage directly and contain it in a double-insulated box with fuses as you suggest and run it through a certified "insulation test". We are using a regular Arduino Uno to handle the inputs. That should be possible with 6 analog ports (3 CT's and 3 AC-AC adapters). 

What do you mean about the voltage drop? The cables are probably about 50 meters or less. Our goal is to give an approximate idea of energy consumption so the accuracy should just be approximate.

You mention Open transformers to measure the voltage. Can you link or elaborate?


Robert Wall's picture

Re: 3 Phases AC monitoring

Thank you for clarifying.

I agree that with 50 m of cable, voltage drop - due to the resistance of the cable - will be small and because you do not need extreme accuracy, you can probably safely ignore it.  (It is 6,4 mV/A/m for 6mm2, so about 10 V, so the power error might be 10%. If needs be, you could program a correction for that in your software - but it would need to be set for each cable length!).

Measuring the voltage directly is no doubt the cheapest way. That is how I would do it. You clearly understand the dangers and it is not a way I would recommend otherwise.

By "open transformers" I meant ones without a plastic casing and a plug top - like http://uk.rs-online.com/web/p/pcb-mounting-transformers/0504486/  or http://uk.rs-online.com/web/p/chassis-mounting-transformers/0504767/

You need to beware of the very small transformers, e.g http://uk.farnell.com/myrra/44049/transformer-1va-6v/dp/1689041 because they are likely to have a lot of distortion in the voltage waveform, which might give too much error in your measurements. Also, there is no value shown for regulation, so the output voltage with no load is a guess. My bet is regulation is at least 25%, so the voltage is at least 7,5 V, and quite likely more, for the 6 V output version. You need to check a sample. Even the larger ones have a value for regulation of about 20%, so you must take that into account in your design.

You might also be interested in using the voltage monitor to power the Arduino. If you use the direct connection, the Atmel application note gives a cheap power supply. If you use transformers, then see here: http://openenergymonitor.blogspot.co.uk/2012/05/emontx-single-ac-power-s... (Though it might mean slightly larger transformers).


Rasmus Fiil's picture

Re: 3 Phases AC monitoring

Hello again,

We are now well away with our project and the first voltage measurement and SD card data logging have been made on a single phase. So far so good.

However we are not quite sure how to connect the adapters (we went with AC-AC adapters). The cable that we are measuring is a 63A cable with 16mm2 wires. Is it possible to use a power splitter or a terminal block to extract a normal 10A cable to connect with each adapter from the responding phases and neutral? Is there a better way to do this? 

In addition we have experienced that certified components an containers are really costly (100 pounds for a box!). If you have a cheaper alternative on where to get (or how to build) a double insulated box it would be very much appreciated. 


Best regards

Rasmus and the boys

Robert Wall's picture

Re: 3 Phases AC monitoring

I do not know how the law applies to your installation. If I was doing it in the UK, I would use an earthed metal box with 10 16mm^2 terminal blocks for incoming and outgoing cables (L1, L2, L3, N, E in and out) with 3 fuses for the low current supply to sockets for each adapter. I think you need to talk to the electrical engineer in charge of the venue.

Pcunha's picture

Re: 3 Phases AC monitoring

I am working on an auto calibration feature for EmonTX.

My scenario:

Assume that i have 3 cts for a 3-phase system connected to the EmonTX.

I Know that the phase order is correct 1,2,3m because i have installed it to the mains. They shoud be 120º apart one of each other.

The phase 1 is connected to the voltage sensor.

Possible scenarios:

Any phase order connected to EmonTX. Any phase connected to voltage sensor.
A 60w Incandescent light bulb connected to each phase for calibration purposes ( FP=1)


I have no idea on how to:

  • Identify phase order.
  • Identify phase connected to voltage sensor.

any algorithm ideas?


JBecker's picture

Re: 3 Phases AC monitoring

any algorithm ideas?

with pure resistive load (as you suggested) you could look for the zero crossings of the voltage and current signal. Something like:

- if ( (lastsampleV1<0) &&(newsampleV1>0) )

where 0 is the neutral value for the high-pass filtered values. Or use 512 for raw-values. The exact value does not matter so much if you can only make sure that it is crossed at all with the loads connected.

You can also store the index (= sample number) of the maximum (or minimum) value measured for each of the four waveforms and then find the phase order from the indeces.

current phase order will always be relative to the voltage phase which can be arbitrarily set as L1 (or is this a problem?)



Pcunha's picture

Re: 3 Phases AC monitoring

 Good idea.

I'll start with that and see what happens


khalidnk83's picture

Re: 3 Phases AC monitoring

Hello friends,

Really glad to see the worked done by openenergymonitor guys and some other folks posting their help on the same website or on their blogs - thanks to everyone participation in openenegry.

I have a scenario of 3 PTs and 3 CTs for three phase.

I want to measure each phase voltage and current (like using PT1 and CT1) seperately; you you guys please help me to start with ??


Look forward to hear from u folks soon.


Thanks and regards,



Robert Wall's picture

Re: 3 Phases AC monitoring

You can either use 3 emonTx's, connected together as MartinR did http://openenergymonitor.org/emon/node/1170 (although this uses the obsolete emonTx V2, you should be able to use the V3.4 though these do not stack), or you can build your own analogue front end with 3 voltage and 3 current inputs, using an Arduino as the processor, or you can look at Robin's offerings at http://mk2pvrouter.co.uk

hanguyen's picture

Re: 3 Phases AC monitoring

I have several time to make a 3 phase Energy monitor, I get the correct Vrms, Irms for each phase, but the wrong power Factor for each phase. Any help for power Factor is appreciate. when pf is 1, my meter is 0.78. when pf is 0,8 it is 0.35, when it is 0.6 , my meter is -0.64

Robert Wall's picture

Re: 3 Phases AC monitoring

I told you in a PM, follow the steps in Resources > Building Blocks > Calibration

If you cannot find the page from that, it is here: http://openenergymonitor.org/emon/buildingblocks/calibration.

If you cannot set the power factor to 1 with sensible value for the phase calibration, then you have a very unsatisfactory voltage transformer, or current transformer, or both.
Using the standard current transformer and ac adapter from the OEM on-line shop, a value of 1.7 for PHASECAL is normally satisfactory.

And don't post the same message twice.

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.