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Thread: Mains power meters

  1. #1
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    Default Mains power meters

    Hi.
    I have some non invasive clamp on power meters that I use to read mains current. I wanted to put one on each ring mains in my house so I can see localised power consumption.

    Something I don't understand is, if I put a clamp meter on single ring mains what stops me sensing the current on all the other ring mains for the entire house? Will I sense current beyond the MCB isolating that ring?

  2. #2
    Moderator toscal's Avatar
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    Interesting.
    I know when you are testing a single ring circuit you are supposed to turn off the others, to avoid for want of a better word cross contamination. One way would be to put in a din rail mounted energy meter after the circuit breaker for the ring, then clamp on the output side of this. Or use the blinking led to measure power consumption.
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    Automated Home Legend paulockenden's Avatar
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    You only sense the current passing through the clamp or CT.

    Think of it a bit like measuring water flowing through a pipe and it might make more sense.

    Ring mains make things complicated.
    Last edited by paulockenden; 21st June 2019 at 02:09 PM.

  4. #4
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    Quote Originally Posted by paulockenden View Post
    You only sense the current passing through the clamp or CT.

    Things of it a bit like measuring water flowing through a pipe and it might make more sense.
    Ah! This explains why you only receive an appliances power usage as the power cable is basically a spur.

    Thanks for the help.

  5. #5
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    Something I was also looking into recently. Unfortunately you can't really just take the current output from the clamp and multiple it by your mains voltage to get power. This will give you a badly inaccurate figure. Simply because you are dealing with AC power. Nor infact can you even sample the current from the clamp and report the current being drawn. it doesn't work that way.

    To get true RMS current you need to sample the current waveform from the clamp many times per cycle, so, for example, assuming 50Hz mains, you might want to sample it at 500Hz or 1Khz. You need to take the RMS, Root, mean, square of those values. That will give you RMS current. It should be noted if you just add average the current over the waveform you will get 0 amps as half the current is negative. The squaring at the end ensures an ABS value, ie. positive.

    Then if you want a power value in watts it gets even more complicated as the voltage is also moving and here's the catch... it will not necessarily be "in phase" with the current. If you have any capacitive or inductive loads, such as motors (inductive) or high power devices with capacitors the current and voltage will be shifted out of phase. So you can't assume the voltage waveform from the current waveform. Smart meters which do this with just a current clamp are inaccurate and are assuming the voltage and current are in phase and that your mains voltage is exactly 240V RMS when it probably won't be. In the UK it can be anything from 230V to 250V.

    Thus, you need to measure the mains voltage as well. It also needs to be sampled and RMS'd. Only then can you get an accurate reading. Note, simply RMS'ing the current and assuming the voltage is in phase and guessing it to be 240V RMS, you might get within 10%.

    Measuring the mains voltage from a micro-controller or raspberry pi is not that easy either. You will need to step it down to something safe using a AC/AC wall adapter power supply, for example, that gives you maybe 9V AC. You need to buy one that has a good datasheet or somehow workout the phase shift caused by it's transformer and account for that shift in your calculations.

    All this said, I decided to not bother attempting to make all this myself and instead am looking at buying one of these:
    https://meters.co.uk/products/single-phase/

    It has a MODBus interface which is fairly easy to integrate with via an RS485 to serial adapter and an arduino/esp or raspberry pi.

    If you hunt around I'm sure you will find plug in variants with some form of hackable interface.
    Last edited by paulca; 29th July 2019 at 04:18 PM.

  6. #6
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    Also, just a quick, but VERY important warning about current clamps.

    NEVER EVER put the clamp around a live wire with it unloaded. Without a burden resistor on a current clamp (transformer) the voltage across the transformer ends can reach many thousands of volts and in the best case fry whatever device you then connect to it and in the worse case give you a nasty shock.

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