extending a signal cable - possible?

  1. johnser

    johnser Member

    Messages:
    1,658
    Location:
    North Cornwall
    Basically, I want to make a signal cable on a proportional controller's clamp meter longer.

    The proportional controller "senses" whether electricity is being exported from your house via a clamp meter and ramps up a resistive load (immersion heater) to stop excess generation (from solar) being exported to the grid. Before anyone suggests I should just "do my bit" and export over generated power, this is for a prospective installation, no FiT and making the most of the generated electricity is the difference between viability of it and not. The diverted power will displace hot water heated by heating oil.

    It's supposed to work at up to 5m cable length. I've already made the cable longer (~10m) with some ugly old speaker cable and that works fine. I'm hoping to extend the cable to almost exactly 50m. Fortunately I have a conduit already.

    The clamp meter cable is thin. 0.081 mm^2 / 28 AWG. It's two wires.

    I think it works by the clamp inducing a current and that current being sensed back at the other end of the device.

    The voltage between the wires is AC and it fluctuates between ~100-300mV
    The current appears to be proportional to the power being drawn and it's on the scale of 0-250μA (microAmps).

    So... having read a little about control signal cables, it suggests that voltage drop may well be an issue with these low mV scale signals but maybe it won't either, especially if it's sensing current at the other end and I can keep the signal together over the distance.

    I don't know enough about exactly how the device is working (nor do I really want to) to make a proper decision based on proper calculations/estimations or even to say that it's a stupid idea. So my only instinct is to try getting the next(ish) size sheilded cable up, wiring up and seeing if it works.

    something like this:
    https://uk.farnell.com/pro-power/cbbr4162/cable-microphone-2core-black/dp/3894680

    However, before I order something like that, anybody any thoughts as to if there would be anything else I could/should try to maximise my chances of success?

    The final issue may be that I try this longer cable and it appears to work but I accidentally introduce some horrible hysteresis into the system. I'll have to see if I can think of a way to test for this...

    If you've read all these waffelings, thanks so much for that alone!
     
    Anguz likes this.
  2. hotponyshoes Member

    Messages:
    846
    Location:
    Somerset. Uk
    I would think 2 core + shield microphone cabke would be ok, I have got a drum here if you are anywhere close and want some?

    I would remove the speaker cable and try and do it in one run.
    It would also be a good idea to take a reading then add the extension and check to see if the reading is changing by adding the longer cable. You might find you can remove the clamp and get it to zero/recalibrate itself after fitting the longer lead?
     
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  3. Farside

    Farside Badly Welded Man

    Messages:
    2,956
    Location:
    Ireland
    If it's a simple current-sensing transformer I would think it might still be readable at that distance by upping the cable gauge sufficiently. Just try it and see. It would be very sensitive to bad connections as time takes its toll, of course.
    If there's any digital signalling involved, the bits might get mushy and unreadable.
     
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  4. Hopefuldave Member

    Messages:
    666
    Location:
    The Shed f Danger, surrey, England
    Use a well-screened cable (double braid or braid over foil), avoid running it with other cables and if you have to get close to any cross at right angles to minimise pickup, should be good to go. Avoid Earthling both ends of the screening as that can lead to circulating earth currents.

    Dave H. (the other one)
     
    johnser likes this.
  5. johnser

    johnser Member

    Messages:
    1,658
    Location:
    North Cornwall
    Thanks for the suggestions.

    Yes, I'll definitely do it in one run. And can easily test it before I try feeding it underground. Connections at both ends will be indoors and well soldered/insulated.

    I don't expect that the cable I linked to is the best sheilded, it was cheap enough. I'll check to see what an extra sheilded version is.

    Unfortunately, it has to run in the conduit with the house supply cable; a big bit of swa on a 100a breaker. It's possible we ran a spare conduit will definitely check!
     
  6. johnser

    johnser Member

    Messages:
    1,658
    Location:
    North Cornwall
    I'm hoping there is nothing very complex involved. The clamp is tiny. No room for any electronics and no power source.
     
  7. garethp Forum Supporter

    Messages:
    772
    sw scotland
    Something rings a bell about using one the twisted pairs in a network cable for something like this, I think I asked the supplier when I fitted our solar diverter but ended up extending the power cable to the immersion instead and running it from a different consumer unit. Its a few years ago which is why I cant remember for sure.
     
    johnser likes this.
  8. johnser

    johnser Member

    Messages:
    1,658
    Location:
    North Cornwall
    Yes, I did reroute the power supply for an diversion controller on a different site previously. However the access to the immersion in this case is very awkward and routing a cable through the building isn't going to happen - so I'd need the wireless capability even after extending the sensor.

    I could use a twisted pair from a network cable, I think the conductor area would be ok but might be better to spend the money on more sheilded rather than redundant wires...

    Edit; just ordered a roll so will see what happens!

    Thanks all for the input once again.
     
  9. Farside

    Farside Badly Welded Man

    Messages:
    2,956
    Location:
    Ireland
    Totally off the wall idea...
    If there's too much loss, you could select a transformer with a greater output that would compensate for the distance, and hopefully bring the readings back into what the device expects to see. Or put two in series at the reading end and knock it into shape with a resistor.
     
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