Voltage drop problem not resolved with bigger cable

[TingleFingers]

Currently working on a job where the client was concerned about the swimming pool heat pump tripping the mcb (the mcb is more than adequate for the job).
The heat pump 4kw and pool pump 1.2kw were drawing 27/28 amps with only 197 volts measured at the pump room. It’s about a 60 meter run.
I replaced the 6mm cable run with 10 mm and the figures are still very much the same - I expected the voltage drop to be much less and therefore the amps to come right down, but no.
Any thoughts??

 

[Shoei]

What's the measured voltage at the load now? Current will only be reduced by increasing the voltage if there is power conversion at the load, like a switch mode power supply, for example. For regular loads like compressors increasing voltage will increase current while they're running.

 

[fairlight]

Is there a high resistance contact somewhere upstream of the cable, perhaps at the switchfuse at the supply end?

 

[DPG]

What figures did the calculations come out at?

 

[Andy-1960]

What is the voltage at the distribution board?
What is the MCB that is installed for the circuit?
What is the Zs at the load end?

 

[TingleFingers]

What's the measured voltage at the load now? Current will only be reduced by increasing the voltage if there is power conversion at the load, like a switch mode power supply, for example. For regular loads like compressors increasing voltage will increase current while they're running.

Have just done some more measurements, volts at main consumer unit only 218 (this is phase 3 of the 3 phase system (heat pump is a single phase)) the other two phases are measuring 230.
Measurements in the pump room - with pool pump only running 220v and 4.3 amps, with pool pump and heat pump it drops to 197v and current shoots up to 29amps. The instructions for the heat pump show normal working current to be 16amps.
Can’t help but think there is a problem with the heat pump as the client said it was always fine for the last two years.

 

[snowhead]

There's a bit of disjointed problem solving going on here.

The client said "it (the installation ) was always fine for the last two years."
So the "undersized cable" hasn't been an issue.
Presumably the MCB has also not been an issue.

Now the MCB is tripping.
What logic decided the cable was undersized and needed replacing?

Was the voltage at the Heat pump, at the MCB and at the Consumer unit ever measured before the cable change, both with and without load?

If so and the 218 /230 /230 was discovered was that difference ever investigated?

 

[Lucien Nunes]

The numbers don't compute. You've replaced the cable with 60m of 10mm^2 which has a VD of 4.4mV/A/m.

29 x 60 x 0.0044 = 7.6 volts drop
218-7.6=210V at the heat pump.
You measured 197V so at least one measurement seems to be in error, the supposed drop being nearly three times as high as expected. This may be due to waveform distortion - are you using true-RMS instruments to take these measurements? Is it possible that the voltage at the pump room is actually higher than your meter indicates?

As a separate deal, the behaviour of the heat pump will depend on whether the compressor is inverter driven. If so, the current may have a complex relationship to the voltage as the control system may adapt the maximum motor power to the available voltage, and simply run it for longer when the voltage is reduced. It might therefore also appear to the client to be working correctly as it performs to spec overall regarding temperature achieved, although the commissioning process ought to have pulled up any supply voltage issues.

Is there a high resistance contact somewhere upstream of the cable

If the excess drop were caused by a single bad joint, it would be dissipating 28 x 13 = 364W and would run very hot indeed and likely burn out. Although, if the voltage readings are correct, high resistance somewhere could account for the disparity in the readings, and also a change in behaviour that has started the MCB tripping, so I'll keep an open mind on this.

 

[TingleFingers]

There's a bit of disjointed problem solving going on here.

The client said "it (the installation ) was always fine for the last two years."
So the "undersized cable" hasn't been an issue.
Presumably the MCB has also not been an issue.

Now the MCB is tripping.
What logic decided the cable was undersized and needed replacing?

Was the voltage at the Heat pump, at the MCB and at the Consumer unit ever measured before the cable change, both with and without load?

If so and the 218 /230 /230 was discovered was that difference ever investigated?

I hear what you’re saying, I’m getting fed info from the client a bit piece meal. I replaced the cable as it should never have been on 6mm in the first place. I’m now uncovering more issues as I go along.
Prime example I’ve got 16mm coming into the fuses, 6mm from the fuses to the meter, then 16mm from the meter to the house.

There's a bit of disjointed problem solving going on here.

The client said "it (the installation ) was always fine for the last two years."
So the "undersized cable" hasn't been an issue.
Presumably the MCB has also not been an issue.

Now the MCB is tripping.
What logic decided the cable was undersized and needed replacing?

Was the voltage at the Heat pump, at the MCB and at the Consumer unit ever measured before the cable change, both with and without load?

If so and the 218 /230 /230 was discovered was that difference ever investigated?

There's a bit of disjointed problem solving going on here.

The client said "it (the installation ) was always fine for the last two years."
So the "undersized cable" hasn't been an issue.
Presumably the MCB has also not been an issue.

Now the MCB is tripping.
What logic decided the cable was undersized and needed replacing?

Was the voltage at the Heat pump, at the MCB and at the Consumer unit ever measured before the cable change, both with and without load?

If so and the 218 /230 /230 was discovered was that difference ever investigated?

i changed the cable as I don’t really know what “it’s been fine for the last two years” means. The online instructions for the heat pump state a 10mm cable is required for that distance of cable. I guess I’ve been a little remiss in not carrying out my own measurements from the get go.
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The numbers don't compute. You've replaced the cable with 60m of 10mm^2 which has a VD of 4.4mV/A/m.

29 x 60 x 0.0044 = 7.6 volts drop
218-7.6=210V at the heat pump.
You measured 197V so at least one measurement seems to be in error, the supposed drop being nearly three times as high as expected. This may be due to waveform distortion - are you using true-RMS instruments to take these measurements? Is it possible that the voltage at the pump room is actually higher than your meter indicates?

As a separate deal, the behaviour of the heat pump will depend on whether the compressor is inverter driven. If so, the current may have a complex relationship to the voltage as the control system may adapt the maximum motor power to the available voltage, and simply run it for longer when the voltage is reduced. It might therefore also appear to the client to be working correctly as it performs to spec overall regarding temperature achieved, although the commissioning process ought to have pulled up any supply voltage issues.



If the excess drop were caused by a single bad joint, it would be dissipating 28 x 13 = 364W and would run very hot indeed and likely burn out. Although, if the voltage readings are correct, high resistance somewhere could account for the disparity in the readings, and also a change in behaviour that has started the MCB tripping, so I'll keep an open mind on this.

Thanks...I’m having to use my trusty clampmeter today but I believe the figures to be correct at the source and destination. Like you say there could well be a resistance problem that I need to investigate.

 

[davesparks]

If the voltage at the incoming supply terminals is 218/230/230 then it sounds like there may be a supply problem.

Have you tried moving this onto a different phase temporarily to rule that out?

 

[Shoei]

Measurements in the pump room - with pool pump only running 220v and 4.3 amps, with pool pump and heat pump it drops to 197v and current shoots up to 29amps. The instructions for the heat pump show normal working current to be 16amps.

Agree with you, that points to a faulty heat pump. I'd measure volts and current at the heat pump alone and compare readings to the spec, sounds like it's taking an excessive amount of power.

 

[TingleFingers]

If the voltage at the incoming supply terminals is 218/230/230 then it sounds like there may be a supply problem.

Have you tried moving this onto a different phase temporarily to rule that out?

Next thing on my list!! Cheers.
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Agree with you, that points to a faulty heat pump. I'd measure volts and current at the heat pump alone and compare readings to the spec, sounds like it's taking an excessive amount of power.

Will do, it hasn’t been serviced in a couple of years either. I’ll check connections inside the heat pump in case there’s a problem there too.

 

[marconi]

If it is a conventional compressor motor using switched start winding and a run winding I suggest you check that the start winding is being switched off once the motor is running. You can ampclamp the start winding to find out. If both are energised it might explain you higher than 16A current draw. If it is not the starter module may be defective. Starter modules use a number of techniques which we can go in to if you find the module is defective.

The same thinking applies to the fan motor.

 

[marconi]

Is there an RCD 'up-front'?

 

[dmxtothemax]

Are you sure that the local grid has the power to power the heat pump ?.
It could simply be that the local supply grid is maxed out,
And therefore the power just aint there !
To know for sure, you need to measure the voltage at the main panel as well and compare.