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Discuss 110V DC Float Earth Ground Fault in the Electrical Engineering Chat area at ElectriciansForums.net

bcm_spark

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Arms
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Good morning all,

A 'ground fault' alarm came in over the weekend on the 110V charging system. We have two 110V fans currently running and I am suspecting that one is at fault but I can't turn it off yet to fault find. I say I suspect this as we have had to change them before because of low insulation resistance to earth. The motors are on a 3 wire DC system.

The measured voltage to earth on the running motors is
A - Earth 76V
AZ - Earth -44V

The output voltage is currently around 121-123V DC.

Now the strange thing for me is that when i turned on the 110V DC lights that run off the same batteries, the fault clears.

I then measured the voltage again at the motors and got the following

A - Earth -44V
AZ - Earth 76V

I am not 100% clued up on DC floating earth systems. If anyone can help educate me I'd appreciate it.

Ben
 
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DPG

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Esteemed
Advent Win
If its a floating earth then surely the DC voltages cannot be measured to earth, so you will get meaningless results? I may have misunderstood the system though.
 

Lucien Nunes

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Mentor
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Esteemed
Floating in the sense that neither wire is solidly earthed, but there is a high impedance connection to earth through the monitoring system. The resistance is low enough that the meter will give a reasonably correct indication of voltage to earth.

It actually sounds like the leakage fault is on the switched line to the lights. With lights switched off, the leakage is pulling the midpoint towards one pole via the lights, when switched on it is pulled an equal amount to the other pole via the switch, hence the voltages to earth swap over.
 

bcm_spark

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Arms
  • Thread Starter Thread Starter
  • #4
Floating in the sense that neither wire is solidly earthed, but there is a high impedance connection to earth through the monitoring system. The resistance is low enough that the meter will give a reasonably correct indication of voltage to earth.

It actually sounds like the leakage fault is on the switched line to the lights. With lights switched off, the leakage is pulling the midpoint towards one pole via the lights, when switched on it is pulled an equal amount to the other pole via the switch, hence the voltages to earth swap over.
Thanks for this!! I should have realised when turning the lights on that this was the issue. Having so many problems with carbon build up from brushes in the past and other issues I went straight to the motors.

Disconnected one of the 110V lighting circuits and the alarm is completely cleared :thumbsup:
 
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