The Mk5 is supplied with plus 63 volts and minus 63 volts from the battery. This is a floating supply centered by a resistor in the PD core.
This system means the installation is safer as there should only ever be 63 volts DC to earth.
The voltage is monitored in the PD core and an alarm is annunciated if the voltage of one leg with respect to earth drops to below the default figure of 34 volts.
This figure is adjustable in the IO Configurator.
If an alarm is shown, it indicates a field earth on one of the legs.
Even without an alarm, the balance should be checked regularly with a multi meter at the PD core and it is recommended that only 10V difference between legs is tolerated (68-58).
With two minor field earths, one on each leg, there could be a full short circuit across the DTBA boards in the MK5.
This has caused serious fires in the past. Earth faults on DC field wiring can be traced by systematically removing the berg jumpers on the DTBA boards and monitoring when the fault clears.
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