Are your DC systems grounded?

Most DC systems in vehicles and UPSs are connected to ground (earth) on one pole. Automobiles in the U.S. ground negative. This eliminates high static floating voltages and minimizes arcing when device insulation begins to fail. It also allows a device to be fed with a single wire and use the chassis as a return.

Since an electric substation control system is considered an emergency system, it is not grounded. It is better to burn up a device than to trip off a circuit and disable a tripping system. Instead, a ground detector is installed. Two relatively high ohm resistors are connected in series across the DC system. The tap between the two resistors is grounded. Voltage is measured by a relay across each resistor. With no inadvertent grounds on the DC system, voltage across each resistor will be equal and about 1/2 the battery voltage. Should one pole be grounded, one resistor will see zero voltage and the other full battery voltage, sending an alarm.

Typical control ckts of DC system (non-automobile application) has no grounding-but the panel has a annunciation system for monitoring +bus/- bus grounding indications and some system have a control pots also. The idea of keeping the system floating is to provide a redundancy and allow the operation engineers time to locate and repair any ground fault -as if there was a -/+ bus already grounded system then any other phase grounding will trip the entire control ckt and will result in total shutdown of systems.

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