Control circuits - grounded or ungrounded?
According to DIN EN 60204-1/VDE 0113-1, ground faults in control circuits must not lead to unintended start-ups or hazardous machine motions and they must not prevent the machine from shutting down. Generally, control circuits are supplied via an isolating transformer with separate windings or a separate power supply unit, considering that the secondary voltage must not exceed 50 V.
The control circuit itself may be operated as a grounded system (TN system) or an ungrounded system (IT system). However, unknown high-resistance ground faults may cause controller faults, especially in the sensitive components of a control circuit.
In a grounded system, these faults are not detected by a fuse since the fault current is too low to trigger the fuse. If the ground fault has a low resistance, the fuse trips and the installation stops. To avoid this, control circuits are operated in ungrounded systems and monitored via an ground-fault monitoring device.