Control circuits - earthed or unearthed?
According to DIN EN 60204-1/VDE 0113-1, earth 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 an earthed system (TN system) or an unearthed system (IT system). However, unknown high-resistance insulation faults may cause controller faults, especially in the sensitive components of a control circuit.
In an earthed system, these faults are not detected by a fuse since the fault current is too low to trigger the fuse. If the insulation fault has a low resistance, the fuse trips and the installation stops. To avoid this, control circuits are operated in unearthed systems and monitored via an insulation monitoring device.