It is an IMD (insulation monitoring device), ohmic insulation relay, which is used to protect and monitor the insulation (kΩ, MΩ) between the phase/s of the receiver and the protection cable reference. It is not a permanent monitor (at rated voltage of the electrical installation). It is only used for loads disconnected from the voltage system. If it detects an installation fault, it prevents the connection of the network, anticipating a trip of the other protections through a circuit breaker with control coil.
Ideal for critical continuous service electrical installations: