1SVR730830R0500 Voltage Monitoring Relay With DPDT Contacts, 1 Phase, Overvoltage. Depending on the configuration, the voltage monitoring relays CM-ESS.1 and CM-ESS.2 can be used for over- or undervoltage monitoring in single-phase AC and/or DC systems. The voltage to be monitored (measured value) is applied to terminals B-C. The devices work according the open-circuit principle.