The Siemens 3RB1257-0KB00-0FM0 is a solid-state overload relay from the SIRIUS 3RB1 series, combining motor overload protection with integrated thermistor (PTC) monitoring and ground-fault detection. It is rated for a current range of 125 to 500 A and operates on a DC 24 V control supply. Ground-fault monitoring is achieved through the connection of an external summation current transformer; the relay provides separate signalling outputs: 1 NO/1 NC for overload/thermistor trip and 1 NO/1 NC for ground-fault alarm. The output contacts are monostable, meaning they return to their de-energised state when the fault clears or the supply is removed. The unit is designed for use with Kloeckner-Moeller ZWA500 summation CTs.
Siemens 3RB1257-0KB00-0FM0 Solid-State Overload Relay, 125-5
MPN3RB1257-0KB00-0FM0
Siemens SIRIUS 3RB1 solid-state overload relay, 3RB1257-0KB00-0FM0, 125-500 A, DC 24 V, integrated thermistor protection and ground-fault monitoring via external summation CT, 1 NO/1 NC contacts.
Sourced new & surplus through independent channelsAuthenticity-screened · ESD-safe packingListing updated Jul 2026
Product details
Frequently asked questions
What is the current range and control voltage for this overload relay?
The relay is rated for a current range of 125 to 500 A and operates on a DC 24 V control supply.
Does this relay include ground-fault monitoring?
Yes, it monitors ground faults through the connection of an external summation current transformer (Kloeckner-Moeller ZWA500) and provides a separate 1 NO/1 NC output for ground-fault alarm.
What type of thermistor protection does it offer?
It has integrated thermistor (PTC) protection that trips the overload/thermistor output (1 NO/1 NC) when the motor winding temperature exceeds the PTC threshold.
MPN
3RB1257-0KB00-0FM0