Why Choose the ABB CM-MSS.31P Thermistor Motor Protection Relay?
The ABB CM-MSS.31P (1SVR740712R1400) is a multifunctional thermistor motor protection relay designed to monitor electric motor winding temperatures and help protect industrial motors against overheating. Part of ABB's CM-MSS measuring and monitoring relay range, it evaluates PTC thermistor sensors installed in motor windings and provides an output signal when an excessive temperature condition is detected.
Operating from a wide-range 24–240 V AC/DC control supply, the CM-MSS.31P provides one PTC thermistor measuring circuit, one normally open (NO) contact and one normally closed (NC) contact. Unlike models with two changeover contacts, this version has separate NO and NC outputs, making accurate contact selection important when designing the control circuit.
The relay incorporates sensor short-circuit detection, dynamic interrupted-wire detection, non-volatile fault storage and configurable automatic, manual or remote reset. ABB's Easy Connect push-in terminal technology, a combined test/reset button and compact 22.5 mm DIN rail housing make the CM-MSS.31P particularly suitable for industrial motor control centres, machinery panels and automation cabinets.
Key Advantages
- Direct motor winding temperature monitoring using PTC thermistor sensors.
- Helps protect motors against overheating caused by overload, insufficient cooling and abnormal operating conditions.
- Wide-range 24–240 V AC/DC supply for flexible industrial installation.
- One PTC sensor measuring circuit for motor thermal protection.
- One normally open and one normally closed output contact for control and fault signalling.
- 250 V / 4 A contact rating for suitable control applications.
- Sensor short-circuit monitoring helps detect defective sensor wiring.
- Dynamic interrupted-wire detection provides additional supervision of the PTC measuring circuit.
- Non-volatile fault storage retains fault information through supply interruptions.
- Selectable automatic, manual or remote reset after a fault clears.
- Combined test/reset button supports commissioning and maintenance.
- Closed-circuit operating principle for fault-responsive switching.
- Easy Connect push-in terminals simplify electrical installation.
- LED indication helps distinguish different fault conditions.
- 22.5 mm DIN rail housing saves control cabinet space.
- Designed according to IEC 60947-8 for thermistor motor protection applications.
Key Features
- ABB CM-MSS.31P thermistor motor protection relay.
- Manufacturer order code: 1SVR740712R1400.
- Rated control supply voltage: 24–240 V AC/DC.
- Monitoring function: Motor winding temperature protection.
- Sensor type: PTC thermistors.
- Sensor circuits: 1.
- Sensor short-circuit detection: Integrated.
- Interrupted-wire monitoring: Dynamic detection.
- Fault memory: Non-volatile storage.
- Reset options: Automatic, manual or remote.
- Local operation: Combined test/reset button.
- Operating principle: Closed-circuit principle.
- Output configuration: 1 NO + 1 NC contact.
- Contact rating: 250 V / 4 A.
- Terminal type: Easy Connect push-in terminals.
- Overvoltage-protected supply and measuring inputs.
- LED indication for operating and fault status.
- Housing width: 22.5 mm.
- Tool-free DIN rail mounting and removal.
- UL 94 V-0 flame-retardant housing material.
- Optional sealable transparent protective cover.
Technical Specifications
- Manufacturer: ABB
- Product series: CM-MSS
- Model: CM-MSS.31P
- ABB order code: 1SVR740712R1400
- ABB product group: 310921-Measuring & Monitoring Relays
- Product type: Thermistor motor protection relay
- Rated control supply voltage: 24–240 V AC/DC
- Supply voltage tolerance: −15% to +10%
- Rated AC supply frequency: 15–400 Hz
- Monitoring function: Motor winding temperature monitoring
- Sensor type: PTC thermistors
- Number of sensor circuits: 1
- Sensor short-circuit detection: Yes
- Dynamic interrupted-wire detection: Yes
- Fault storage: Non-volatile
- Reset modes: Automatic, manual or remote
- Local test/reset: Combined front-mounted button
- Operating principle: Closed-circuit principle
- Output contacts: 1 normally open (NO) + 1 normally closed (NC)
- Contact rating: 250 V / 4 A
- Maximum UL/CSA output operating voltage: 300 V AC
- Power failure buffering time: 20 ms
- Supply and measuring circuit isolation: Electrically isolated
- Input protection: Overvoltage-protected supply and measuring inputs
- Terminal type: Easy Connect push-in terminals
- Connection technology: Screwless spring-clamp connection
- Applicable product standard: IEC 60947-8
- Housing flammability classification: UL 94 V-0
- Mounting type: DIN rail mounting
- Operating ambient temperature: −25 to +60 °C
- Storage temperature: −40 to +85 °C
- Housing width: 22.5 mm
- Customs tariff number: 85364900
Advanced Thermistor-Based Motor Temperature Protection
The ABB CM-MSS.31P provides direct thermal monitoring by evaluating the resistance of PTC thermistor sensors embedded in electric motor windings. As the winding temperature approaches the thermistor's rated response temperature, sensor resistance rises sharply. The relay detects this increase and changes its output state to indicate an overheating condition.
Because PTC thermistors respond to the motor's actual temperature, this monitoring method can identify thermal stress associated with mechanical overload, repeated starting, inadequate ventilation and elevated ambient temperature. It complements conventional current-based motor protection by supervising the thermal condition of the motor itself.
The CM-MSS.31P operates according to the closed-circuit principle, supporting fault-responsive switching in a correctly designed control system. Its separate normally open and normally closed contacts can be incorporated into suitable contactor control, PLC monitoring and alarm circuits.
The relay does not directly interrupt the motor's main power conductors. It provides control contacts that must be connected to appropriately rated external switching and protective equipment.
Sensor Circuit Diagnostics and Non-Volatile Fault Storage
A key advantage of the CM-MSS.31P is its integrated monitoring of the thermistor measuring circuit. In addition to detecting excessive motor temperature, it provides sensor short-circuit monitoring and dynamic interrupted-wire detection.
These diagnostic functions help distinguish motor overheating from faults in the sensor wiring. This is particularly useful in industrial installations where damaged cables, loose connections or defective sensors could compromise temperature monitoring.
The relay also incorporates non-volatile fault storage, allowing fault information to be retained when the control supply is interrupted. This supports maintenance investigations and helps avoid losing evidence of a detected fault following a temporary loss of power.
LED indicators provide visual information about the relay's operating state and detected fault conditions, helping maintenance personnel identify whether further investigation of the motor or its sensor circuit is required.
Flexible Reset and Test Functions
The ABB CM-MSS.31P supports automatic, manual and remote reset after the relevant fault condition has been removed. These options allow the relay to be configured for different industrial operating requirements.
Automatic reset allows the device to return to its normal operating state after the motor has cooled and the monitored sensor condition is acceptable. Manual reset requires deliberate operator intervention, while remote reset supports integration with an external control arrangement.
The combined test/reset button provides convenient local access for checking relay functionality and resetting eligible fault conditions. The exact reset behaviour depends on the selected operating mode and the status of the monitored PTC sensor circuit.
Where unexpected motor restarting could create a hazard, the complete motor control system must include suitable restart prevention and safety interlocking. The relay's reset function alone does not constitute a complete machinery safety system.
Easy Connect Push-In Terminals and Compact Installation
The CM-MSS.31P uses ABB's Easy Connect Technology, featuring screwless push-in terminals designed for secure, maintenance-free electrical connections when correctly installed.
The push-in connection system simplifies control cabinet wiring and reduces the need for conventional screw terminal tightening. It is particularly useful for OEM machinery, production equipment and industrial automation installations where efficient panel assembly is important.
Its compact 22.5 mm housing supports space-efficient DIN rail installation. The device also features overvoltage-protected supply and measuring inputs, electrical isolation between the supply and measuring circuits, and housing material classified UL 94 V-0.
Typical Applications
- Motor winding temperature protection in industrial machinery.
- Thermal monitoring of motors equipped with PTC thermistors.
- Motor control centres and electrical control cabinets.
- Pump motor overheating protection.
- Fan and ventilation motor thermal supervision.
- Compressor motor protection.
- Conveyor and material handling machinery.
- Manufacturing and processing equipment.
- Industrial motors subject to frequent starting and stopping.
- Applications requiring sensor short-circuit and interrupted-wire diagnostics.
- PLC-based motor temperature fault signalling.
- Motor control systems requiring non-volatile fault storage.
- OEM machinery with wide-range AC/DC control supplies.
- Industrial installations requiring local or remote reset.
- Electrical panels requiring compact push-in terminal monitoring relays.
