Technical Specifications

FAQ

1. What is the main function of the ABB CM-MPS.21 monitoring relay?

The ABB CM-MPS.21 is a multifunctional three-phase monitoring relay designed to continuously supervise important electrical parameters in three-phase AC systems. It can detect phase sequence errors, phase failure, overvoltage, undervoltage, phase unbalance and interrupted neutral conditions. The relay uses a True RMS measuring principle to evaluate the monitored voltage and provides relay outputs that can be connected to contactors, alarms, controllers or other protection circuits. Because several monitoring functions are integrated into one device, the CM-MPS.21 can be used as a compact protection and monitoring solution in industrial control cabinets and electrical distribution systems. The monitored thresholds can be adjusted from the front panel, allowing the relay to be adapted to different operating conditions. When an abnormal voltage or phase condition is detected, the output contacts change state after the configured delay, allowing the associated equipment to be disconnected or an alarm to be generated.

2. What voltage range does the CM-MPS.21 monitor?

The CM-MPS.21 is designed for monitoring a three-phase AC system in the 180–280 V AC range. The measuring voltage is also used as the control supply, so the relay is powered directly from the monitored circuit rather than requiring a separate auxiliary power supply. This arrangement simplifies installation in many industrial applications. The relay continuously evaluates the monitored voltage and compares it with the configured overvoltage and undervoltage thresholds. When the voltage moves outside the permitted range for longer than the configured delay, the output relay changes state. Before installation, the actual system voltage should be checked to ensure that it falls within the specified measuring range. Incorrect voltage selection can result in improper operation or damage to the monitoring relay. The phase connections should also be made according to the recommended wiring arrangement to ensure that phase sequence, phase failure and unbalance monitoring operate correctly.

3. Which electrical faults can the CM-MPS.21 detect?

The CM-MPS.21 can detect several common three-phase power-system abnormalities. Its monitoring functions include phase sequence, phase failure, overvoltage, undervoltage, phase unbalance and interrupted neutral detection. Phase sequence monitoring can identify incorrect phase rotation, which is particularly useful for applications involving three-phase motors where incorrect rotation could cause undesirable mechanical operation. Phase failure detection identifies the loss of one phase, while overvoltage and undervoltage monitoring protect connected equipment from sustained abnormal voltage conditions. The phase unbalance function monitors differences between the three phase voltages and can be configured according to the required protection level. The CM-MPS.21 also includes interrupted neutral monitoring, which is an important feature for detecting neutral conductor problems in suitable systems. These functions allow one monitoring relay to supervise several electrical conditions simultaneously instead of requiring separate relays for every individual fault.

4. Can the CM-MPS.21 be used to monitor a single-phase power supply?

Yes. The CM-MPS.21 can also be used for single-phase monitoring when configured correctly. For single-phase operation, the three external conductor inputs are connected together and used as a common monitored conductor. Phase sequence monitoring must be disabled because there is no three-phase rotation to evaluate in a single-phase system. The phase unbalance threshold also needs to be configured appropriately, with the recommended maximum setting used for this operating mode. Correct wiring and configuration are essential because the relay is primarily designed as a multifunctional three-phase monitoring device. The single-phase application should therefore be implemented according to the appropriate connection diagram and configuration requirements. When converting the device from a three-phase application to a single-phase application, technicians should verify the wiring and function-selection settings before energizing the circuit. Incorrect configuration can result in false fault indications or prevent the intended monitoring function from operating correctly.

5. How does the adjustable tripping delay work on the CM-MPS.21?

The CM-MPS.21 provides an adjustable tripping delay that allows temporary electrical disturbances to be distinguished from sustained faults. The delay can be set to 0 seconds or adjusted from approximately 0.1 to 30 seconds using the front-panel adjustment. Depending on the selected function, the relay can operate with an ON-delay or OFF-delay response. This is useful in industrial applications because voltage fluctuations can occur during motor starting, switching operations or other transient conditions. Without an appropriate delay, a short-duration voltage disturbance could cause unnecessary relay operation and potentially interrupt the controlled equipment. Conversely, setting the delay too long could allow a genuine electrical fault to remain present for an undesirable period. The correct delay should therefore be selected according to the characteristics of the monitored electrical system and the response requirements of the connected equipment. During commissioning, the actual system behavior should be observed to ensure that the selected delay provides adequate protection without causing unnecessary trips.

6. What do the three LEDs on the CM-MPS.21 indicate?

The CM-MPS.21 uses three front-panel LEDs to provide a convenient indication of the relay’s operating and fault status. The indicators allow technicians to determine whether the device is operating normally and whether a monitored electrical abnormality has been detected. The relay-status indicator provides information about the operating or timing condition, while the fault indicators are associated with detected monitoring faults. These visual indicators are particularly useful during commissioning because they provide immediate information without requiring additional diagnostic equipment. However, the LEDs should not be considered a replacement for electrical measurements when troubleshooting. If a fault LED is illuminated, the technician should identify which monitoring condition has been triggered and then measure the relevant phase voltages to determine whether the fault is actually present in the electrical system. Checking the LED state together with measured phase voltage, phase sequence and relay output status provides a more reliable method of determining whether the problem is caused by the monitored power system or by the monitoring relay itself.

7. Why might the CM-MPS.21 repeatedly trip even when the power system appears normal?

Repeated unwanted tripping can have several causes. The most common possibilities include incorrectly adjusted overvoltage or undervoltage thresholds, excessive phase unbalance, an inappropriate tripping delay, incorrect phase wiring or short-duration voltage disturbances that are not obvious during a basic voltage check. Loose terminals or damaged wiring can also cause intermittent voltage loss on one phase, resulting in repeated phase-failure detection. If the system is operating close to the configured voltage threshold, normal variations may be interpreted as an abnormal condition. The first troubleshooting step should be to measure all three phase voltages under normal operating conditions and compare them with the relay settings. The phase sequence should also be verified if phase-sequence monitoring is enabled. Next, inspect the wiring and terminals for loose connections or signs of overheating. If the electrical system is confirmed to be stable, the relay’s threshold and timing settings should be reviewed. An appropriately selected delay can prevent brief transients from causing unnecessary relay operation.

8. What should be checked when replacing an ABB CM-MPS.21?

Before replacing a CM-MPS.21, the technician should first determine whether the relay itself has failed or whether an external electrical fault is causing the abnormal indication. The three-phase supply, phase sequence, phase voltages, neutral connection and wiring should be checked first. The existing threshold settings and tripping-delay settings should also be recorded before removing the original device so that the replacement can be configured consistently. The correct model and order number should be verified, particularly when working with older installations. For this product, the specified model is CM-MPS.21 and the order number is 1SVR630885R3300. During installation, the phase conductors and neutral connection should be connected to the appropriate terminals, and the relay should be securely mounted on the DIN rail. After installation, the operating voltage, phase sequence, monitoring thresholds and relay output behavior should be tested. Where possible, controlled fault simulations should be used to confirm phase-failure, voltage-limit and unbalance detection. The connected contactor, alarm circuit or controller should also be checked to ensure that the relay output is correctly interpreted by the rest of the control system.