Motor Protection Relay Symbol
Definition: The Motor Protection Relay symbol represents an electronic or electromechanical protective relay that monitors motor current via IN+ and IN- sensing terminals and trips the motor circuit by operating Trip NO (normally open) and Trip NC (normally closed) output contacts when it detects overcurrent, thermal overload, phase loss, or other fault conditions, per IEC 60255 and IEC 60947-4-1.
Also known as: thermal overload relay, electronic overload relay, motor protection unit, motor protective relay, overcurrent relay, MPR.
What the Motor Protection Relay symbol means
The Motor Protection Relay symbol in a wiring diagram indicates a dedicated protective relay device that continuously monitors the motor supply current and trips the motor circuit when a fault condition is detected. The relay receives motor current information through its IN+ and IN- current-sensing terminals (connected via a current transformer or direct series connection), and uses this to calculate thermal accumulation, detect phase imbalance, or identify locked-rotor conditions.
The symbol communicates that this device is distinct from a simple thermal overload relay: it is an intelligent relay with multiple protection functions, adjustable settings, and separate output contacts (Trip NO and Trip NC) used to trip the motor contactor coil circuit and provide fault indication. Motor protection relays are used in applications where a simple bimetal overload relay is insufficient due to high motor value or complex load characteristics.
How to identify the Motor Protection Relay symbol
The Motor Protection Relay symbol is drawn as a rectangle with four terminals: IN+ at the top left and IN- at the top right (current sensing inputs), and Trip NO at the bottom left and Trip NC at the bottom right (output contacts). The block is labelled 'Motor Protection', 'MPR', or 'Overload Relay'. The IN+ and IN- terminals represent the current measurement circuit; the Trip contacts are wired into the motor contactor control circuit to de-energise the contactor on trip.
Function in a circuit
The Motor Protection Relay measures the motor phase currents through IN+ and IN- terminals and applies protection algorithms to detect: thermal overload (simulated bimetal thermal model), phase loss or phase unbalance (>10% current asymmetry), locked-rotor (sustained high current at start), ground fault (residual current), thermistor overtemperature (if PTC input available), and stall or undercurrent conditions. On detection of a fault, the relay's Trip NO contact closes (activating an alarm or indication) and the Trip NC contact opens (de-energising the motor contactor to stop the motor). The relay is manually or automatically resettable after a defined cooling period.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60255-8 defines thermal electrical relays; IEC 60255-149 covers thermal protection for electrical machines. IEC 60947-4-1 governs overload relays as part of motor starter combinations. The motor protection relay symbol follows IEC 60617 general relay block conventions. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE C37.2 defines protective relay function numbers; a thermal overload relay corresponds to ANSI device number 49 (machine or transformer thermal relay) and overcurrent protection to device number 51. ANSI Y32.2 / IEEE 315-1975 uses a rectangle with labelled current input and contact output terminals. |
| Key difference | IEC 60255 / IEC 60617 labels the sensing terminals IN+/IN- and the output contacts Trip NO/NC. ANSI/IEEE C37.2 uses function numbers (49, 51) on the relay block; current inputs may be labelled I1/I2 or CT1/CT2 in ANSI practice. Both use a rectangular block symbol. |
Terminals / pins
| Pin | Name |
|---|---|
| in_p | IN+ |
| in_n | IN- |
| trip_no | Trip NO |
| trip_nc | Trip NC |
Typical values
Current setting range: 0.1–100 A (adjustable); trip class: Class 10, 20, or 30 (IEC 60947-4-1); thermal memory retention: 30–60 minutes (simulates motor thermal state); phase unbalance threshold: typically 10–40% asymmetry (adjustable); output contact rating: 250 V AC, 3–5 A.
Where the Motor Protection Relay symbol is used
- High-value motor installations: electronic motor protection relays protect expensive compressor and pump motors where simple bimetal relays are insufficient
- Phase-loss sensitive applications: motor protection relays with phase unbalance detection protect motors in areas with unstable supply quality
- Process pumps in chemical plants: motor protection relays with undercurrent detection identify pump dry-running conditions before cavitation damage occurs
- Multi-function motor starters: combined with contactors in smart motor control centres, motor protection relays provide all protective functions in one device
- HVAC chiller motor protection: motor protection relays monitor chiller compressor motors for overload, phase loss, and stall conditions
- Mining and heavy industry conveyor drives: motor protection relays protect high-inertia conveyor motor drives with adjustable Class 20/30 trip characteristics
Example
In a 37 kW pump motor control circuit, the Motor Protection Relay symbol (IN+ and IN- connected in series with the motor supply conductors; Trip NC contact wired in series with contactor coil K1) is set to Class 10 tripping at the motor FLA of 72 A. When the pump jams and the motor draws locked-rotor current (7× FLA = 504 A), the relay's Trip NC contact opens within 10 seconds, de-energising K1, dropping the motor, and closing the Trip NO contact to illuminate a fault indicator on the panel.
Key facts
- The Motor Protection Relay symbol represents an electronic or electromechanical relay that monitors motor current and trips the motor contactor on overload, phase loss, or other fault conditions per IEC 60255 and IEC 60947-4-1.
- The four standard terminals are IN+ and IN- (current sensing inputs, top) and Trip NO and Trip NC (output contacts, bottom).
- The Trip NC (normally closed) contact is wired in series with the motor contactor coil: it opens on trip, de-energising the contactor and stopping the motor.
- The Trip NO (normally open) contact closes on trip and is typically used to activate an alarm indicator, PLC input, or remote monitoring signal.
- IEC device classification: motor protection relays with thermal and overcurrent functions correspond to ANSI/IEEE C37.2 device numbers 49 (thermal) and 51 (overcurrent).
- Electronic motor protection relays (e.g. Siemens 3RB, ABB EF series) simulate a thermal model of the motor, maintaining thermal memory even through power cycles to prevent restarting before the motor has cooled.
- Trip class per IEC 60947-4-1: Class 10 trips within 10 s at 7.2× rated current; Class 20 within 20 s; Class 30 within 30 s — choose based on motor starting time.
Frequently asked questions
What does the motor protection relay symbol mean in a wiring diagram?
The motor protection relay symbol means the diagram includes a protective relay that continuously monitors motor current and opens the motor circuit when it detects overcurrent, thermal overload, phase loss, or other fault conditions. The Trip NC contact in the relay de-energises the contactor to stop the motor.
What does the motor protection relay symbol look like?
The motor protection relay symbol is a rectangle with IN+ and IN- current sensing terminals at the top and Trip NO (normally open) and Trip NC (normally closed) contact terminals at the bottom. It is labelled 'MPR', 'Overload Relay', or with the device product number.
What is the difference between a motor protection relay and a thermal overload relay?
A thermal overload relay uses a bimetal strip to trip on sustained overcurrent; it provides basic overload protection only. A motor protection relay is electronic, simulates a thermal model, and adds phase-loss, phase-unbalance, ground-fault, and locked-rotor protection functions in a single adjustable device.
What do Trip NO and Trip NC mean on a motor protection relay?
Trip NC (normally closed) is the contact that is closed during normal operation and opens when the relay trips, de-energising the motor contactor coil. Trip NO (normally open) is closed only when the relay trips, used to trigger an alarm or fault indication signal.
What IEC standard covers motor protection relays?
Motor protection relays are governed by IEC 60255-8 (thermal electrical relays) and IEC 60947-4-1 (overload relays as part of motor starters). ANSI/IEEE C37.2 device numbers 49 (thermal protection) and 51 (overcurrent) correspond to motor protection relay functions.
How do you select the right trip class for a motor protection relay?
Trip class selection is based on motor starting time: Class 10 suits motors that start within 3–4 seconds (typical for pumps and fans); Class 20 suits higher-inertia loads starting in 10–15 seconds (conveyor drives); Class 30 suits very high inertia loads. The trip class must allow the motor to reach running speed without a nuisance trip.
What is the ANSI device number for a motor protection relay?
ANSI/IEEE C37.2 assigns device number 49 to machine thermal relays (overtemperature protection) and device number 51 to overcurrent relays (time-overcurrent protection). A combined motor protection relay may be labelled 49/51 in ANSI drawings.
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