Protective Relay Reference - IEEE C37.2 Standard
ANSI Device Numbers are standardized numerical codes (1-99) that identify the function of protective relays and control devices in electrical power systems. Defined by IEEE Standard C37.2, these numbers provide a universal language for engineers, technicians, and manufacturers.
A device number defines a function, not a physical device. Modern microprocessor-based relays (multifunction devices, ANSI 11) can perform dozens of protective functions in a single unit.
Master elements, time delays, interlocks
Distance, sync check, voltage, thermal
Field, phase balance, overcurrent, overvoltage
Balance, directional OC, reclosing
Frequency, lockout, differential
Regulating, trip-free, user-defined
Distance
Sync Check
Undervoltage
Inst. OC
Time OC
AC Breaker
Overvoltage
Dir. OC
Recloser
Frequency
Lockout
Differential
| Device # | Function Name | Description | Category |
|---|---|---|---|
| 1 | Master Element | Control switch that places equipment in/out of operation | Control |
| 2 | Time Delay Starting/Closing Relay | Provides time delay in switching sequence | Control |
| 3 | Checking or Interlocking Relay | Operates based on position of other devices | Control |
| 4 | Master Contactor | Main contactor controlled by Device 1 | Switching |
| 5 | Stopping Device | Shuts down equipment and holds out of operation | Control |
| 6 | Starting Circuit Breaker | Connects machine to starting voltage source | Switching |
| 7 | Rate of Change Relay | Operates when rate of change exceeds preset | Protection |
| 8 | Control Power Disconnecting Device | Disconnects control power | Switching |
| 9 | Reversing Device | Reverses motor field or phase sequence | Control |
| 10 | Unit Sequence Switch | Changes sequence of units in equipment | Control |
| 11 | Multifunction DeviceCOMMON | Device performing multiple functions (digital relays) | Protection |
| 12 | Overspeed Device | Operates when speed exceeds preset | Protection |
| 13 | Synchronous-Speed Device | Operates at synchronous speed | Control |
| 14 | Underspeed Device | Operates when speed falls below preset | Protection |
| 15 | Speed/Frequency Matching Device | Matches speed/frequency to system | Control |
| 16 | Data Communications Device | Serial/Ethernet communications (added 2008) | Auxiliary |
| 17 | Shunting or Discharge Switch | Opens/closes shunting circuit | Switching |
| 18 | Accelerating/Decelerating Device | Accelerates or decelerates machine | Control |
| 19 | Starting-to-Running Transition Contactor | Transfers from starting to running | Switching |
| 20 | Electrically Operated Valve | Solenoid or motor-operated valve | Auxiliary |
| 21 | Distance RelayCOMMON | Operates on impedance/admittance/reactance | Protection |
| 22 | Equalizer Circuit Breaker | Connects equalizer circuits | Switching |
| 23 | Temperature Control Device | Raises/lowers temperature | Control |
| 24 | Volts per Hertz Relay | Operates on V/Hz ratio (overexcitation) | Protection |
| 25 | Synchronizing/Sync Check RelayCOMMON | Checks synchronism before closing | Protection |
| 26 | Apparatus Thermal Device | Operates on apparatus temperature | Protection |
| 27 | Undervoltage RelayCOMMON | Operates when voltage falls below preset | Protection |
| 28 | Flame Detector | Monitors pilot or main flame | Protection |
| 29 | Isolating Contactor | Disconnects circuits for safety | Switching |
| 30 | Annunciator Relay | Gives alarm indication | Auxiliary |
| 31 | Separate Excitation Device | Connects to separate excitation | Control |
| 32 | Directional Power RelayCOMMON | Operates on power flow direction (reverse power) | Protection |
| 33 | Position Switch | Indicates position of device | Auxiliary |
| 34 | Master Sequence Device | Establishes operating sequence | Control |
| 35 | Brush-Operating Device | Raises/lowers brushes or short-circuits slip rings | Auxiliary |
| 36 | Polarity/Polarizing Voltage Device | Operates on polarity or voltage | Protection |
| 37 | Undercurrent/Underpower Relay | Operates when current/power falls below preset | Protection |
| 38 | Bearing Protective Device | Operates on bearing temperature/wear | Protection |
| 39 | Mechanical Condition Monitor | Monitors vibration, eccentricity | Protection |
| 40 | Field Relay (Loss of Excitation)COMMON | Operates on loss of field excitation | Protection |
| 41 | Field Circuit Breaker | Applies or removes field excitation | Switching |
| 42 | Running Circuit Breaker | Connects machine to running voltage | Switching |
| 43 | Manual Transfer/Selector DeviceCOMMON | Transfers control between sources | Control |
| 44 | Unit Sequence Starting Relay | Starts next unit in sequence | Control |
| 45 | Atmospheric Condition Monitor | Monitors humidity, etc. | Auxiliary |
| 46 | Reverse Phase/Phase Balance RelayCOMMON | Operates on negative sequence current | Protection |
| 47 | Phase Sequence Voltage Relay | Operates on negative sequence voltage | Protection |
| 48 | Incomplete Sequence Relay | Returns equipment if sequence incomplete | Protection |
| 49 | Machine/Transformer Thermal RelayCOMMON | Thermal overload protection | Protection |
| 50 | Instantaneous OvercurrentCOMMON | Operates instantly on overcurrent | Protection |
| 51 | AC Time Overcurrent RelayCOMMON | Time delay overcurrent (inverse time) | Protection |
| 52 | AC Circuit BreakerCOMMON | Closes and interrupts AC power circuit | Switching |
| 53 | Exciter/DC Generator Relay | Forces field excitation buildup | Protection |
| 54 | Reserved | Reserved for future application | Reserved |
| 55 | Power Factor Relay | Operates on power factor | Protection |
| 56 | Field Application Relay | Controls field excitation application | Control |
| 57 | Short-Circuiting/Grounding Device | Grounds circuit for safety | Switching |
| 58 | Rectification Failure Relay | Monitors rectifier failure | Protection |
| 59 | Overvoltage RelayCOMMON | Operates when voltage exceeds preset | Protection |
| 60 | Voltage/Current Balance Relay | Operates on voltage/current difference | Protection |
| 61 | Density Switch/Sensor | Operates on SF6 gas density | Protection |
| 62 | Time-Delay Stopping/Opening Relay | Time delay for stopping | Control |
| 63 | Pressure SwitchCOMMON | Operates on pressure (Buchholz) | Protection |
| 64 | Ground Detector RelayCOMMON | Detects ground faults | Protection |
| 65 | Governor | Controls speed/power | Control |
| 66 | Notching/Jogging Device | Allows specified operations | Control |
| 67 | AC Directional OvercurrentCOMMON | Directional overcurrent protection | Protection |
| 68 | Blocking/Out-of-Step Relay | Blocks tripping or out-of-step tripping | Protection |
| 69 | Permissive Control Device | Permits closing or startup | Control |
| 70 | Rheostat | Variable resistance | Auxiliary |
| 71 | Level Switch | Operates on liquid/gas level | Auxiliary |
| 72 | DC Circuit Breaker | DC power circuit breaker | Switching |
| 73 | Load-Resistor Contactor | Controls load limiting resistance | Switching |
| 74 | Alarm Relay | Indicates alarm condition | Auxiliary |
| 75 | Position Changing Mechanism | Moves OLTC to next position | Auxiliary |
| 76 | DC Overcurrent Relay | DC overcurrent protection | Protection |
| 77 | Telemetering Device | Telemetering transmitter/receiver | Metering |
| 78 | Phase-Angle Measuring Relay | Out-of-step detection | Protection |
| 79 | AC Reclosing RelayCOMMON | Controls automatic reclosing | Control |
| 80 | Flow Switch | Operates on flow | Auxiliary |
| 81 | Frequency RelayCOMMON | Over/underfrequency protection | Protection |
| 82 | DC Reclosing Relay | DC automatic reclosing | Control |
| 83 | Automatic Selective Control Relay | Selects between sources | Control |
| 84 | Operating Mechanism | Motor driving mechanism | Auxiliary |
| 85 | Carrier/Pilot-Wire Relay | Communications channel relay | Protection |
| 86 | Lockout RelayCOMMON | Hand-reset lockout relay | Protection |
| 87 | Differential Protective RelayCOMMON | Differential current protection | Protection |
| 88 | Auxiliary Motor/Generator | Operates auxiliary equipment | Auxiliary |
| 89 | Line Switch | Isolating/disconnecting switch | Switching |
| 90 | Regulating Device | Regulates to predetermined value | Control |
| 91 | Voltage Directional Relay | Directional voltage relay | Protection |
| 92 | Voltage & Power Directional Relay | Directional voltage/power relay | Protection |
| 93 | Field-Changing Contactor | Changes field excitation | Switching |
| 94 | Tripping/Trip-Free Relay | Trips breaker or prevents reclosing | Protection |
| 95-99 | User-Defined | Specific applications where 1-94 not suitable | Special |
Trips instantly when current exceeds pickup. Used for high-magnitude faults. Typical pickup: 6-10x FLA.
Trips with inverse time delay - higher currents trip faster. Allows coordination between devices.
Trips when voltage falls below pickup. Protects motors from reduced voltage operation. Set at 80-90% nominal.
Trips when voltage exceeds pickup. Protects insulation from damaging overvoltages. Set at 110% nominal.
Hand-reset relay that locks equipment out after a trip. Requires manual reset before re-energizing.
Compares current in vs out of protected zone. Any difference = internal fault. Fast and selective.
Measures impedance (Z=V/I) to detect faults. Multiple zones for coordination. Primary for transmission lines.
Auto-recloses breaker after fault. Most line faults are temporary. Typical: 1 fast + 2 delayed shots.
Over/underfrequency protection. 81U for load shedding (59.5-59.0 Hz). 81O for generator protection.
OC relay that only operates for faults in one direction. Essential for loop systems and parallel feeders.
51G = Ground OC | 87G = Generator differential
51N = Neutral OC | 59N = Neutral overvoltage
87T = Transformer diff | 86T = Transformer lockout
87B = Bus differential | 86B = Bus lockout
87M = Motor differential | 86M = Motor lockout
87L = Line differential | 21L = Line distance
50P = Phase inst. OC | 51P = Phase time OC
64F = Field ground | Also indicates fuse protection
X, Y, Z used for auxiliary relays in same circuit
51-1, 51-2 = Multiple same devices in equipment
50BF = Backup if breaker fails to trip
81U = Underfrequency | 81O = Overfrequency
Device numbers combined when multiple functions: 50/51 = Inst./Time OC, 27/59 = UV/OV
| Designation | Function | Application |
|---|---|---|
| 50/51 | Instantaneous/Time Overcurrent | Standard feeder protection |
| 50/51N | Inst./Time Overcurrent (Neutral) | Ground fault via neutral CT |
| 50/51G | Inst./Time Overcurrent (Ground) | Ground fault via ground CT |
| 27/59 | Undervoltage/Overvoltage | Voltage protection package |
| 50BF | Breaker Failure | Backup for failed breaker |
| 67/67N | Directional OC (Phase/Neutral) | Loop systems, parallel feeders |
| 81U/81O | Under/Over Frequency | Load shedding, generator |
| 87T | Transformer Differential | Primary transformer protection |
| 87G | Generator Differential | Primary generator protection |
| 86G/86T | Generator/Transformer Lockout | Lockout after fault trip |
| Symbol | Meaning | Example |
|---|---|---|
| a | Normally Open (NO) | 52a |
| b | Normally Closed (NC) | 52b |
| aa | NO, Maintained | 86aa |
| bb | NC, Maintained | 86bb |
| TC | Trip Coil | 52TC |
| CC | Close Coil | 52CC |
| CS | Control Switch | 52CS |
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