A Guide to ANSI Standard Device Numbers
Have you ever looked at a substation single-line diagram and felt like you
were reading a secret code? If a drawing is scattered with numbers like 50,
51, or 87, you are looking at the universal language of power system
protection: ANSI Standard Device Numbers (IEEE C37.2).
Instead of cluttering a schematic with long names like "Instantaneous
Over-current Relay," engineers use these standardized numbers. Whether you
are dealing with equipment from GE, Siemens, or SEL, this numerical language
remains exactly the same.
Here is a quick breakdown of how the system works and a few of the "VIPs"
you will encounter most often.
ANSI Standard Suffixes
| Suffixes | Description |
|---|---|
| _1 | Positive-Sequence |
| _2 | Negative-Sequence |
| A | Alarm, Auxiliary Power |
| AC | Alternating Current |
| AN | Anode |
| B | Bus, Battery, or Blower |
| BF | Breaker Failure |
| BK | Brake |
| BL | Block (Valve) |
| BP | Bypass |
| BT | Bus Tie |
| BU | Backup |
| C | Capacitor, Condenser, Compensator, Carrier Current, Case, or Compressor |
| CA | Cathode |
| CH | Check (Valve) |
| D | Discharge (Valve) |
| DC | Direct Current |
| DCB | Directional Comparison Blocking |
| DCUB | Directional Comparison Unblocking |
| DD | Disturbance Detector |
| DUTT | Direct Underreaching Transfer Trip |
| E | Exciter |
| F | Feeder, Field, Filament, Filter, or Fan |
| G | Ground or Generator |
| GC | Ground Check |
| H | Heater or Housing |
| L | Line or Logic |
| M | Motor or Metering |
| MOC | Mechanism Operated Contact |
| N | Neutral or Network |
| O | Over |
| P | Phase or Pump |
| PC | Phase Comparison |
| POTT | Pott: Permissive Overreaching Transfer Trip |
| PUTT | Putt: Permissive Underreaching Transfer Trip |
| R | Reactor, Rectifier, or Room |
| S | Synchronizing, Secondary, Strainer, Sump, or Suction (Valve) |
| SOTF | Switch On To Fault |
| T | Transformer or Thyratron |
| TD | Time Delay |
| TDC | Time-Delay Closing Contact |
| TDDO | Time Delayed Relay Coil Drop-Out |
| TDO | Time-Delay Opening Contact |
| TDPU | Time Delayed Relay Coil Pickup |
| THD | Total Harmonic Distortion |
| TH | Transformer (High-Voltage Side) |
| TL | Transformer (Low-Voltage Side) |
| TM | Telemeter |
| TT | Transformer (Tertiary-Voltage Side) |
| U | Under or Unit |
| X | Auxiliary |
| Z | Impedance |
Common Acronyms
| Acronyms | Description |
|---|---|
| AFD | Arc Flash Detector |
| CLK | Clock or Timing Source |
| CLP | Cold Load Pickup |
| DDR | Dynamic Disturbance Recorder |
| DFR | Digital Fault Recorder |
| ENV | Environmental Data |
| HIZ | High Impedance Fault Detector |
| HMI | Human Machine Interface |
| HST | Historian |
| MET | Substation Metering |
| PDC | Phasor Data Concentrator |
| PMU | Phasor Measurement Unit |
| PQM | Power Quality Monitor |
| RIO | Remote Input/Output Device |
| RTD | Resistance Temperature Detector |
| RTU | Remote Terminal Unit / Data Concentrator |
| SER | Sequence of Events Recorder |
| TCM | Trip Circuit Monitor |
| VTFF | Vt Fuse Fail |
ANSI Standard Device Numbers
| Device No. | Description |
|---|---|
| 1 | Master Element |
| 2 | Time Delay Starting or Closing Relay |
| 3 | Checking or Interlocking Relay |
| 4 | Master Contactor |
| 5 | Stopping Device |
| 6 | Starting Circuit Breaker |
| 7 | Rate of Change Relay |
| 8 | Control Power Disconnecting Device |
| 9 | Reversing Device |
| 10 | Unit Sequence Switch |
| 11 | Multifunction Device |
| 12 | Overspeed Device/Protection |
| 13 | Synchronous-Speed Device |
| 14 | Underspeed Device |
| 15 | Speed or Frequency Matching Device |
| 16 | Communication Networking Device |
| 17 | Shunting or Discharge Switch |
| 18 | Accelerating or Decelerating Device |
| 19 | Motor Starter / Starting-to-Running Transition Contactor |
| 20 | Electrically-Operated Valve |
| 21 | Distance Relay |
| 21G | Ground Distance |
| 21P | Phase Distance |
| 22 | Equalizer Circuit Breaker |
| 23 | Temperature Control Device |
| 24 | Volts-per-Hertz Relay / Overfluxing |
| 25 | Synchronizing or Synchronism-Check Device |
| 26 | Apparatus Thermal Device |
| 27 | Undervoltage Relay |
| 27P | Phase Undervoltage |
| 27TN | Third Harmonic Neutral Undervoltage |
| 27X | Auxiliary Undervoltage |
| 27 AUX | Undervoltage Auxiliary Input |
| 27/27X | Bus/Line Undervoltage |
| 28 | Flame Detector |
| 29 | Isolating Contactor |
| 30 | Annunciator Relay |
| 31 | Separate Excitation Device |
| 32 | Directional Power Relay |
| 32L | Low Forward Power |
| 32N | Wattmetric Zero-Sequence Directional |
| 32P | Directional Power |
| 32R | Reverse Power |
| 33 | Position Switch |
| 34 | Master Sequence Device |
| 35 | Brush-Operating or Slip-ring Short Circuiting Device |
| 36 | Polarity or Polarizing Voltage Device |
| 37 | Undercurrent or Underpower Relay |
| 37P | Underpower |
| 38 | Bearing Protective Device / Bearing Rtd |
| 39 | Mechanical Condition Monitor |
| 40 | Field Relay / Loss of Excitation |
| 41 | Field Circuit Breaker |
| 42 | Running Circuit Breaker |
| 43 | Manual Transfer or Selector Device |
| 44 | Unit Sequence Starting Relay |
| 45 | Atmospheric Condition Monitor |
| 46 | Reverse-Phase or Phase Balance Current Relay or Stator Current Unbalance |
| 47 | Phase-Sequence or Phase Balance Voltage Relay |
| 48 | Incomplete Sequence Relay / Blocked Rotor |
| 49 | Machine or Transformer Thermal Relay / Thermal Overload |
| 49RTD | RTD Biased Thermal Overload |
| 50 | Instantaneous Overcurrent Relay |
| 50BF | Breaker Failure |
| 50DD | Current Disturbance Detector |
| 50G | Ground Instantaneous Overcurrent |
| 50N | Neutral Instantaneous Overcurrent |
| 50P | Phase Instantaneous Overcurrent |
| 50_2 | Negative Sequence Instantaneous Overcurrent |
| 50/27 | Accidental Energization |
| 50/74 | Ct Trouble / Ct Supervision |
| 50/87 | Instantaneous Differential |
| 50EF | End Fault Protection |
| 50IG | Isolated Ground Instantaneous Overcurrent |
| 50LR | Acceleration Time |
| 50NBF | Neutral Instantaneous Breaker Failure |
| 50SG | Sensitive Ground Instantaneous Overcurrent |
| 50SP | Split Phase Instantaneous Current |
| 51 | Ac Time Overcurrent Relay |
| 51 | Overload |
| 51G | Ground Time Overcurrent |
| 51N | Neutral Time Overcurrent |
| 51P | Phase Time Overcurrent |
| 51V | Voltage Restrained Time Overcurrent |
| 51R | Locked / Stalled Rotor |
| 51_2 | Negative Sequence Time Overcurrent |
| 52 | Ac Circuit Breaker |
| 53 | Exciter or Dc Generator Relay |
| 54 | Turning Gear Engaging Device |
| 55 | Power Factor Relay |
| 56 | Field Application Relay |
| 57 | Short-Circuiting or Grounding Device |
| 58 | Rectification Failure Relay |
| 59 | Overvoltage Relay |
| 59B | Bank Phase Overvoltage |
| 59P | Phase Overvoltage |
| 59N | Neutral Overvoltage |
| 59NU | Neutral Voltage Unbalance |
| 59X | Auxiliary Overvoltage |
| 59_2 | Negative Sequence Overvoltage |
| 60 | Voltage or Current Balance Relay |
| 60N | Neutral Current Unbalance |
| 60P | Phase Current Unbalance |
| 61 | Density Switch or Sensor |
| 62 | Time-Delay Stopping or Opening Relay |
| 63 | Pressure Switch Detector |
| 64 | Ground Protective Relay |
| 64F | Field Ground Protection |
| 64S | Sub-harmonic Stator Ground Protection |
| 64TN | 100% Stator Ground |
| 65 | Governor |
| 66 | Notching or Jogging Device/Maximum Starting Rate/Starts Per Hour/Time Between Starts |
| 67 | Ac Directional Overcurrent Relay |
| 67G | Ground Directional Overcurrent |
| 67N | Neutral Directional Overcurrent |
| 67P | Phase Directional Overcurrent |
| 67SG | Sensitive Ground Directional Overcurrent |
| 67_2 | Negative Sequence Directional Overcurrent |
| 68 | Blocking Relay / Power Swing Blocking |
| 69 | Permissive Control Device |
| 70 | Rheostat |
| 71 | Liquid Switch |
| 72 | Dc Circuit Breaker |
| 73 | Load-Resistor Contactor |
| 74 | Alarm Relay |
| 75 | Position Changing Mechanism |
| 76 | Dc Overcurrent Relay |
| 77 | Telemetering Device |
| 78 | Phase Angle Measuring or Out-of-Step Protective Relay |
| 78V | Loss of Mains |
| 79 | Ac Reclosing Relay / Auto Reclose |
| 80 | Liquid or Gas Flow Relay |
| 81 | Frequency Relay |
| 81O | Over Frequency |
| 81R | Rate-of-Change Frequency |
| 81U | Under Frequency |
| 82 | Dc Reclosing Relay |
| 83 | Automatic Selective Control or Transfer Relay |
| 84 | Operating Mechanism |
| 85 | Carrier or Pilot-Wire Receiver Relay |
| 86 | Locking-Out Relay |
| 87 | Differential Protective Relay |
| 87B | Bus Differential |
| 87G | Generator Differential |
| 87GT | Generator/Transformer Differential |
| 87LG | Ground Line Current Differential |
| 87S | Stator Differential / Percent Differential |
| 87L | Segregated Line Current Differential |
| 87M | Motor Differential |
| 87O | Overall Differential |
| 87PC | Phase Comparison |
| 87RGF | Restricted Ground Fault |
| 87T | Transformer Differential |
| 87V | Voltage Differential |
| 88 | Auxiliary Motor or Motor Generator |
| 89 | Line Switch |
| 90 | Regulating Device |
| 91 | Voltage Directional Relay |
| 92 | Voltage And Power Directional Relay |
| 93 | Field-Changing Contactor |
| 94 | Tripping or Trip-Free Relay |
| 50/74 | CT Supervision |
| 27/50 | Accidental Generator Energization |
| 27TN/59N | 100% Stator Earth Fault |
Fluency in ANSI device numbers isn't just about reading schematics faster; it
is about
safety and effective troubleshooting. When a fault occurs in a substation, knowing instantly whether a 50
(instantaneous) or a 51 (time-delayed) relay triggered tells you a massive
amount about the nature and location of the fault.
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