 1 - Master Element  32P - Directional Power

 2 - Time-delay Starting or Closing Relay  32R - Reverse Power

 3 - Checking or Interlocking Relay  33 - Position Switch
 4 - Master Contactor  34 - Master Sequence Device
 5 - Stopping Device  35 - Brush-Operating or Slip-ring Short
 6 - Starting Circuit Breaker Circuiting Device
 7 - Rate of Change Relay  36 - Polarity or Polarizing Voltage Device
 8 - Control Power Disconnecting Device  37 - Undercurrent or Underpower Relay
 9 - Reversing Device  37P - Underpower
 10 - Unit Sequence Switch  38 - Bearing Protective Device / Bearing
 11 - Multifunction Device Rtd
 12 - Overspeed Device  39 - Mechanical Condition Monitor
 13 - Synchronous-Speed Device  40 - Field Relay / Loss of Excitation
 14 - Underspeed Device  41 - Field Circuit Breaker
 15 - Speed or Frequency Matching Device  42 - Running Circuit Breaker
 16 - Data Communications Device  43 - Manual Transfer or Selector Device
 17 - Shunting or Discharge Switch  44 - Unit Sequence Starting Relay
 18 - Accelerating or Decelerating Device  45 - Atmospheric Condition Monitor
 19 - Starting-to-Running Transition  46 - Reverse-Phase or Phase Balance
Contactor Current Relay or Stator Current
 20 - Electrically-Operated Valve Unbalance
 21 - Distance Relay  47 - Phase-Sequence or Phase Balance
 21G - Ground Distance Voltage Relay
 21P - Phase Distance  48 - Incomplete Sequence Relay / Blocked
Rotor
 22 – Equalizer circuit breaker
 49 - Machine or Transformer Thermal
 23 – Temperature control device
Relay / Thermal Overload
 24 – Volts per hertz relay
 49RTD - RTD Biased Thermal Overload
 25 – Synchronizing or synchronism-check
 50 - Instantaneous Overcurrent Relay
device
 50BF - Breaker Failure
 26 – Apparatus thermal device
 50DD - Current Disturbance Detector
 27 – Undervoltage relay
 50EF - End Fault Protection
 27P - Phase Undervoltage
 50G - Ground Instantaneous Overcurrent
 27S - DC undervoltage relay
 50IG - Isolated Ground Instantaneous
 27TN - Third Harmonic Neutral
Overcurrent
Undervoltage
 50LR - Acceleration Time
 27TN/59N - 100% Stator Earth Fault
 50N - Neutral Instantaneous Overcurrent
 27X - Auxiliary Undervoltage
 50NBF - Neutral Instantaneous Breaker
 27 AUX - Undervoltage Auxiliary Input
Failure
 27/27X - Bus/Line Undervoltage
 50P - Phase Instantaneous Overcurrent
 27/50 - Accidental Generator Energization
 50SG - Sensitive Ground Instantaneous
 28 - Flame Detector
Overcurrent
 29 - Isolating Contactor
 50SP - Split Phase Instantaneous Current
 30 - Annunciator Relay
 50Q - Negative Sequence Instantaneous
 31 - Separate Excitation Device Overcurrent
 32 - Directional Power Relay  50/27 - Accidental Energization
 32L - Low Forward Power  50/51 - Instantaneous / Time-delay
 32N - Wattmetric Zero-Sequence Overcurrent relay
Directional
 50Ns/51Ns - Sensitive earth-fault  64S - Sub-harmonic Stator Ground
protection Protection
 50/74 - Ct Trouble  64TN - 100% Stator Ground
 50/87 - Instantaneous Differential  65 - Governor
 51 - AC Time Overcurrent Relay  66 - Notching or Jogging
 51 - Overload Device/Maximum Starting Rate/Starts Per
 51G - Ground Time Overcurrent Hour/Time Between Starts
 51LR - AC inverse time overcurrent  67 - AC Directional Overcurrent Relay
(locked rotor) protection relay  67G - Ground Directional Overcurrent
 51N - Neutral Time Overcurrent  67N - Neutral Directional Overcurrent
 51P - Phase Time Overcurrent  67Ns – Earth fault directional
 51R - Locked / Stalled Rotor  67P - Phase Directional Overcurrent
 51V - Voltage Restrained Time  67SG - Sensitive Ground Directional
Overcurrent Overcurrent
 51Q - Negative Sequence Time  67Q - Negative Sequence Directional
Overcurrent Overcurrent
 52 – AC circuit breaker  68 - Blocking Relay / Power Swing
 52a - AC circuit breaker position (contact Blocking
open when circuit breaker open)  69 - Permissive Control Device
 52b - AC circuit breaker position (contact  70 - Rheostat
closed when circuit breaker open)  71 - Liquid Switch
 53 - Exciter or Dc Generator Relay  72 - DC Circuit Breaker
 54 - Turning Gear Engaging Device  73 - Load-Resistor Contactor
 55 - Power Factor Relay  74 - Alarm Relay
 56 - Field Application Relay  75 - Position Changing Mechanism
 57 - Short-Circuiting or Grounding Device  76 - DC Overcurrent Relay
 58 - Rectification Failure Relay  77 - Telemetering Device
 59 - Overvoltage Relay  78 - Phase Angle Measuring or Out-of-
 59B - Bank Phase Overvoltage Step Protective Relay
 59P - Phase Overvoltage  78V - Loss of Mains
 59N - Neutral Overvoltage  79 - AC Reclosing Relay / Auto Reclose
 59NU - Neutral Voltage Unbalance  80 - Liquid or Gas Flow Relay
 59P - Phase Overvoltage  81 - Frequency Relay
 59X - Auxiliary Overvoltage  81O - Over Frequency
 59Q - Negative Sequence Overvoltage  81R - Rate-of-Change Frequency
 60 – Voltage or current balance relay  81U - Under Frequency
 60 - Voltage or Current Balance Relay  82 - DC Reclosing Relay
 60N - Neutral Current Unbalance  83 - Automatic Selective Control or
 60P - Phase Current Unbalance Transfer Relay
 61 - Density Switch or Sensor  84 - Operating Mechanism
 62 - Time-Delay Stopping or Opening  85 - Pilot Communications, Carrier or
Relay Pilot-Wire Relay
 63 - Pressure Switch Detector  86 - Lock-Out Relay, Master Trip Relay
 64 - Ground Protective Relay  87 - Differential Protective Relay
 64F - Field Ground Protection  87B - Bus Differential
 64R – Rotor earth fault  87G - Generator Differential
 64REF – Restricted earth fault differential  87GT - Generator/Transformer
 64S – Stator earth fault Differential
 87L - Segregated Line Current Differential  SER – Sequence of Events Recorder
 87LG - Ground Line Current Differential  TCM – Trip Circuit Monitor
 87M - Motor Differential  LRSS – Local/Remote selector switch
 87O - Overall Differential  VTFF - Vt Fuse Fail
 87PC - Phase Comparison Suffixes Description
 87RGF - Restricted Ground Fault
 87S - Stator Differential  _1 - Positive-Sequence
 87S - Percent Differential  _2 - Negative-Sequence
 87T - Transformer Differential  A - Alarm, Auxiliary Power
 87V - Voltage Differential  AC - Alternating Current
 88 - Auxiliary Motor or Motor Generator  AN - Anode
 89 - Line Switch  B - Bus, Battery, or Blower
 90 - Regulating Device  BF - Breaker Failure
 91 - Voltage Directional Relay  BK - Brake
 92 - Voltage And Power Directional Relay  BL - Block (Valve)
 93 - Field-Changing Contactor  BP - Bypass
 94 - Tripping or Trip-Free Relay  BT - Bus Tie
 95 – For specific applications where other  BU - Backup
numbers are not suitable  C - Capacitor, Condenser, Compensator,
 96 – For specific applications where other Carrier Current, Case or Compressor
numbers are not suitable  CA - Cathode
 97 – For specific applications where other  CH - Check (Valve)
numbers are not suitable  D - Discharge (Valve)
 98 – For specific applications where other  DC - Direct Current
numbers are not suitable
 DCB - Directional Comparison Blocking
 99 – For specific applications where other
 DCUB - Directional Comparison
numbers are not suitable
Unblocking
Acronyms Description  DD - Disturbance Detector
 DUTT - Direct Underreaching Transfer Trip
 AFD - Arc Flash Detector  E - Exciter
 CLK - Clock or Timing Source  F - Feeder, Field, Filament, Filter, or Fan
 CLP - Cold Load Pickup  G - Ground or Generator
 DDR – Dynamic Disturbance Recorder  GC - Ground Check
 DFR – Digital Fault Recorder  H - Heater or Housing
 DME – Disturbance Monitor Equipment  L - Line or Logic
 ENV – Environmental data  M - Motor or Metering
 HIZ – High Impedance Fault Detector  MOC - Mechanism Operated Contact
 HMI – Human Machine Interface  N - Neutral or Network
 HST – Historian  O - Over
 LGC – Scheme Logic  P - Phase or Pump
 MET – Substation Metering  PC - Phase Comparison
 PDC – Phasor Data Concentrator  POTT - Pott: Permissive Overreaching
 PMU – Phasor Measurement Unit Transfer Trip
 PQM – Power Quality Monitor  PUTT - Putt: Permissive Underreaching
 RIO – Remote Input/Output Device Transfer Trip
 RTD - Resistance Temperature Detector  R - Reactor, Rectifier, or Room
 RTU – Remote Terminal Unit/Data
Concentrator
 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