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| Sample Application in Generator Protection (Photo Credit: General Electic) |
In power system, the protection and control of equipment is represented by ANSI device numbers, with corresponding suffix letters when necessary, in relation to the functions they perform. The numbers are based on a system that is adopted by a standard for automatic switchgear by Institute of Electrical and Electronics Engineers (IEEE), and incorporated in American Standard C37.2-1996.
This system is used along with the diagrams that can be found in instruction books and in specifications. The International Electrotechnical Commission (IEC) standards 617 and 60617 also provide different symbols and terminology for most of the device numbers that are defined by C37.2.
The second portion of this document provides a brief overview of a few of the more common IEC symbols used.
Read: Related Articles in Fault Analysis
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 - Under-speed 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
27 TN - Phase Undervoltage
27 X - Third Harmonic Neutral Undervoltage
27 AUX - Undervoltage Auxiliary Input
27/ 27 X - Bus/Line Undervoltage
28 - Flame Detector
29 - Isolating Contactor
30 - Annunciator Relay
31 - Separate Excitation Device
32 - Directional Power Relay
32 L - Low Forward Power
32 N - Wattmetric Zero-Sequence Directiona
32 P - Directional Power
32 R - 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
49 - RTD RTD Biased Thermal Overload
Read: What are the Conditions in Selecting Current Transformer in Protective Relaying
50 - Instantaneous Overcurrent Relay 50BF Breaker Failure
50 - DD 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
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
59P - Phase Overvoltage
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
87S - 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 2
7/50 - Accidental Generator Energization
27TN/59N - 100% Stator Earth Fault
