From HwB

This table shows name of protection devices used in substations and electrical plants. Number can be combined with letter to describe a function.

Device function

Function Description
7 Sudden Change In Phase/Residual Current Protection
21 Line Distance
21G Ground Distance
21P Phase Distance
24 Overexcitation (Volts per Hertz)
25 Synchrocheck
25A Automatic Synchronizing
26 Overload
27 Undervoltage
27N Ground Fault Undervoltage
27P Phase Undervoltage
27X Auxiliary Undervoltage
32 Power
37 Undercurrent
40 Loss Of Field
46 Phase Balance Current / Current Plausibility Check
46N Negative Sequence Overcurrent
47 Phase Sequence Voltage
47N Negative Sequence Voltage
49 Thermal
50 Instantaneous Overcurrent
50BF Breaker Failure
50DD Adaptive Fault Detector
50G Ground Instantaneous Overcurrent
50N Neutral Instantaneous Overcurrent
50P Phase Instantaneous Overcurrent
50_2 Negative Sequence Instantaneous Overcurrent
51 Time Overcurrent
51G Ground Time Overcurrent
51N Neutral Time Overcurrent
51P Phase Time Overcurrent
51_2 Negative Sequence Time Overcurrent
51/27C Voltage Controlled Time Overcurrent
51/27R Voltage Restrained Time Overcurrent
52 AC Circuit Breaker
59 Overvoltage
59N Neutral Overvoltage
59P Phase Overvoltage
59X Auxiliary Overvoltage
60 Voltage Balance
60FL Voltage Transformer Supervision
64F Field Ground
64G 100% Stator Ground Inputs
67 Ac Directional Overcurrent
67N Neutral Directional Overcurrent
67P Phase Directional Overcurrent
67_2 Negative Sequence Directional Overcurrent
68 Power Swing Blocking
74TC Trip Circuit / Control Circuit Supervision
78 Out-of-Step / Pole Slip
79 Automatic Recloser
81 Frequency
87 Differential Protective
87B Bus Differential Protective
87L Segregated Line Current Differential
87T Transformer Differential Protective
87N Low Impedance Differential Protection
87V Voltage Differential Protection
90 Transformer Control


  • IEEE C37.2-1991 (ANSI)