Protective Relaying by Equipment

The protective functions commonly applied to transformers, motors, generators, buses, feeders and capacitor banks, and the IEEE guide behind each.

87T	Differential — internal faults, with harmonic restraint for inrush.
50/51	Phase overcurrent on the primary as backup and through-fault protection.
51G / 50N/51N	Ground overcurrent on the neutral or residual, per winding grounding.
63 / 63SPR	Sudden pressure or gas (Buchholz) relay for liquid-filled units.
49 / 26	Winding or oil temperature.
24	Volts per hertz where overexcitation is possible (generator step-up units).

Small distribution transformers are commonly protected by fuses or overcurrent alone; differential is typical from roughly 5 to 10 MVA upward.

49 / 51	Thermal overload model and locked-rotor or long-start protection.
50	Instantaneous phase overcurrent for short circuits (not with contactor-only interruption beyond its rating).
50G / 51G	Ground fault, often from a zero-sequence (window) CT.
46	Current unbalance or negative sequence, including single phasing.
27 / 59	Under- and overvoltage.
66 / 48	Starts per hour and incomplete sequence.
37	Undercurrent or underpower, for pumps running dry or broken couplings.
38 / 49R	Bearing and stator RTD temperatures on larger machines.
87M	Differential for large or critical motors.

Low-voltage motors usually rely on the starter overload relay and the branch circuit device; the full list applies to medium-voltage motors.

87G	Stator differential.
64G / 59N / 27TN	Stator ground fault; 59N covers most of the winding, third-harmonic 27TN extends coverage to the neutral end.
40	Loss of field (excitation).
32	Reverse power, protecting the prime mover from motoring.
46	Negative-sequence current heating the rotor.
51V / 21	Voltage-restrained overcurrent or distance as backup for system faults.
24	Overexcitation (volts per hertz).
81O/U	Over- and underfrequency.
59 / 27	Over- and undervoltage.
78	Out-of-step for large machines on strong systems.

Emergency and standby sets under NFPA 110 often use a reduced set with alarms instead of trips for some functions.

87B	Bus differential (low- or high-impedance).
50/51 partial differential	Summed source currents as a lower-cost alternative.
50BF	Breaker failure, clearing the bus when a feeder breaker fails to open.
Zone interlocking	Fast bus tripping by blocking signals from feeder relays.

Arc-flash reduction often drives faster bus protection (differential, zone interlocking or maintenance switch).

50/51	Phase overcurrent, coordinated with downstream devices.
50N/51N or 50G/51G	Ground overcurrent.
67 / 67N	Directional overcurrent in looped or multi-source systems.
79	Automatic reclosing on overhead lines.
21	Distance protection on transmission and sub-transmission lines.
87L	Line current differential over a communication channel.

Radial industrial feeders usually need only 50/51 and ground protection; directional and pilot schemes appear with multiple sources.

50/51	Bank overcurrent for major faults.
60 / 59N unbalance	Unbalance detection for failed units or elements.
59 / 27	Overvoltage, and undervoltage to trip before re-energizing a charged bank.

Fuses (external or internal) protect individual capacitor units.

Sources: IEEE C37.2 (device numbers) and the IEEE C37 application guides named on each card — described in our own words. © RokBench.