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 Device No.
Definition and Function
51
AC time overcurrent relay has either a definite or inverse time characteristic that
functions when the current in an AC circuit exceeds a predetermined value.
55
Power factor relay operates when the power factor in an AC circuit rises above or
below a predetermined value.
56
Field application relay automatically controls the application of the field
excitation to an AC motor at some predetermined point in the slip cycle.
59
Overvoltage relay functions on a given value of overvoltage.
60
Voltage or current balance relay operates on a given difference in voltage, or
current input, or output of two circuits.
63
Liquid, gas pressure, or vacuum relay operates on given values or on a given rate
of change of pressure.
64
Ground protective relay functions on failure of the insulation of a machine,
transformer or of other apparatus to ground, or on flashover of a DC machine to
ground.
NOTE: This function is assigned only to a relay that detects the flow of current
from the frame of a machine or enclosing case or structure of a piece of apparatus
to ground, or detects a ground on a normally ungrounded winding or circuit.
It is not applied to a device connected in the secondary circuit or secondary
neutral of a current transformer, or in the secondary neutral of current
transformers connected in the power circuit of a normally grounded system.
67
AC directional overcurrent relay functions on a desired value of AC overcurrent
flowing in a predetermined direction.
81
Frequency relay functions on a predetermined value of frequency. Either under,
over, or on normal system frequency (or rate of change of frequency).
85
Carrier or pilot-wire receiver relay is operated or restrained by a signal used in
connection with carrier-current or DC pilot-wire fault directional relaying.
86
Locking-out relay shuts down, or withholds from service, certain equipment, if
abnormal conditions persist. It can be reset electronically (remote) or manually.
87
Differential protective relay functions on a percentage, phase angle, or other
quantitative difference of two currents or other electrical quantities.
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