Definition and Function
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.
Power factor relay operates when the power factor in an AC circuit rises above or
below a predetermined value.
Field application relay automatically controls the application of the field
excitation to an AC motor at some predetermined point in the slip cycle.
Overvoltage relay functions on a given value of overvoltage.
Voltage or current balance relay operates on a given difference in voltage, or
current input, or output of two circuits.
Liquid, gas pressure, or vacuum relay operates on given values or on a given rate
of change of pressure.
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
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.
AC directional overcurrent relay functions on a desired value of AC overcurrent
flowing in a predetermined direction.
Frequency relay functions on a predetermined value of frequency. Either under,
over, or on normal system frequency (or rate of change of frequency).
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.
Locking-out relay shuts down, or withholds from service, certain equipment, if
abnormal conditions persist. It can be reset electronically (remote) or manually.
Differential protective relay functions on a percentage, phase angle, or other
quantitative difference of two currents or other electrical quantities.