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 Device No.
Definition and Function
21
Distance relay functions when the circuit admittance, impedance, or reactance
increases or decreases beyond predetermined limits.
25
Synchronizing or synchronism-check relay functions when two alternating current
sources become electrically separated or displaced by an unacceptable phase
angle.
27
Undervoltage relay functions on a given value of undervoltage.
32
Directional power relay functions on a desired value of power flow in a given
direction, or upon reverse power resulting from arc back in the anode or cathode
circuits of a power rectifier.
37
Undercurrent or underpower relay functions when the current or power flow
decreases below a predetermined value.
40
Field relay functions on a given, or abnormally low, value or failure of machine
field current, or on an excessive value of the reactive component of armature
current in an AC machine indicating abnormally low field excitation.
46
Reverse-phase or phase-balance current relay functions when the polyphase
currents are of reverse-phase sequence, or when the polyphase currents are
unbalanced or contain negative phase-sequence components above a given
amount.
47
Phase-sequence voltage relay functions upon a predetermined value of polyphase
voltage in the desired phase sequence.
48
Incomplete sequence relay generally returns equipment to the normal (off)
position and locks it out if the normal starting, operating, or stopping sequence is
not properly completed within a predetermined time. If the device is used for
alarm purposes only, it should preferably be designated as 48A (alarm).
49
Machine or transformer thermal relay functions when the temperature of a
machine armature, or other load carrying winding or element of a machine, or the
temperature of a power rectifier or power transformer (including a power rectifier
transformer) exceeds a predetermined value.
50
Instantaneous overcurrent or rate-of-rise relay functions instantaneously on an
excessive value of current, or on an excessive rate of current rise. This indicates a
fault in the apparatus or circuit being protected.
4-15
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