37
thermal stress value, I
2
x t, until all three phase currents have fallen below the set
pickup value. When the calculated value exceeds the reference value, (48/14
PICKUP)
2
x (48/14 TDLY), the element will generate a trip signal. The element will
be reset in 240 ms after all three phase currents have fallen below the set pickup
value of the element.
The operate time is calculated as below. However, the shortest possible operate time
of element (48/14 PICKUP)
2
x (48/14 TDLY) is ~300 ms.
t s
PICKUP
TDLY
I
⎡⎣ ⎤⎦
×
=
(
/
)
(
/
)
48 14
2
48 14
2
(1)
t
= operate time
48/14 PICKUP
= set start-up current of motor
48/14 TDLY
= set start-up time of motor
I
= phase current value
An advantage of start-up supervision based on thermal stress calculation is that the
operate time will be automatically extended during low-voltage conditions as it
depends on the start-up current of the motor.
Element (48/14 PICKUP)
2
x (48/14 TDLY) cannot be used
concurrently with element 48/14.
Start-up supervision with speed switch
In case the safe stall time is shorter than the start-up time of the motor stated by the
motor manufacturer, as with motors of ExE-type, for instance, a speed switch on the
motor shaft is required to give information on whether the motor is accelerating
during motor start up. The speed switch should be open at standstill and closed
during acceleration.
Elements 48/14 and (48/14 PICKUP)
2
x (48/14 TDLY) will be blocked on
activation of the speed switch input.
5.1.4.4.
Short-circuit protection
The non-directional short-circuit protection detects overcurrent caused by
interwinding, phase-to-phase and phase-to-ground short circuits.
When one or several phase currents exceed the set pickup value of element 50P, the
element will generate a pickup signal after a ~ 50 ms
’
pickup time. When the set
operate time at definite-time characteristic elapses, the element will generate a trip
signal. The high-set overcurrent element can be given an instantaneous
Motor Protection Relay
Technical Reference Manual - ANSI Version
REM 610
REM 610
1MRS755537
Summary of Contents for REM 610
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