Areva MiCom P120, Technical Manual, Page: 234 / 668

P12x/EN AP/A96 Application Guide
Page 12/80
MiCOM P120/P121/P122/P123
2.5 Reset timer
The first phase overcurrent threshold [I>/tI>] ([Ie>/tIe>] for the earth) has a reset timer.
The value that is set for this reset timer corresponds to the minimum time during which the
current value needs to be lower than 95% of the phase (or earth) threshold before the
corresponding phase (or earth) time delay is reset.
NOTE: This rule doesn’t apply when the protection triggers. When the
protection triggers, the time delay tI> (or tIe>) is immediately reset.
The value of this reset timer depends on the type of timer associated with the first phase
(Earth) threshold.
Reset Timer
Type of timer associated
with the first & second
phase (earth) threshold
P120, P121 P122, P123
DMT (see note below) 0 ms 0 ms to 600 s
Rectifier, IDMT IEC or RI 50 ms Setting range from 0 to 600 s
RXIDG - Setting range from 0 to 600 s
IDMT IEEE or CO 50 ms
Setting range from 0 to 600 s
or
Inverse Time
(Choice of 5 IEEE curves)
(*) first and second earth threshold only
2.5.1 Reset timer ( P122 & P123 only)
For the first phase and earth overcurrent stages, MiCOM P122 and P123 have a timer hold
facility "t Reset", which can be set to a definite time value or to an inverse time characteristic
(IEEE/ANSI curves only). This may be useful for some applications, for example when
grading with upstream electromechanical overcurrent relays which have inherent reset time
delays.
This timer hold facility used to reduce the time to clear a fault is also useful in situations
where intermittent faults occur. This may occur for example in a plastic insulated cable. In
this case, the fault energy may provocate the cable insulation to melt and reseal, thereby
extinguishing the fault. This process repeats itself a couple of times giving a succession of
fault current pulses, each one of increasing duration with reducing intervals between the
pulses, until the fault becomes permanent.
When the reset time of the overcurrent relay is minimum the relay will be repeatedly reset
and not be able to trip until the fault becomes permanent. By using the Timer Hold facility,
the relay will integrate the fault current pulses, thereby reducing fault clearance time.