Phase Overcurrent Protection - GE Agile P50 Series Technical Manual

6 Protection Functions
Another possible situation where the timer hold facility may be used to reduce fault clearance times is
for intermittent faults. An example of this may occur in a plastic insulated cable. In this application it is
possible for the fault energy to melt and reseal the cable insulation, thereby extinguishing the fault.
This process repeats to give a succession of fault current pulses, each of increasing duration with
reducing intervals between the pulses, until the fault becomes permanent.
When the reset time is instantaneous, the device will repeatedly reset and not be able to trip until the
fault becomes permanent. By using the Timer Hold facility the device will integrate the fault current
pulses, thereby reducing fault clearance time.
The timer hold facility is available to all three stages of OC and EF functions.
The Definite Time Reset characteristic is applicable for IEC curves / DT.
The value of the Reset Timer depends on the type of the timer associated to the pick-up phase (Earth)
threshold.
Type of timer associated with phase (earth)
- For DT time delay
- For IDMT IEC time delay
- For IDMT IEEE or CO time delay
The mathematical formula applicable to the five curves is:
t = RTMS x K
--------------------
1- (I / Is)
Where:
t =
K =
I =
Is =
α
=
RTMS
Description
Moderate Inverse
Very Inverse
Extremely Inverse
US Inverse
US Short Time Inverse
2.2

Phase Overcurrent Protection

Phase current faults are faults where fault current flows between two or more phases of a three-phase
power system. The fault current may be between the phase conductors only or, between two or more
phase conductors and earth. There are three types of phase fault:
• Line to Line (accounting for approximately 8% of all faults)
6-6
threshold
α
Reset time
Factor (see table)
Value of the measured current
Value of the programmed threshold (pick-up value)
Factor (see table)
Reset time multiplier (RTMS) setting is between 0.025 and 1.2
Standard
IEEE
IEEE
IEEE
CO8
CO2
Reset Timer
DT Reset Characteristic
0 – 100s
0 – 100s
0 – 100s
α
K
4.85 2
21.6 2
29.1 2
5.95 2
2.261 2
P50 Agile P153
IDMT Reset Characteristic
Not available
Not available
Based on RTMS value
(0.025-1.2)
P153/EN M/B