Utility Relay Company AC-PRO-II, Instruction Manual

Page: 40 / 56

AC-PRO-II
®
Instruction Manual www.utilityrelay.com
Page 38
With I
2
T off or for currents greater than 10 X LT pick-up
setting, the ST trip time is a constant equal to the ST time
band setting.
With I
2
T on and for currents less than 10 X LT pick-up
setting, the ST trip time is determined by the following
equation:
T
= TBC
ST
X²
Where:
T = time to trip in seconds (center of the band)
X = current in multiples of the LT pick-up
TBC
ST
= the ST Time Band Constant
**** NOTE ****
The ST Time Band Constant (TBC
ST
) =
40 for the .40S Time Band
30 for the .30S Time Band
20 for the .20S Time Band
15 for the .15S Time Band
10 for the .10S Time Band
7 for the .07S Time Band
EXAMPLE #2:
CT Rating 1600A
LT pick-up 1200A
ST pick-up 6000A
ST delay .20S I²T ON
Overload Current 7200A
TBC
ST
= 20
and X = overload current = 7200A = 6
LT Pick-Up 1200A
therefore:
trip time = T = TBC
ST
or 20 = 20
X² 6² 36
= .556 seconds
**** IN SUMMARY ****
To calculate the ST I
2
T trip time:
1) Determine the ST Time Band Constant (TBC
ST
)
2) Calculate "X" where
X = overload current
LT Pick-Up
3) Solve the equation:
trip time(sec) = TBC
ST
X
2
18.2 Short Time (ST) Trip Time