Utility Relay Company AC-PRO-II, Instruction Manual

Page: 42 / 56

AC-PRO-II
®
Instruction Manual www.utilityrelay.com
Page 40
With the GF slope set to OFF, the GF trip time is a constant
equal to the GF Time Band setting.
With the GF slope set to I
2
T:
For GF currents greater than 0.6 times the CT rating, the
GF trip time is a constant equal to the GF Time Band
setting.
For GF currents less than 0.6 times the CT rating, the GF
trip time is determined by the equations below.
With the GF slope set to I
5
T:
For GF currents greater than 4.0 times the GF Pickup, the
GF trip time is a constant equal to the GF Time Band
setting.
For GF currents less than 4.0 times the GF Pickup, the GF
trip time is determined by the equations below:
for I
2
T OR for I
5
T
Where:
T = time to trip in seconds (center of the band)
X
GF
= ground fault current
CT rating
X5
GF
= ground fault current
GF Pickup
TB2C
GF
= the GF I
2
T Time Band Constant
TB5C
GF
= the GF I
5
T Time Band Constant
**** NOTE ****
The GF I
2
T Time Band Constant (TB2C
GF
) =
0.18 for the .50S Time Band
0.144 for the .40S Time Band
0.108 for the .30S Time Band
0.072 for the .20S Time Band
0.036 for the .10S Time Band
**** NOTE ****
The GF I
5
T Time Band Constant (TB5C
GF
) =
512 for the .50S Time Band
409.6 for the .40S Time Band
307.2 for the .30S Time Band
204.8 for the .20S Time Band
102.4 for the .10S Time Band
EXAMPLE #3 (I
2
T Slope):
CT Rating 1600A
LT pick-up 1200A
GF pick-up 640A
GF time band .20S I²T Slope
Ground Fault Current 800A
TB2C
GF
= 0.072
and X
GF
= ground fault current = 800A
CT Rating 1600A
= 0.5
therefore:
trip time = T = TB2C
GF
or 0.072 = 0.072
X
GF
² (0.5)² 0.25
= 0.288 sec
**** IN SUMMARY ****
To calculate the GF I
2
T trip time:
1) Determine the GF Time Band Constant (TB2C
GF
)
2) Calculate "X
GF
" where
X
GF
= ground fault current
CT Rating
3) Solve the equation:
trip time(sec) = TB2C
GF
X
GF
2
EXAMPLE #4 (I
5
T Slope):
CT Rating 2000A
LT pick-up 2000A
GF pick-up 800A
GF time band .30S I
5
T Slope
Ground Fault Current 2400A
TB5C
GF
= 307.2
and X5
GF
= ground fault current = 2400A
GF Pickup 800A
= 3.0
therefore:
trip time = T = TB5C
GF
or 307.2 = 307.2
X5
GF
² (3.0)
5
243
= 1.264 sec
18.4 Neutral Overload (NOL) Trip Time
The NOL trip time is calculated in the same manner as the
LT trip time.
For NOL currents, the "I²T = Constant" equation can be
restated as follows:
T = TBC
NOL
X²
Where:
T = time to trip in seconds (center of the band)
X = current in multiples of the NOL pick-up setting
TBC
NOL
= the NOL Time Band Constant
= 36 X NOL time band setting
**** NOTE ****
The NOL Time Band Constant (TBC
NOL
) is by definition 36
times the NOL Time Band Setting in seconds.
EXAMPLE #5:
CT Rating 1600A
NOL pick-up 800A
NOL time band 15.0S
NOL Current 2400A
TBC
NOL
= 36 X NOL Time Band Setting
= 36 X 15.0
= 540
and X = overload current = 2400A = 3
NOL pick-up 800A
therefore:
trip time = T = TBC
NOL
or 540 = 540
X² 3² 9
= 60 seconds
**** IN SUMMARY ****
To calculate the NOL trip time:
1) Calculate the NOL Time Band Constant (TBC
NOL
)
2) Calculate "X" where
X = overload current
NOL pick-up Setting
3) Solve the equation:
trip time(sec) = TBC
NOL
X
2
18.3 Ground Fault (GF) Trip Time
T = TB2C
GF
X
GF
²
T = TB5C
GF
X5
GF
5