The E/M Reset Time is defined by the following equation:
t
r
= (Reset Time * Time Multiplier) / (1 - M
2
)
Where,
• Reset Time is entered by the user. Some standard cur ves have default
Reset Time
values.
• M = I
load current
/ MAX(I
minimum pickup
, I
low current cutoff
)
The D/T Reset Time is not affected by the Time Multiplier or load current, so the
equation simplifies to: t
r
= Reset Time.
Table 2. Examples of Reset Time Tests for Phase, Ground, and Negative
Sequence Overcurrent Elements
Time
Multiplier
Reset
Characteristic
Reset Time
Setting
Expected Reset
Time
Actual Time to Reset
(Test Result)
Test 1
1
D/T
21 .6
Just under 21 .6 s
21 .2 s
Test 2
1
E/M
21 .6
Longer than 21 .6 s
22 .6 s
Test 3
1
D/T
10 .8
Just under 10 .8 s
10 .6 s
Test 4
1
E/M
10 .8
Longer than 10 .8 s
11 .3 s
Test 5
0 .5
D/T
21 .6
Just under 21 .6 s
21 .2 s
Test 6
0 .5
E/M
21 .6
Longer than 10 .8 s
11 .3 s
The six test cases in Table 2 demonstrate the influences of Reset Characteristic, Reset
Time, and Time Multiplier. For each test, 50 amperes of current were used to start the
test under normal load conditions using an IEEE VI curve and a 185-A minimum trip. The
current was increased to 300 A and then removed just prior to the element tripping (14.4 s
for Time Multiplier of 1.0, or 7.2 s for a Time Multiplier of 0.5), and returned to 50 A of load.
• The D/T Reset Characteristic is not affected by the 50 A of load, so the expected time
to reset is just under the
Reset Time
setting, as confirmed by Tests 1 and 3.
• The E/M Reset Characteristic is slowed down by the existence of 50 A of load
current, so the expected time to reset is a bit longer than the
Reset Time
setting, as
confirmed by Tests 2 and 4.
• When the Time Multiplier is off-nominal, it affects the actual reset time proportionally
for the E/M Reset Characteristic only, as confirmed by Tests 5 and 6.
S&C Instruction Sheet 766-530 51
Protection Setup
