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489 GENERATOR MANAGEMENT RELAY – INSTRUCTION MANUAL
CHAPTER 5: SETPOINTS
ANSI Curves
The ANSI time overcurrent curve shapes conform to industry standard curves and fit into
the ANSI C37.90 curve classifications for extremely, very, normally, and moderately
inverse. The 489 ANSI curves are derived from the formula:
(EQ 0.4)
where:
T
= Trip Time in seconds
M
= Multiplier setpoint
I
= Input Current
I
pickup
= Pickup Current setpoint
A
,
B
,
C
,
D
,
E
= Constants
IEC Curves
For European applications, the relay offers the four standard curves defined in IEC 255-4
and British standard BS142. These are defined as IEC Curve A, IEC Curve B, IEC Curve C, and
Short Inverse. The formula for these curves is:
(EQ 0.5)
where:
T
= Trip Time in seconds
M
= Multiplier setpoint
I
= Input Current
I
pickup
= Pickup Current setpoint
K
,
E
= Constants
Table 5–2: ANSI Inverse Time Curve Constants
ANSI Curve Shape
Constants
A
B
C
D
E
Extremely Inverse
0.0399
0.2294
0.5000
3.0094
0.7222
Very Inverse
0.0615
0.7989
0.3400
–0.2840
4.0505
Normally Inverse
0.0274
2.2614
0.3000
–4.1899
9.1272
Moderately Inverse
0.1735
0.6791
0.8000
–0.0800
0.1271
Table 5–3: IEC (BS) Inverse Time Curve Constants
IEC (BS) Curve Shape
Constants
K
E
IEC Curve A (BS142)
0.140
0.020
IEC Curve B (BS142)
13.500
1.000
IEC Curve C (BS142)
80.000
2.000
Short Inverse
0.050
0.040
T
M
A
B
I I
pickup
⁄
(
)
C
–
------------------------------------
D
I I
pickup
⁄
(
)
C
–
(
)
2
-------------------------------------------
E
I I
pickup
⁄
(
)
C
–
(
)
3
-------------------------------------------
+
+
+
⎝
⎠
⎜
⎟
⎛
⎞
×
=
T
M
K
I I
pickup
⁄
(
)
E
1
–
--------------------------------------
⎝
⎠
⎜
⎟
⎛
⎞
×
=