scheider electric MiCOM P746, Technical Manual

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Operation P746/EN OP/G31
MiCOM P746 (OP) 5-
15
1.4 Three Phase Overcurrent Protection
1.4.1.1 Inverse Time (IDMT) Characteristic
IDMT characteristics are selectable from a choice of four IEC/UK and five IEEE/US curves
as shown in the table below.
The IEC/UK IDMT curves conform to the following formula:
( )
⎟
⎠
⎞
⎜
⎝
⎛
+ × =
−
α
L
K
T t
1 Is I
The IEEE/US IDMT curves conform to the following formula:
( )
⎟
⎠
⎞
⎜
⎝
⎛
+ × =
−
L
K
TD t
Is I 1
α
Where: t = Operation time
K = Constant
Ι
= Measured current
Ι
S
= Current threshold setting
α
= Constant
L = ANSI/IEEE constant (zero for IEC/UK curves)
T = Time Multiplier Setting for IEC/UK curves
TD = Time Dial Setting for IEEE/US curves
OP
IDMT Curve description Standard K Constant
α
Constant
L Constant
Standard Inverse IEC 0.14 0.02 0
Very Inverse IEC 13.5 1 0
Extremely Inverse IEC 80 2 0
Long Time Inverse UK 120 1 0
Moderately Inverse IEEE 0.0515 0.02 0.114
Very Inverse IEEE 19.61 2 0.491
Extremely Inverse IEEE 28.2 2 0.1217
Inverse US-C08 5.95 2 0.18
Short Time Inverse US-C02 0.02394 0.02 0.01694
1.4.1.2 Reset Characteristics
For all IEC/UK curves, the reset characteristic is definite time only.
For all IEEE/US curves, the reset characteristic can be selected as either inverse curve or
definite time.
The definite time can be set (as defined in IEC) to zero. Range 0 to 100 seconds in steps of
0.01 seconds.
The Inverse Reset characteristics are dependent upon the selected IEEE/US IDMT curve as
shown in the table below.