GE c60 Instruction Manual page 306

GROUPED ELEMENTS
5.7.3.2 Inverse TOC curve characteristics
The inverse time overcurrent curves used by the time overcurrent elements are the IEEE, IEC, GE Type IAC, and I
curve shapes. This allows for simplified coordination with downstream devices.
If none of these curve shapes is adequate, FlexCurves can be used to customize the inverse time curve characteristics. The
definite time curve is also an option that can be appropriate if only simple protection is required.
Table 5-26: Overcurrent curve types
IEEE
IEEE Extremely Inverse
IEEE Very Inverse
IEEE Moderately Inverse
A time dial multiplier setting allows selection of a multiple of the base curve shape (where the time dial multiplier = 1) with
the curve shape (
CURVE
to the time multiplier (
or base curve values. Setting the multiplier to zero results in an instantaneous response to all current levels above pickup.
Time overcurrent time calculations are made with an internal energy capacity memory variable. When this variable
indicates that the energy capacity has reached 100%, a time overcurrent element operates. If less than 100% energy
capacity is accumulated in this variable and the current falls below the dropout threshold of 97 to 98% of the pickup value,
the variable must be reduced. Two methods of this resetting operation are available: "Instantaneous" and "Timed." The
"Instantaneous" selection is intended for applications with other relays, such as most static relays, which set the energy
capacity directly to zero when the current falls below the reset threshold. The "Timed" selection can be used where the
5
relay must coordinate with electromechanical relays.
IEEE curves
The IEEE time overcurrent curve shapes conform to industry standards and the IEEE C37.112-1996 curve classifications for
extremely, very, and moderately inverse. The IEEE curves are derived from the formulae:
where
T = operate time (in seconds)
TDM = Multiplier setting
I = input current
I = Pickup Current setting
pickup
A, B, p = constants
T = reset time in seconds (assuming energy capacity is 100% and RESET is "Timed")
RESET
t = characteristic constant
r
Table 5-27: IEEE inverse time curve constants
IEEE curve shape
IEEE Extremely Inverse
IEEE Very Inverse
IEEE Moderately Inverse
5-180
IEC
IEC Curve A (BS142)
IEC Curve B (BS142)
IEC Curve C (BS142)
IEC Short Inverse
) setting. Unlike the electromechanical time dial equivalent, operate times are directly proportional
) setting value. For example, all times for a multiplier of 10 are 10 times the multiplier 1
TD MULTIPLIER
A
------------------------------ - B
p
I

T = TDM  
------------- - 1
–
 
I
pickup
A B
28.2 0.1217
19.61 0.491
0.0515 0.1140
GE type IAC
IAC Extremely Inverse
IAC Very Inverse
IAC Inverse
IAC Short Inverse
+
------------------------------- -

, T = TDM

RESET
1 ------------- -
–

I
p t
r
2.0000 29.1
2.0000 21.6
0.02000 4.85
C60 BREAKER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS
Other
2
I t
FlexCurves A, B, C, and D
Recloser Curves
Definite Time
t
r
I 2


pickup
2
t standard
Eq. 5-8