The inverse time overcurrent curves used by the TOC (time overcurrent) Current Elements are the IEEE, IEC, GE Type
IAC, and I
t standard curve shapes. This allows for simplified coordination with downstream devices. If however, none of
these curve shapes is adequate, the FlexCurve™ may be used to customize the inverse time curve characteristics. The
Definite Time curve is also an option that may be appropriate if only simple protection is required.
Table 5–15: OVERCURRENT CURVE TYPES
IEEE Extremely Inv.
IEEE Very Inverse
IEEE Moderately Inv.
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 (
) setting. Unlike the electromechanical time dial equivalent, operate times are directly proportional
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 indi-
cates that the energy capacity has reached 100%, a time overcurrent element will operate. If less than 100% energy capac-
ity 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 Instan-
taneous 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 relay must
coordinate with electromechanical relays. With this setting, the energy capacity variable is decremented according to the
Graphs of standard time-current curves on 11" × 17" log-log graph paper are available upon request from
the GE Power Management literature department. The original files are also available in PDF format on the
UR Software Installation CD and the GE Power Management Web Page.
GE Power Management
5.5.11 INVERSE TIME OVERCURRENT CURVE CHARACTERISTICS
IEC Curve A (BS142)
IEC Curve B (BS142)
IEC Curve C (BS142)
IEC Short Inverse
) setting value. For example, all times for a multiplier of 10 are 10 times the multiplier 1
L90 Line Differential Relay
GE TYPE IAC
IAC Extremely Inv.
IAC Very Inverse
IAC Short Inverse
5.5 GROUPED ELEMENTS