I.B. 17555
Page 21
Effective 03/97
The Time Multiplier setting is used to select a predeter-
mined amount of time a sustained overload condition
will be carried before the breaker trips. For the Thermal
Curves, a value of (3 x I
n
) is the reference point where
the programmed time multiplier setting is fixed on the
curve. A wide range of time settings are available and
depend upon the curve shape selection. Refer to Tables
2.2 and 2.3 for a complete list of available time multiplier
settings. As Time Multiplier settings are varied, the Time
Multiplier portion of the overall curve is moved vertically
up or down on the time current grid. This movement is
also independent of the other portions of the curve.
Figure 3-5 graphically illustrates the vertical time line
movement with an I
2
t curve shape selection. Similar
movement occurs for the remaining curve shapes.
Short Time Protection
Short time (fault) protection responds to short circuit
conditions. Similar to the inverse time overcurrent func-
tion, the short time function is comprised of a short time
current pickup setting and a short delay time setting.
The Short Delay pickup setting establishes the current
level at which the relay’s short time tripping function
begins timing. The Short Delay Time setting establishes
the amount of time a short-circuit will be carried before
TIME
Minimum
Pickup Setting
MULTIPLES OF I
n
I
n
Figure 3-4 Typical Inverse Time Overcurrent Pickup
Horizontal Movement
TIME
CURRENT
(Pickup
Accuracy)
(Trip Point)
(Timing
Accuracy)
Figure 3-3 Sample Electronic Trip Curves
TIME
CURRENT
(Trip Point)
Typical Thermal Curve
Typical ANSI and IEC Curves
Note: Pickup and Tripping will occur within ±% of any selected trip point.
