S&C Vista SD, Programming

Page: 28 / 34

One final consideration in the selection of ANSI/US and IEC curve families to achieve
proper power system coordination is the “I
2
t security characteristic” of the S&C Over­
current Control, included with controls having firmware version R300 or later. The
control will self­protect its internal power supply and current­sensing circuitry when a
Pickup set ting (in amperes) and/or a Time Dial setting exceeds the thermal limits of
the electronic cir cuitry (in both the electronics module and burden board assemblies).
Figure 1 shows the “I
2
t security characteristic” of the S&C Overcurrent Control. When
the “I
2
t security char acteristic” is enabled, the control may implement a definite­time
response characteristic of 0.25 seconds for any current magnitude above 14.5 kA. This
matches the performance ratings of the control listed in the “Specifications” section.
The “I
2
t security characteristic” does not apply to the E, K, Coordinating­Tap, and
Coordinating­Main curve families under all settings.
S&C Overcurrent Control I
2
t Security Characteristic
Ti
me(s)
Current (kA)
Maximum Rating
20 15 10 5 0 25
10
5
0
Figure 1. S&C Overcurrent Control maximum current rating adjusted for C.T. saturation.
Table 1 on page 29 should be used to determine whether the “I
2
t security characteris­
tic” will become enabled during the maximum calculated fault levels. In most cases, the
Pickup Current and Time Dial settings selected will be well below the values listed in
the table. To prevent the “I
2
t security characteristic” from becoming enabled, the pickup
current and Time Dial settings must be less than the limits listed in the following table.
The operation time of the U.S. and IEC curves is a function of three variables: the
Pickup setting, the Time Dial setting, and the fault current (measured in multiples
of the Pickup setting). With this information, the overcurrent control calculates the
curve opera tion time and the I
2
t security characteristic operation time, compares the
two, and oper ates according to the faster time. The table provides example curve
settings at sample fault current levels that would cause the curve operation time to
be slower than the I
2
t security characteristic operation time for a 660:1 CT.
Appendix D
28 S&C Instruction Sheet 695-515