IEC 60255 Curves
Equation:
Where,
TMS denoted time multiplier setting
Is
denoted the current setting.
I
denoted the actual current.
k
and α denoted the curve type constants.
See table below.
|
Curve type |
k |
α |
|
Standard
inverse |
0.140 |
0.020 |
|
Very inverse |
13.5 |
1 |
|
Extremely
inverse |
80 |
2 |
|
Long time
standard inverse |
120 |
1 |
The objective of protective device coordination study
is to effect a time-current coordination among the devices that achieve the
expected system protection and electrical service continuity goals.
As presented on
the attached, these graphs not only plot the time-current curves (TCC) of
Relays, Circuit Breakers and Fuses on the log-log paper but also damage curve
of equipment and current landmarks. In addition, there is an associated one
line diagram located at right side of graph to show the settings and ratings.
According to
these curves and landmarks on the graphs, it can tell their coordination situations
among protective devices easily. Any dis-coordination includes cross or
overlaps even disorder of curves shall be fixed and repaired to get properly
interval. The results and remarks of PD coordination refer to the attached.
The motor starting curves on the time-current curve graph
is also modeled in the SKM.
Besides short circuit study, the protective devices
coordination also play a vital role to enable Arc Flash Study could be
continuous.
