GE MIFII GEK-106237P, Instruction Manual, Page: 181 / 245

A-6 MIFII Digital Feeder Relay GEK-106237P
A.5 THERMAL CURVE APPENDIX A
A
A.5THERMAL CURVE
The time needed for an element to rise its temperature (from an initial temperature
θ
0
) the 63% of
θ
(
θ
is the temperature
difference between the initial and the final temperature) is called “Time Constant”, and it is represented by the letter
τ
.
Using an equation, it is the time it takes to reach an intermediate temperature
θ
i where:
If
θ
0
is the temperature origin, at a given time, the temperature can be written as:
Where:
θ
Temperature increase at a given time
θ
N Rated temperature (temperature reached if I = IN)
In Protected element rated current
I Current flowing through the protected element
T Time
τ
Time constant
θ
t
63%
100%
θ
0
θ
∞
θ
i
τ
θ
t
63%
100%
θ
0
θ
∞
θ
i
τ