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6 F 2 S 0 8 5 7
where:
I
P
= prior load current.
In fact, the cold curve is simply a special case of the hot curve where prior load current I
P
= 0, catering for
the situation where a cold system is switched on to an immediate overload.
Figure N-2 shows a typical thermal profile for a system which initially carries normal load current, and is
then subjected to an overload condition until a trip results, before finally cooling to ambient temperature.
θ (%)
t (s)
100%
Normal Load
Current Condition
Cooling Curve
Overload Current
Condition
Trip at 100%
Figure N-2 (1) Thermal Curve without Prior Load Current
θ (%)
t (s)
100%
Normal Load
Current Condition
Cooling Curve
Overload Current
Condition
Trip at 100%
80%
Figure N-2 (2) Thermal curve with Prior Load Current (
θ=80%)