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2.18.2 Thermal characteristic
THM characteristics are determined with two equations. Equation (2.18-3) is used to define
the cold state. Equation (2.18-4) is used to define the hot state:
where t is time to trip for a constant overload current of I (minutes), I is the overload current
(largest phase current in amperes), I
AOL
is the permitted overload current (amperes), Ip is the
pre-load current (amperes), τ is the thermal time constant (minutes), and Ln is the natural
logarithm.
In Equation (2.18-3), cold curves are special versions of the hot curves where Ip is zero,
catering for the situation where a cold system is switched onto an immediate overload.
Figure 2.18-1.a shows the cold state where an overload has been switched onto a
previously un-loaded system. Figure 2.18-1.b shows the hot state where an overload is switched
onto a system that has previously been loaded to 90% of its capacity. The figures are in
conformity with the IEC 60255-149 curves, and these curves are drawn in a range of settings
of the time constant ().
a.Thermal Curves (Cold Curve - no prior load)
b. Thermal Curves (Hot Curve - 90% prior load))
Figure 2.18-1 Thermal curves
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