Toshiba GR-200 Series Instruction Manual page 144

Thermal characteristic
2.9.2
THM characteristics are determined with two equations. Equation (2.9-3) is used to define the
cold state. Equation (2.9-4) 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
pre-load current (amperes), τ is the thermal time constant (minutes), and Ln is the natural
logarithm.
In Equation (2.9-3), 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 2.9-1.a shows the cold state where an overload has been switched onto a previously
un-loaded system. Figure 2.9-1.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)
2
��
t = τ ∙ Ln [
2
�� − ��
2
�� − ��
t = τ ∙ Ln [
2
�� − ��
is the permitted overload current (amperes), Ip is the
AOL
b. Thermal Curves (Hot Curve - 90% prior load))
Figure 2.9-1 Thermal curves
- 125 -
]
2
������
2
��
]
2
������
6F2T0200 (0.15)
(2.9-3)
(2.9-4)

100
50
20
10
5
1
GRE200 (1,2)