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Ln = natural logarithm
Figure 2.7.1.1 illustrates the IEC60255-8 curves for a range of time constant settings. The
left-hand chart shows the 'cold' condition where an overload has been switched onto a previously
un-loaded system. The right-hand chart shows the 'hot' condition where an overload is switched
onto a system that has previously been loaded to 90% of its capacity.
Thermal Curves (Cold Curve -
no prior load)
1000
100
10
1
0.1
0.01
1
Overload Current (Multiple of I
2.7.1
Scheme Logic
Figure 2.7.1.2 shows the scheme logic of the thermal overload protection.
The thermal overload element THM has independent thresholds for alarm and trip, and outputs
alarm signal THM ALARM and trip signal THM TRIP. The alarming threshold level is set as a
percentage of the tripping threshold.
The alarming and tripping can be disabled by the scheme switches [THMAL] and [THMT]
respectively or binary input signals THMA BLOCK and THM BLOCK.
Thermal Curves (Hot Curve -
90% prior load)
1000
100
10
1
τ
100
50
0.1
20
10
0.01
5
2
1
0.001
10
1
)
Overload Current (Multiple of
AOL
Figure 2.7.1.1 Thermal Curves
⎯ 37 ⎯
τ
100
50
20
10
5
2
1
10
I
)
AOL
6 F 2 S 0 8 2 8
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