Toshiba GRZ100 B Series, Instruction Manual

Page: 71 / 447

⎯
70
⎯
6 F 2 S 0 8 4 6
t = τ ·
Ln
I
I I
AOL
2
2 2
−
⎡
⎣
⎢
⎤
⎦
⎥
(2)
t = τ ·
Ln
I I
I I
P
AOL
2
2
2 2
−
−
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
(3)
where:
t = time to trip for constant overload current I (seconds)
I = overload current (largest phase current) (amps)
I
AOL
= allowable overload current (amps)
I
P
= previous load current (amps)
τ
= thermal time constant (seconds)
Ln = natural logarithm
Figure 2.4.6.1 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)
0.01
0.1
1
10
100
1000
1 10
Overload Current (Multiple of I
AOL
)
Operate Ti
m
e
(m
inutes)
Thermal Curves (Hot Curve - 90%
prior load)
0.001
0.01
0.1
1
10
100
1000
1 10
Overload Current (Multiple of I
AOL
)
Operate Time (minutes)
Figure 2.4.6.1 Thermal Curves
Scheme Logic
Figure 2.4.6.2 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.
τ
100
50
20
10
5
2
1
τ
100
50
20
10
5
2
1