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Technical specifications
6.4 Derating and temperature compensation
Example 1: Correction of setting values as a function of ambient temperature
Starting point:
On a 3VA1 100 A molded case circuit breaker with a TM240 100 A trip unit, the real tripping
threshold for various different service temperatures must be set to a required system current
of I = 90 A.
● Ambient temperature 60 °C:
Setting factor EF(50 °C) = (90 / 100) = 0.9
Correction factor K = 0.58 (see chart above)
Temperature compensation TK(60 °C) = (0.58 · (60 °C to 50 °C) / 100) + 1 = 1.058
Setting value I
● Ambient temperature 40 °C:
Setting factor EF(50 °C) = (90 / 100) = 0.9
Correction factor K = 0.58
Temperature compensation TK(40 °C) = (0.58 · (40 °C ... 50 °C) / 100) + 1 = 0.942
Setting value I
If the setting value calculated for the thermal trip unit I
range, an appropriate molded case circuit breaker with a higher or lower rated operational
current must be selected and the compensation calculation then performed again.
Note
These values do not apply to all installation situations of the molded case circuit breaker and
may deviate depending upon factors such as cables and busbars, packing density,
ventilation, etc.
Please observe the applicable overriding guidelines (e.g. IEC 61439).
Example 2: Calculation of the tripping time
Starting point:
Based on the example above, the tripping time for an overcurrent I
calculated.
To compensate for the ambient temperature of 60 °C, rather than 50 °C, the thermal trip unit
has been set I
The first step is to calculate the ratio between the overcurrent and the system current:
I
/I = 360 A / 90 A = 4
o
634
= 100 A · 0.9 · 1.058 = 95 A
r(60 °C)
= 100 A · 0.9 · 0.942 = 85 A
r(40 °C)
= 95 A for a system current I = 90 A.
r(60 °C)
is outside the possible setting
r(TU)
= 360 A will now be
O
3VA molded case circuit breakers with IEC certificate
Manual, 03/2019, A5E03603177010-03
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