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Temperature compensation with thermal-magnetic trip units of the TM 2-series

The trip units for 3VA1 molded case circuit breakers are factory-calibrated to a temperature of 50 °C. The thermal response threshold of the trip unit changes when the circuit breaker is operated in a higher or lower ambient temperature. To obtain the characteristic tripping times at a specific ambient temperature, it is necessary to apply correction factors when setting the thermal trip units.

Correction factor TK is applied to compensate for the ambient temperature.

The first step in calculating the correction factor TK is to determine the setting factor at 50 °C:

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EF(50 °C)

Setting factor at 50 °C

I

System current

In

Rated operational current of the molded case circuit breaker

Using the setting factor calculated at 50 °C, it is possible to read the correction factor K off the following charts:

Using the two calculated values, it is possible to determine the temperature compensation TK and finally the setting value for the trip unit Ir(TU):

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TK(TU)

Temperature compensation for molded case circuit breaker with service temperature TU

K

Correction factor

Tu

Service temperature of molded case circuit breaker

Trip unit setting value:

Ir(TU) = In · EF(50 °C) · TK

Ir(TU)

Trip unit setting value with service temperature Tu

The operational current must never exceed the maximum rated operational current In of the molded case circuit breaker. If the current calculated for the operating conditions is higher than the rated operational current of the unit, a molded case circuit breaker with the appropriate rated operational current must be deployed.

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.

If the setting value calculated for the thermal trip unit Ir(TU) is outside the possible setting 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 mounting conditions for molded case circuit breakers 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 IO = 360 A will now be calculated.
To compensate for the ambient temperature of 60 °C, rather than 50 °C, the thermal trip unit has been set Ir(60 °C) = 95 A for a system current I = 90 A.

The first step is to calculate the ratio between the overcurrent and the system current:

Io/I = 360 A / 90 A = 4

Using this value, it is now possible to read the tripping time from the characteristics chart:

Inclusion of accessories

None of the accessories from the extensive range available for 3VA1 molded case circuit breakers has an influence on the thermal response thresholds of the switching devices except for two accessories that require additional correction factors. These are:

  • Plug-in version / draw-out version

  • Residual current devices

None of the other accessories has an influence on the thermal response thresholds of trip units.

The table below shows the correction factors of setting value Ir for the trip units of 3VA1 molded case circuit breakers in plug-in / draw-out technology and for 3VA1 molded case circuit breakers combined with residual current devices:

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Example

Starting point:

The current loading of a 3VA1 molded case circuit breaker is calculated to be 90 A and the ambient temperature 65 °C for a given application. The plug-in version of the molded case circuit breaker will be used.

The appropriate size of molded case circuit breaker is determined first on the basis of the correction factor for 3VA1 plug-in technology (see table above):
Design of assembly at 50°C: Ir= (90 A / 0.9) = 100 A

It is therefore necessary to select a molded case circuit breaker that allows a setting value of 100 A on the thermal trip unit at a calibration temperature of 50 °C. A 3VA1 molded case circuit breaker 160 A with In = 125 A is therefore used for the application.

The temperature compensation is then calculated according to the method described above:

  • Ambient temperature 65 °C:
    Setting factor EF(50 °C) = (100 / 125) = 0.8
    Correction factor K = 0.69 (from chart)
    Temperature compensation of switching device TK(65 °C) = (0.69 · (65 °C - 50 °C) / 100) + 1 = 1.1
    Setting value Ir(65 °C) = 125 A · 0.9 · 1.1 = 124 A

NOTE

These values do not apply to all mounting conditions for molded case circuit breakers 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).

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No derating up to the rated operational current In of the molded case circuit breaker

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Tripping time (schematic representation)

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