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Derating for the electronic trip units of 3VA2 molded case circuit breakers

The current measuring circuits of the electronic trip units of 3VA2 molded case circuit breakers are not affected by the ambient temperature. The Rogowski coils integrated in the trip unit measure the current and compare it to the set tripping threshold. Higher or lower ambient temperatures do not influence the tripping threshold, which means that it is not necessary to apply correction factors.

Limitation of the rated operational current

The temperature inside the molded case circuit breaker is nonetheless influenced by the ambient temperature and the current-dependent power loss generated inside the unit. To prevent overloading of molded case circuit breakers under difficult thermal conditions, the maximum rated continuous operational current must therefore be limited as a function of the ambient temperature in some cases:

The table below specifies the maximum rated continuous operational current as a function of the ambient temperature and the installed accessories. The operational current must never exceed the maximum rated operational current of the molded case circuit breaker.

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).

When terminals with auxiliary conductor connection are used, the total current from the main circuit and auxiliary conductor must not exceed the maximum rated operational current of the molded case circuit breaker.

Example 1

Starting point:
A plug-in version of a 3VA2 molded case circuit breaker 160 A with an ETU350 LSI trip unit is to be operated at 70 °C:

Ir(max 70° C) = 160 A

No derating is required.

Example 2

Starting point:

A plug-in version of a 3VA2 molded case circuit breaker 250 A with an ETU850 LSI trip unit is to be operated at 60 °C with the COM060 communication module:

Ir(max 60 °C)205 A

COM060

COM060 communication module

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

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