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Current limitation

The term "current limitation" refers to the action of limiting the peak value of the prospective impulse short-circuit current ip to a lower let-through current iD.

The compact design of the breakers has been made possible by their excellent current limiting capabilities. In the event of a short circuit, the molded case circuit breaker significantly reduces the magnitude of let-through current, i.e. it reduces the current loading of downstream equipment (lower thermal load, fewer dynamic forces). The level of let-through energy is also reduced to a considerable extent.

3VA molded case circuit breakers are designed with a current-limiting capability. IEC EN 60947‑2 (VDE 0660‑101), section 2.3, page 12, defines current-limiting molded case circuit breakers in the following way:

"Circuit breaker that prevents the let-through current from achieving the prospective peak value over a specific current range, and that limits the let-through energy (I2t) to a value that is lower than the let-through energy of one half-wave of the symmetrical prospective current."

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Unlimited current

Peak value of limited current

Limited current

iD

Let-through current

ip

Unlimited impulse short-circuit current

ta

Break time

Double-rotary contact system

To achieve excellent current limiting, the 3VA molded case circuit breakers are equipped with a double-rotatory contact system that opens dynamically on its own above the specified disengaging currents on the principle of magnetic repulsion before the expected peak value of the short-circuit current is reached. These limits have been coordinated and optimized to suit the overall device characteristics. This substantially reduces the thermal and mechanical loading on the molded case circuit breaker and the electrical installation.

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Fixed contacts

Rotary contact system

Arc splitter chute

The switching pole cassettes are optimized for high breaking capacity, and their double-rotary contact system design enables extremely good current limiting thanks to the build-up of peak arc voltage generated at both contacts in the event of a short-circuit. This results in significant limitation of the let-through energy I²t and the let-through current.