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The art of engineering

Complex technology in the miniature circuit breaker

ABB expert knowledge is not only needed for devices that switch high currents and voltages, but also for the more commonly used miniature circuit breaker – a sophisticated piece of engineering.  “During development, designers for the mechanical parts work closely together with the electrical engineers,” says Joachim Becker, Product Manager at ABB STOTZ-KONTAKT. “The development of a new generation takes about three years.  Today not even a computer simulation can show everything. That’s why various elements and steps must be tested separately, whenever possible, such as the arc entering the arc chamber.”

A miniature circuit breaker S201-B16 trips a short-circuit

In the beginning

Within a half millisecond (ms) a short circuit is released, the coil registers the increased current due to the fault. The hammer trip forces the contacts to open.

An arc forms

Between the fixed and the moving contacts an arc rises during the opening process. Current continues to flow through this arc.

Contacts open

The contacts are open and the arc is clearly visible.

Arc in arc extinguishing chamber

The unequal pressure across the arc causes the plasma wave to be driven away from the contacts into the arc extinguishing chamber.

Dispersing the arc flash

As the arc enters the evenly spaced arc chute deviders, it is broken up into smaller 30V arcs that are individually extinguished.

The toggle trips

In less than 3ms after the short circuit current is released, the fault is safely cleared. Due to its mass inertia, it takes the toggle 10ms to reach its end position.

The switch arc

Basically, an electrical arc exists during a self-sustaining gas discharge between two electrodes. If the open contacts are not far enough from each other during the switching operation, it only takes little electric field strength to consistently ionize the air.  The result is the forming of an arc, just like lightening during a thunderstorm. In alternating current, the arc extinguishes at the zero crossing of the sinusoidal current wave. If the contacts are not far enough apart upon re- tensioning, the arc ignites again. The goal of various systems is to avoid this from happening and to quickly extinguish the arc.  For example, compressed air could blow out the arc or various switching methods could with high dielectric strength, in a vacuum or with hexafluoride gas.

An arc fault that is not quickly extinguished can cause great damage such as fire and explosion, which can occur if the arc fault lasts up to 300 milliseconds.  Depending on the design of the switchgear, personnel can be seriously injured.

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