Unpacking the air circuit breaker – applications and development

March 17th, 2014, Published in Articles: Vector

by Paul Louw, ABB

The development and applications of low voltage electrical circuit breakers, from the post-World War II days to the incorporation of the de-ionisation chamber in circuit breaker design in the 1940s, and the development and applications of the air circuit breaker.

Reconstruction after World War II resulted in the modernisation of industrial and agricultural equipment. Alongside the resumption of normal production, special attention was paid to raising productivity and to developing new technologies to support these initiatives.

A technology destined to play a significant role in low voltage switchgear, the de-ionisation chamber, was invented during this period and adopted for use by the Italian company Sace.

Switchgear manufacturers have always faced challenges when interrupting high current; as contacts of a circuit breaker are opened under load, the surrounding air temperature rises rapidly and ionises, changing its electrical property from an insulator to a conductor. This phenomenon results in the formation of an electric arc, developing thermal energy proportional to the size of the current to be interrupted.

Fig. 1: New generation ACBs offer more than circuit protection with a wide range of easily installable accessories.

Fig. 1: New generation ACBs offer more than circuit protection with a wide range of easily installable accessories.

This electric arc must be quenched effectively, safely and repeatedly in LV switchgear, and the de-ionisation chamber proved to be a solution to this problem.

Sace investigated incorporation de-ionising chambers into circuit breaker design from 1949. Through the use of a suitably shaped contact, the electric arc which is subjected to electromagnetic forces is deformed until it enters the de-ionisation chamber, where non-ionised air, channelled through the chamber, cools and de-ionises the conducting air. This extinguishes the electric arc.

The heat and electromagnetic power generated through the passage of high current in short-circuit conditions and opening under load are of such a nature that they cannot be contained within the confined space or by materials typical of moulded-case circuit breakers.

Air circuit breakers provide a solution as their larger size and metallic structures are particularly robust and resistant to thermal and dynamic stresses found in these conditions.

This advance in technology allowed for the production of circuit breakers capable of protecting electric machines (generators, motors, transformers and capacitors) used primarily in industry where short-time current and high breaking capacities are required. Non-industrial applications such as secondary distribution in high-power installations and primary LV distribution protection are also suited to air circuit breakers.

Larger loads are commonplace today as more electricity is used, and moulded-case circuit breakers are unable to provide the circuit protection required. This paved the way for the increased use of the larger-sized air circuit breaker.

Air circuit breakers have multiple applications: normal, current-limiting, selective and selective current-limiting.

In the event of a short circuit, current-limiting circuit breakers limit the let-through current to be lower than the prospective value.

Fig. 2: The ABB Novomax air circuit breaker, circa 1971, compact and solidly engineered.

Fig. 2: The ABB Novomax air circuit breaker, circa 1971, compact and solidly engineered.

Selective circuit breakers can withstand high let-through currents for periods in excess of those normally needed to open a circuit breaker, allowing for downstream circuit breakers to open. The tripping of the protection circuit breaker, which is electrically closer to the fault, guarantees continuity of service in the balance of the installation.

Selective, current-limiting circuit breakers combine both these characteristics. By exploiting the current-limiting capacity, these circuit breakers cause an increase in the resistance to the specific let-through energy, enabling them to withstand the fault current for the time needed to guarantee selectivity of the installation.

The SACE product line commenced in 1960 with the Otomax. This air circuit breaker was designed to guarantee selective protection and was suitable for protecting generators, large motors and outgoing feeders. Following the establishment of ABB, the Megamax range was launched in 1989, breaking ground with the introduction of retrofitting kits for air circuit breakers.

The new century saw the introduction of the Emax range in 2004. Emax X1 followed shortly thereafter. It provides air circuit breaker performance packaged into a space one would expect a moulded-case circuit breaker would occupy. Variations have followed and solutions for direct current applications, variable frequency and extreme environmental conditions are produced. The Emax evolved into a power manager with the introduction of Emax 2 in 2013.

Contact Derek Sleep, ABB, Tel 010 202-5061, lp@za.abb.com

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