ABB celebrates the miniature circuit breaker’s 90th anniversary. The invention, patented in Germany in November 1924 by Hugo Stotz and his chief engineer, Heinrich Schachtner, made the rapidly increasing electrification of private homes safer and more efficient.

Today, nearly every household is equipped with miniature circuit breakers to stop the flow of electricity, protecting people and equipment from electrical fire. Like a police officer who halts traffic at an intersection when there’s an accident, these breakers interrupt the current when something in a home’s or building’s electricity system goes wrong.

German patent issued 90 years ago this year

When Stotz and Schachtner were toiling away on their landmark invention in the early 1920s, homes in Germany and beyond were increasingly being electrified. At the time, fuses, where a metal wire melts when too much current flows through it, represented cutting-edge technology when it came to shielding buildings from electrical fires.

The problem was, the fuse had to be replaced every time there was a fault. Here’s where Stotz and Schachtner produced a revolution: They combined thermal and magnetic trips into a single, reusable unit capable of switching off high currents without requiring devices to be replaced repeatedly. 

Today’s world of ubiquitous, convenient electricity wouldn’t be possible without a miniature circuit breaker because a fuse would have to be replaced with each overload. Lamps, refrigerators and other household appliances can operate safely because harmful currents are detected by the miniature circuit breaker and almost instantaneously interrupted.

Temperatures sufficient to melt rocks

Today, when an ABB miniature circuit breaker reacts to short circuits or overloads, it trips and interrupts the current within 10 milliseconds, 10 times faster than the blink of an eye. When this happens, the breaker is exposed to intense heat ranging from 5,000 degrees to 6,000 degrees Celsius, temperatures capable of melting rocks.

Even after all that, homeowners simply must flip the switch that has been tripped, and power resumes its flow.

^{1. 1923 First miniature circuit breaker (MCB) | 2. 1928 Legacy MCB with K-characteristics for motors | 3. 1957 High-efficiency legacy MCB S201-K4 | 4. 1961 Legacy MCB S161 | 5. 2012 MCB from System pro M compact S200}
 

Sold to Brown Boveri amid economic turbulence

Hugo Stotz, who died at age 66 in 1935 in Mannheim, Germany, was an energetic inventor and entrepreneur who founded his first company with a partner in 1891.

It was in the wake of economic turbulence that accompanied World War I that he sold his business in 1918 to Switzerland’s Brown Boveri & Cie, but remained manager until 1929.

In the intervening years, the business now known as ABB Stotz-Kontakt has manufactured more than 950 million miniature circuit breakers. At a rate of 42 million miniature circuit breakers annually at its production plant in Heidelberg, Germany, ABB will break the one billion mark by 2015 as it expands its leadership position in this technology.

“Our mission has been to continuously improve miniature circuit breaker technology since its introduction 90 years ago, along with the creation of the state-of-the-art manufacturing we've installed in Heidelberg today,” says Frank-Andreas Winter, ABB’s Global Product Manager for Enclosures and DIN-Rail Products. “The latest innovative feature is the real contact position indication, which makes it easy to identify the MCB that has tripped. This feature saves time and effort for the installer to analyze the fault and to take the appropriate corrective actions.”

The mere pressure of a finger

Incidentally, it's Schachtner’s name that appears on the U.S. patent - No. 1,629,640 - for the miniature circuit breaker, issued May 24, 1927.

"My invention relates to automatic electric circuit breakers of the kind in which two contact pieces normally pressed together by springs are separated by the interposition of an insulating slide,” Schachtner wrote, according to the U.S. Patent Office document. “When the overload in the apparatus ceases, switching-on can be effected by the mere pressure of the finger without the necessity of unscrewing the apparatus from its socket.”

The first serial production began in 1928 at the Stotz facility in southern German and was met with great success, as its device could easily be screwed into the existing fuse base. No changes to the electrical installation were required.

A legacy of pushing the technology further

That year, Stotz and his team were already pushing the technology further.

He developed a special circuit breaker for coping with loads of higher starting currents, including motor applications. This opened the door for industrial applications, where the miniature circuit breaker is found today as abundantly as in households.

Over the years, ABB has continuously improved the breaker technology and kept pace with standards such as the DIN-Rail in 1970 that made the installation of multiple circuit breakers in a single panel easier.

In the process, ABB Stotz-Kontakt has remained loyal to one of its slogans from the 1920s: “All brands are developed for practical considerations with the objective of simplification and acceleration of assembly.”