Miniature breakers stop overloads, short circuits

In every modern home you’ll find them: Miniature circuit breakers, the devices that protect occupants and their belongings from fire, shock and damage

Behind these important safety aspirations is plenty of technology. Here’s a primer to explain these essential little gadgets – and help show why they’re a big reason why buildings have grown safer over the years.

First, a little background: The invention of the miniature circuit breaker, which ABB produces in factories in Germany, Bulgaria, India, Argentina, Indonesia and China, stretches back to the early days of electrification. Pioneering engineers figured out that electrical installations needed to be protected from short-circuits and overloads.

Overloads, short circuits

An overload results when many devices consuming electricity from a single electrical circuit draw too much current. Here, the miniature circuit breaker, or MCB, performs one of its critical tasks: To trip before the overload damages the cable.

Alternatively, a short circuit is a defect within the circuit where electricity takes a “short cut” via a faulty path where there is a lower resistance than the device being powered. The result? A sudden surge in current, within milliseconds. You might be able to feel or see it, as wires become hot or sparks fly.

Such an event could damage the circuit as well as any connected devices – if an MCB weren’t installed to keep them safe.

Don’t blow a fuse

Inventor Thomas Edison actually patented an idea for a circuit breaker, back in the late 1870s.

Still, electricians from his era and beyond resorted to fuses to protect cables from overloads and short circuits.

The idea behind them is pretty simple: It’s a low-resistance wire or strip that melts when too much current flows through it. A fuse can handle less current than the main cable within a circuit, and is designed to be sacrificed to protect the system from short circuits, poorly matched loads, overloads and instances where a device fails. A fuse has a predetermined breaking point – so before the cable breaks, the fuse burns through or “trips.”

There is only one problem: When a fuse trips, it must be replaced.

Back in 1923, however, an ABB employee, Hugo Stotz, along with his chief engineer, came up with the first MCB to protect a cable from overcurrent and short-circuit, while eliminating the hassle of replacing the fuses. You just pushed a button to reset the breaker, once the fault had been cleared.

Here’s an interesting fact: Stotz’s first MCBs looked a lot like one of those fuses it was destined to replace. That’s because it had to fit into the same sockets as the fuses. Only later was the compact MCB developed to fit on DIN-rails that now prevail inside modern consumer enclosures.

Multiple circuits

You may wonder why there’s more than one MCB inside a consumer unit. This is because your utility sends electricity to your home or building via a large cable. It’s then routed to smaller units, one cable for the living room, another for the dining room, and so on. Each of these smaller cables must be protected separately.

This also boosts flexibility: In the case of a defective device like a vacuum cleaner connected to the living room circuit, it will trip only the MCB of the living room, not the rest of the house. This is also why circuits are generally required to have clear and legible labels describing their destination.

In overload tripping, excessive current heats up a strip made up of two metals, called a bimetal (pictured in the yellow area.) When the current exceeds the rating of the MCB, the bimetal bends and eventually trips the MCB. Depending on intensity of the current, this may happen after seconds or even minutes.

With short-circuit tripping, however, the MCB must trip as quickly as possible. The bimetal is just too slow. That’s why ABB MCB’s have a coil (pictured in green) that reacts almost instantly to sudden current surges. This triggers an armature and a hammer; the armature trips a toggle, the hammer pushes the MCB’s contacts apart to break the circuit in a split second.

In both cases, however, an electrical arc is produced between the contacts inside the MCB. This arc is several thousand degrees and must be cleared immediately, to avoid risk of fire. The arc is directed toward the MCB’s extinguishing chamber (the blue area), where small metal plates interrupt the arc by dividing it into smaller arcs that dissipate quickly.

Despite the improvements over the years, the basic concept behind ABB’s miniature has stood the test of time. Since Stotz’ invention 90 years ago, 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 produced annually at its plant in Heidelberg, Germany, ABB Stotz-Kontakt is on track to break the one billion mark by 2015.