The solid-state circuit breaker: interruption in microseconds, without an arc

Every low-voltage circuit breaker we know — from the miniature MCB to the 4000 A air breaker — works on the same principle: two contacts separate, an arc strikes between them, the arc is stretched, cooled and extinguished as the current passes through zero. That takes from half a cycle to several cycles — 10 to 50 milliseconds. During that time the short-circuit current reaches its full value, and the cables, busbars and equipment downstream absorb all of its thermal and dynamic energy.

A solid-state circuit breaker has no contacts in the classical sense. Power transistors sit in the current path and, on command, stop conducting — within a few microseconds. There is no arc, no wear, and the short-circuit current is interrupted on its rising edge, long before the peak. The idea is old; what made it feasible at scale is silicon carbide.

Why silicon carbide

An ordinary silicon transistor placed in a power circuit has one problem — on-state resistance. At rated current it heats continuously, losses are a percentage of the transmitted power and cooling becomes impractical. Silicon carbide (SiC) has a band gap about three times wider than silicon, a breakdown field ten times higher and three times better thermal conductivity. In practice that means a 1200 V transistor with resistance many times lower, which tolerates a higher junction temperature.

The result: on-state losses fall to the order of 0.1–0.3 % of the transmitted power — still not zero, but within reasonable cooling. For a 400 V three-phase circuit, 1200 V devices are used, which leaves margin for switching overvoltages.

What changes in protection

• Current limiting. Interruption in microseconds means the short-circuit current never reaches its prospective peak. Let-through energy I²t is one to two orders of magnitude lower than with a mechanical breaker. Downstream cables and busbars can be sized to rated current with a far smaller check for thermal withstand.
• Arc flash. Arc-flash incident energy is proportional to time and to current. Both fall drastically, which changes the category of protective equipment for work on the switchgear.
• Selectivity. The tripping characteristic is programmable — not a curve built into a bimetal and a solenoid, but a table in a controller. Selectivity between levels is set and changed without replacing the device.
• Endurance. A mechanical breaker has a limited number of short-circuit operations — usually a few. A solid-state one has no switching wear.

What is still missing

Three things keep this technology out of the standard switchboard for now.

Galvanic isolation. A transistor in the off state is not a disconnector. Safe isolation of a circuit for work on it requires a mechanical contact with a visible gap. So every solid-state breaker is paired with a disconnector — built in or separate — and that complicates the device.

Interruption energy. Current in an inductive circuit cannot stop instantly. On interruption in microseconds, the energy ½LI² stored in the inductance of cables and busbars has to go somewhere. It is absorbed by varistors and snubbers sized for the worst case — which on long routes is not small.

Cost and cooling. At present a solid-state breaker costs many times its mechanical equivalent and needs a heat sink or forced cooling. Hybrid designs — a mechanical contact for normal operation with a parallel semiconductor branch that carries the current only for the duration of interruption — reduce losses but add complexity.

Where it already makes sense

DC systems come first. There is no current zero to extinguish an arc, mechanical breakers are cumbersome and expensive, and semiconductor interruption has a clear advantage. Battery storage systems, charging infrastructure, DC busbars in data centres — these are the markets where the first production products from ABB, Atom Power and others are already in service.

In AC industrial switchgear the case is wherever one arc flash or one burnt-out board costs more than the price difference — critical consumers, places with frequent staff access, extensions of old switchgear with insufficient thermal withstand. The main board of the hall will stay mechanical for years yet. But the feeder to the most important machine may not.

Related area: Switchboards

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