A Square D shunt trip breaker is a type of circuit breaker with a built-in indicator and remote tripping mechanism. The indicator is a clear plastic window that shows a black line when the circuit is in good condition and a red line when the circuit is broken.
Square D shunt trip breakers are manufactured by Schneider Electric, a company that introduced many innovations in circuit breaker technology. Schneider Electric owns the patent for the Square D's indicator technology, making this product line the only circuit breaker on the market with such a feature. The Square D also trips faster than any other available circuit breaker, breaking the circuit one-sixtieth of a second after a fault occurs.
Like all circuit breakers, the Square D relies on a protective relay to function. Protective relays are incredibly sensitive electrical relays that can detect even minute changes in the electrical circuit to which they are connected. When a circuit has a fault, an electrical arc forms inside the protective relay's contacts. The protective relay then activates a solenoid inside of the circuit breaker, which interrupts the circuit.
After the conditions in the circuit return to normal, the shunt trip activates the circuit breaker, restoring the circuit. Unlike the circuit breaker itself, which has an internal power source, the shunt trip device requires external power.