interrupting time

Interrupting time is the elapsed time from the instant a trip signal energizes the breaker's release mechanism until the arc between the main contacts is fully extinguished. In practice, this is measured in milliseconds, typically between 20 and 150 milliseconds depending on breaker type and voltage class. It is a critical performance metric because the longer an arc burns, the more energy it dissipates as heat and the more damage it inflicts on the contact surfaces and surrounding equipment.

The process itself unfolds in discrete stages. When the trip coil energizes, a latch mechanism releases and allows a spring or stored energy system to drive the movable contact arm open at high speed. As the contacts separate, an arc forms across the gap and continues to conduct current until the contacts are far enough apart or the current naturally falls through zero (in AC systems). The arc is then quenched, usually aided by arc chutes, magnetic blow-out coils, or gas puffing chambers that cool and deionize the arc column.

Variants and measurement

Low-voltage breakers typically interrupt in 50 to 80 milliseconds. Medium-voltage breakers may take 100 to 200 milliseconds because the higher voltage sustains the arc longer and requires more aggressive extinction methods. Medium-voltage breakers often employ separate arc-quenching chambers and may use vacuum or SF6 gas to achieve rapid arc collapse. High-voltage circuit breakers use even larger interrupting chambers and can take several hundred milliseconds. Interrupting time is always measured under rated load or fault conditions specified in standards like ANSI C37.04 or IEC 60947.

Do not confuse interrupting time with opening time, which is the pure mechanical delay before the contacts begin to separate, or with arcing time, the duration the arc actually burns. A breaker can have a short opening time but a long interrupting time if the arc is stubborn to extinguish. Conversely, a breaker designed with aggressive arc quenching may have a shorter interrupting time even if its mechanical opening is slightly slower.

In short-circuit protection schemes, interrupting time matters because it sets a hard limit on how much energy the breaker's contacts and surrounding components must withstand. A breaker rated for 10 kA interrupting current will successfully stop a 10 kA fault in its rated interrupting time, but attempting to interrupt a much larger current can cause contact welding, arc chute failure, or violent expansion of hot gas. This is why circuit breakers are always selected not only for nominal current but for their interrupting rating at the expected fault level on the circuit.