It means a space between the edge of a exhaust port and panel component ( such as wall or duct) of a circuit breaker. The arc space is needed for smoothly discharging ionized gas from a exhaust port of circuit breaker power supply side which occurs when the circuit breaker trips short circuit current. ( In the case that the gas is not discharged smoothly, some troubles such as decreasing the breaking performance of the circuit breaker may occur. Therefore, it is necessary to secure enough space in front of a exhaust port.)