An arc monitor for tripping a circuit breaker in the event of an arc occurrence, the arc monitor including at least one arc detector and arranged to be operatively connected to at least one circuit breaker. The arc monitor includes a base unit and at least one extension unit with at least one arc detector input terminal, the at least one arc detector being connectable to the at least one arc detector input terminal, and the base unit includes a control unit. The extension unit is operatively connectable to and arranged to send an arc occurrence signal to the control unit, which is arranged to send a trip signal for tripping the at least one circuit breaker, in the event of an arc occurrence. Also an electrical installation including an arc monitor, and the use of an arc monitor as a supervision and safety equipment in an electrical installation.