By sending a command for contact opening very accurately before the instant of current zero crossing to provide a high velocity of the contact moving. It is done to ensure a sufficient distance between contacts by the instant of current zero and the arc extinction to guarantee the required electrical strength of a contact gap and eliminate arc reigniting. The accuracy of the command sent for CB opening is determined by a switching controller, while a high velocity of the contact moving is provided by a special CB mechanism.
Fig shows principles of controlled opening. A command for CB opening is generated at a random time t command and send to a switching controller. This command is delayed by the controller for a certain period T total.According to ➊, the time interval T total consists of a controller response time Tresp and an intentional synchronization time Tsync. The time interval Tsync is determined in relation to the instant of current zero and depends on an opening time of a CB Topening and a time of the contact separation t separate for a specified distance to provide the required electric strength of a contact gap.
- Ttotal = Tresp+Tsync
- Tsync =Tzero–Tarcing–Topening