Ac contactors are used for high-current load applications they are designed to control and reduce the arc produced when the heavy motor currents are interrupted.  Other than the low current contacts, they are also setup with Normally Open contacts.  These are devices which handle more than 20 Amperes current and over 100 Kilo Watts power.

When current is passed through the contactor, the electromagnet starts to build up, producing a magnetic field. Thus the core of the contactor starts to wind up. This process helps in energizing the moving contact. Thus the moving and fixed contacts make a short circuit. Thus the current is passed through them to the next circuit. The armature coil brings in high current in the initial position. This reduces as soon as the metal core enters the coil. When the current is stopped, the coil gets de-energized and thus the contacts get open circuited.