Contactors are used to switch the mains supply on and off. Contactors are activated by the Charge Controller. Switching the mains supply is a safety measure. If an EV is not actually charging, the contactor in the EV Charger is switched off and the cable to the car and the car itself are disconnected form the mains supply.
The capacity of the contactor should be inline with the maximum current of the EV Charger. As often a system current of 16 or 32 ampère is used, the contactors do have a rating of 25 and 40 ampère respectively. Why is that?
The rating of a contactor is the thermal capacity of the contactor. It’s electrical rating is lower and dependent on the reactive power that the converter in the EV draws. The manufacturer does specify this. It is denoted as the ac-1, ac-2 and ac-3 rating. For a 25 amp (thermal) contactor, it is not unusual that the ac-3 rating is only 9 ampères.
Contactors are sometimes also referred to as relays.