If you don't have them installed: 300 is your limit.
Switch position (it's not a switch but a button) doesn't matter, because ECAM and the button tells you if the fans are working or not. If there is a problem you have an ECAM error message.
The difference comes from the fact that the temperature sensor is an air temperature sensor, and if there is more airflow through the brakes discs through the fan, there is more colder ambient air flowing around the sensor, which lowers the indicated temperature. 300 with fans off and 150 with fans on gives you about the same disc temps.
Dani