A propulsor (101) for an aircraft is shown. The propulsor comprises a propulsive fan (106), and an electric machine (108) configured to drive the propulsive fan. The electric machine has a casing containing electrical and electromechanical components, a shaft which extends outside of the casing and which is connected to the propulsive fan, and a seal to seal the casing around the shaft. A depressurisation system depressurises the casing below an external pressure to prevent electrical breakdown within gas in the casing of the electric machine.