06-05-2016, 10:19 AM
On 600V underground trains they used to have compressed air puffers to blow the arc out. The brake pump always used to hum when they were in a station and even modern ones with solid state speed controllers still hum when in a station. The air tanks needed to be at full pressure when they pulled out of the station for a good reason. They needed it to keep the relays going. Many switches are marked AC only and for a good reason.
