You can correct the arc (EMF feed back) problem by adding a diode across the Snap Disc's tab. ...
Maybe Sean or other electronic tech can chime in before I get back.
Be glad to. What Gerald is talking about is the tendency for any inductive DC load (meaning something with electromagnetic coils in it, such as a motor) to generate a high voltage on its operating circuit when it is switched off. This is due to the collapse of the magnetic field.
The voltages thus generated can be so high that they are known to fry other components on the same circuit, such as an indicator lamp, for example. More importantly, the voltage can cause an electric arc across the contact points of the switch as it is being opened. This tends to shorten the life of the switch, and/or cause resistance in the switch (when closed) which adversely impacts performance of the device.
One simple way to deal with this is to install a "snubber" across the switch contacts. Commonly, this is a diode with a reverse voltage rating a little higher than the operating voltage of the device. If you have a "24 volt" bus, you'd want a diode with a reverse bias greater than 30 volts. The diode is installed parallel to the switch, "backwards," by which I mean in such a way that it will not conduct current to operate the motor.
All of this, of course, applies to DC circuits only. If you have an AC fan motor, this is not applicable.