My whole house attic fan has a three way switch in the house. In the late fall I cover the fan with an insulation blanket to keep heat from escaping from the house into the attic.
As a safety device, I installed an on/off switch in the attic as well.
This year I failed to toggle it to the off position.
Sometime this winter the switch in the hall got toggled to high speed. The blanket was keeping the fan from spinning.