SK serial three-speed switch SK-3 serial three-speed switch of coil pipes of the blowing machine is used for controlling the three speeds, high/medium/low, of the coil pipes of the blowing machine in the central air-conditioning system (The maximum electric current is 3A.). SK-3D electronic three-speed switch, elegant in appearance, has a digital display of working conditions with a luminotron. (The maximum electric current is 3A.). SK-3J knob-type mechanical three-speed switch is simple but elegant in appearance (The maximum electric current is 3A.).