On/Off Switch
In a petrol-powered chainsaw, the on/off switch stops the engine running by preventing the ignition coil from firing. It must be clearly marked with the stop position. There must be a positive click action, so that there is no chance that the switch will change position accidentally, even while the saw is vibrating under heavy load.
If the switch were to move to the off position while the operator was in the middle of a critical cut while felling a tree, there would at least be a delay, which increases the chance that the tree might fall in an uncontrolled manner. Also, the saw might jam in the cut, requiring that the operator spend extra time under the unsafe tree freeing it. A switch failing "on" would also present a hazard, since the saw may then start inappropriately, such as when testing compression or assessing starter function.
If the operator is injured while using the saw, a bystander might have to move in to turn the saw off. The bystander may be unfamiliar with the saw and needs to be able to identify the on/off switch by its markings.
The Stihl model shown combines the choke, throttle start setting and ignition switch into one unified control lever. Other brands of saw usually have separate controls for all three, or a combined throttle start setting and choke control with a separate ignition switch.
If the switch fails to operate, a saw may be reliably stopped by operating the choke control to flood the engine.
Read more about this topic: Chainsaw Safety Features
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