It detects electricity pretty much. I’ve set one off twice. First time i tried to cut formica laminate with aluminum in it, the metal creates an electric charge so it kicked the stop.
Second time I was cutting a flat pvc sheet and the static electricity from the plastic kicked it.
I know a guy who knicked his finger with one, it just barely broke his skin.
You can turn the stop off with a key if you want to cut off your finger.
I was doing a project at a freinds house, he handed me a peice of wood and asked if i could chop six inches off the end of it, the wood was slightly damp because we were working outside.
Id never seen an auto-stop before and didnt know he had one. As soon as the damp wood touched the blade there was a loud bang and the blade was gone. I thought the blade had hit a nail or something and shattered.
Scared the hell out of me, cost him close to $100 for a new blade and stop. Seemed effective though, the wood barely had a mark on it.
Capacitance.
But before we discuss that, let’s touch on conductivity.
If those safety stops worked on conductivity, then wearing rubber soles boots, which are a better insulator than wood, would disable this safety.
Rubber soles boots don’t disable this safety, so that’s not the correct answer.
Capacitance is the correct answer. Let’s get into this.
A capacitor holds an electric charge (like a battery), but losses that charge quickly (unlike a battery).
Most things act like a capacitor, holding a bit of charge. This means if you touch most things to a battery, there will be a tiny, but measurable, current flow out of the battery.
dry wood is a crappy capacitor. It holds very little charge.
The human body, or a hot dog, or a bucket of salt water, all of these are also crappy capacitors, but they are orders of magnitude better than dry wood.
So if you electrically isolate the blade, and tricke a bit of voltage into the blade, and measure how much current is flowing, most of the time it’ll be almost none.
But if you get a little jump in current, that means something with better capacitance than dry wood has touched the blade, like maybe a finger, or a hot dog. Trigger the brake.
This is irrespective of how well insulated your boots are, since it’s measuring capacitance, not conductivity.
It’s also why wet wood can sometimes trigger that safety. Wet wood has a higher capacitance than dry wood.
If for some reason you want to test this, hold a hot dog to the blade, not your finger… 😉
In a similar way a modern touch screen can tell the difference between a finger and a finger with a glove on it: Capacitance.
Or, like you’re 5, the way electricity flows on what is touching the saw blade.
Flesh has a unique enough interaction with electricity that can trip a sensor fast enough to stop a spinning saw blade before a deep cut happens.
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