This question crops up about November, every year. Colder dryer air allows static to build up and cause sparks.
Some info on how static builds up:
With rotating dissimilar insulating objects that have a small contact area (car tires,grinding wheels,VandeGraaff generators), electrons are removed from one surface and build up on the other. Normally, they bleed back the overflow and all is even. When the conditions don't allow the bleed-off (dry, cold, fast moving surfaces) ,the build up can get into the 1,000 to 10,000 volt range. At this point, nature takes over and the imbalance cancels itself by jumping to a larger reservoir -the earth- which you are connected to. This is what a spark, or lightning, is.
To prevent it you have to allow the different potentials to constantly equalize.
Raising the humidity (pan of water on the stove was one common way to keep down static in households.), changing the dissimilarity of the objects (leather soled shoes, different rugs/floor covering, different clothing), or providing a path for the electrons to flow through ( ground straps, static strips).A simple ground strap is the easiest way to deal with it in the shop. There are some things that need to be dealt with though. First, it is imperative that all equipment be properly grounded to an earth ground. Water pipes don't do it well anymore. Second, an anti-static mat will help transfer the charge from you to the floor more efficiently. Third, if using a ground strap, it must allow the static BUT NOT THE AC current to flow. This is done by putting a 1 megohm resister in the circuit. Attach a 1 meg resister in the center of four foot piece of ball chain (hardware store).Wrap it in electrical tape to strengthen the joint. Attach a good size alligator clip to one end, and a 1X3" piece of copper to the other end. Stick the copper strip wherever it gets a good body ground ( In the boot/shoe side, in the waistband,etc.). Clip the chain to the machine, out of the way of all rotating parts.
This set up will allow the chain to unclip if you walk away, or the chain to break if caught in rotating equipment. Most importantly, it prevents the flow of AC if the motor should develop a ground fault. My strap has a clip on both ends, and I put a "U" shaped piece of copper in my waistband,clipping the chain to that. This allows the chain to just slip off if I walk away.This whole setup can be made for about $5, and will completely eliminate static shocks. Changing the shoes, using an anti-static mat, adding humidity and warmth to the air will all help,too.
OK, science lesson is over for today.
Stacy
