Shop Tips - Electrostatic Discharge ESD

20 Jun 2015

Even though it is summer in the northern USA, with good humidity levels, some new flooring and polyurethane wheels on my shop stool have me dealing with electrostatic discharge. I first noticed the problem when I reached for a tool in the cabinet and the Fluke Volt Alert tester triggered with a bright red flash. A series of experiments were performed and it was confirmed that the chair and the new flooring were perfect for destruction of electronic components.

The polyurethane wheels of the shop stool were stripping electrons from the flooring material and as a result the chair and my body were being charged to a high enough voltage to create a detectable EM field when the charge jumped from my finger to the metal of the tools.

Have you ever noticed that large trucks, particularly those transporting flammable materials, will have straps hanging down from their metal structure and touching the ground. These are discharge straps and they work by returning the accumulated charge that is produced by the tires to the roadway. The strap completes the circuit and prevents the voltage build up on the frame of the truck. This was also the first approach that I took to solve my problem. In my case a small brass bead chain was used. The bead chain was looped around the metal structure of the stool and secured with a small nylon strap. The chain was made just long enough so it dragged slightly on the floor. There was no need to make a good mechanical connection as the voltages involved are high enough to bridge any inherent resistances.

This fix solved the immediate problem and I could no longer trigger the Fluke Volt Alert regardless of how I slid and shuffled the stool around the office. However, the warm humid weather of northern Wisconsin is relatively short lived and by October the cold will return and with it the very dry static producing air of Winter. Therefore a second level of protection was needed to properly protect the electronics experimentation areas in the shop.

One of the established solutions to ESD is a wrist band that is worn by the person at the bench that ties to ground and prevents static discharge. I have tried this method in the past and still use it when I am dealing with critical low voltage components and circuits, however I find it clumsy, confining and uncomfortable. Since working with mains power is often part of my work, the wrist band makes me feel like a lightning rod in a storm. I am more comfortable feeling that I can let go of ground if necessary.

Therefore I have decided to add a copper strip to each work bench that is resistively linked to ground. The copper strip sits at the front of the bench and is frequently in contact with the arms and wrists. There are 4 areas in the shop that will need these strips. The benches are 150 cm long and a strip that is 1.25 cm wide will be just right for my application.

I found some long 1.25 cm wide copper strips from Jason Carver at a company in Utah called Metal Remnants

http://www.metalremnants.com/

While the copper pieces were cut very nice and straight they had bur edges that were sharp and dangerous if not removed. While my shop isn't really equipped to deal with this type of metal work I found that if I stretched the strip tight between two vises and then used a sanding block I could burnish the edge and make them safe to use.

The strips were screwed to the metal framework of the table on one side and then stretched across the top of the table roughly 5 cm from the edge. At the other end a turnbuckle was tied to the framework of the table and tightened to pull the strips as tight as a guitar string.

The strips on the workbenches are finally tied to Earth Ground through a 10K Resistor. Since it is not uncommon for me to build or repair equipment with mains voltage I did not want to tie the copper strip directly to Earth Ground. The 10K floating ground of the strip will quickly dissipate any static that may be on my body but at the same time will limit mains current to less than 10 mA should I accidently find myself an unwilling conductor. A second precaution that I use when working with mains voltage is isolation. The mains voltage on the bench that is used for testing does not have a neutral line tied to Earth Ground and as such produces very little potential difference to the copper static discharge strip. Never, however, do I make any risky assumptions about mains power. Always assuming that it can kill you is the safest attitude.

The drop chains from the shop stool and the Static Discharge Strips on the benches will give me the protection from static that is needed to work with sensitive electronic components. For those of you who have come to expect it of me, I labeled the strips.