How do shoe grounders work?
Answer: Shoe grounders use the force of gravity to bring electricity that is static in our shoes and lower body to ground.
Static electrons can sit on the surface of our skin. This can cause us to hold a positive or negative charge. The charge can then accumulate on us with greater mass if we are electrically isolated from the ground underneath our feet. This can happen when we are wearing insulating shoes- like tennis shoes or other rubber-based footwear.
Materials like wood and wool are also insulating. Walking up and down a wool carpet with tennis shoes can easily cause us to hold static on our skin and clothes.
When we go to handle electronics we want to make sure we are not electrically isolated, but instead electrically bonded. This allows those static electrons to discharge from our clothes and skin to the ground beneath our feet. Wearing a shoe grounder connected to an electrically bonded floor or mat is a common way to “ground” large assembly lines.
1. Heel Grounders
Heel Grounders are a specific type of shoe grounder that use a conductive black carbon rubber heel to ground the shoe.
Connected to the rubber heel is a conductive silver thread that tucks into your sock. The thread draws static electricity from your skin’s surface so that it grounds you and the shoe as one unit.
The way this works is that as the shoe grounders make contact with ground the electricity that was static on your shoes and skin is dissipated- literally falls into the Earth through the ground.
However, this means you should be wearing the heel grounder while standing on a grounded surface- typically a floor coated in ESD Safe paint or padded with ESD Safe matting. This way, you know the floor is not stopping the static from dissipating into the Earth.
2. Testing ESD Shoe Grounders
Shoe grounders should be worn and tested regularly if many people are working near electronics frequently. A charge can build up on clothes, skin, the floor, and tables causing unexplained machine shutdowns and device failures. In fact, one study conducted in the 1980s found that as much as 75% of their unknown device failures were attributable to ESD.
To test shoe grounders you would typically use a tester with a single metal plate.
ESD test equipment, like the combo tester, is itself typically setup to meet the requirements of ESD TR53.
This means most testers are typically looking for a reading from the shoe grounder of less than 1 x 109 (1 billion or 1,000,000,000) ohms / square. This verifies that the shoe grounder is, at a minimum, static dissipative.
1. Western Electric North Andover Works (1983). “ESD – How Often Does It happen?” G.T. Dangelmayer, EOS-5, pp.1-5, 1983.