ESD Affects People, Too

In June, we published a Made By Monkeys blog by Dwight Bues about his experiences with ESD (Electro-Static Discharge). ESD, while not necessarily permanently harmful to people, is still a pain (no pun intended). When possible (and most times, it is), a machine design should handle ESD to prevent discharge through people or other equipment.

Recently, I designed the controls for a material handling system, which fabricated 4-ft x 8-ft composite panels. The panels had composite top and bottom skins and a honeycomb core. To make the panels, operators started with a large flat aluminum plate, then layered a sheet of sacrificial paper, the bottom composite skin, the honeycomb core, the top composite skin, another layer of sacrificial paper, and then a top plate. Up to 12 of these stacks would be put into a multi-opening press; the press would apply pressure and heat until the panels were fully cured.

When it came time to remove the panels from tooling, the operators would peel off and discard the paper from the top and bottom. They complained of being continuously shocked as they peeled the paper away. These shocks were as bad as the one you might get crossing a carpeted floor in winter time and then touching a metal surface (or your sister's ear), but instead of a single shock, they'd receive dozens. This problem was raised to HR/safety status rather quickly; their solution was to give the operators static discharge wristbands.

What's wrong with that solution? Static discharge bands prevent a charge from moving from a human to whatever needs ESD protection; they prevent the charge from traveling from the human to the sensitive device. The ESD wristbands will do nothing to help the human when the charge moves in the other direction; the human is still part of the electrical circuit.

After watching the operators perform their task several times, I hunted down a piece of copper pipe, clamped some stranded 14-gauge copper wire to one end, and clamped the other end of the wire to a nearby steel building column. I had the two operators hold the pipe right next to the panel/paper interface as they peeled the paper off, moving the pipe along as they peeled. They commented that the static discharge problem, while still there, was greatly reduced. The pipe and wire had lower resistance than the human operators; the charge flowed through the copper instead of the humans. It was a somewhat awkward solution, having to wave the pipe along the panel as they peeled, but they weren't being painfully shocked anymore.

I recommended to the operations manager that the pipe be replaced with a proper active static eliminator, one that emits positive ions at a negatively charged surface. There are a number of manufacturers that make such systems; any of them would have completely neutralized the static charge, which built up as the paper was peeled.

Knowing which way the charge moves is important when deciding how to dissipate static electricity.

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