Static charge is more than a slight nuisance and a brief, surprising shock. It can cause damage to sensitive electronic components, raise the electromagnetic interference (EMI) or radio frequency interference (RFI) noise floor, and, in extreme cases, lead to fires and explosions.

Static charge buildup is unavoidable. If two objects slide past each other, electrons will be stripped from one object, as some of them are attracted to the atoms on the other object. There is no way to prevent static charge from occurring. The best plan is to find ways to bleed off the charge and prevent current from flowing at the wrong time and to the wrong place.

Static mitigation through grounding equipment

Static is a major threat and can cause problems in virtually any industrial operation. What can be done about electrostatic discharge (ESD)?

Most static mitigation techniques are done through engineering controls. For some operations, ensuring metal-to-metal contact between all components ensures that the components are grounded, and thus kept at the same potential. When this occurs, there is no way for a charge to build up, reducing the chance that static will try to jump across an air gap to equalize the charge between components.

Equipment can be statically grounded as well. For example, powder bin feeders have numerous small particles rubbing past one another, creating static charge. Through the use of proper grounding, static electricity will not spark. It also has the added benefit of limiting the “clumping” of powder due to static cling.

Chemical and fuel storage tanks are also grounded. All components are electrically tied together. One famous example of failed grounding was at the Barton Solvents facility, where a float had a rocking hinge that did not make constant contact with the cable that suspended it over the fuel. Over time, a charge buildup between the cable and the float sent a spark across the flammable vapor and caused an explosion.

Vehicles are particularly vulnerable to static discharge. Trucks, railcars, shipping containers and aircraft must be grounded before flammables are transferred. Source: Newson-Gale

In especially sensitive environments, there may be metal panels, tracks or flooring. Push carts and other moving equipment may have a chain or steel cable that drags along the metal flooring to reduce charge buildup on the cart. This is particularly true in semiconductor cleanrooms and pharmaceutical cleanrooms, where static on a cart could damage a product or cause powders to clump.

Grounding straps on plant personnel

For dealing with sensitive electronics, especially when technicians are handling material, they can reduce the static charge buildup on their bodies through the use of grounding straps. Grounding straps are wrapped around the wrist and connected to ground with a metal clip. The “ground” could be a special grounding rod near the workbench, a metal table, or any number of large, metal objects. The grounding strap is attached at the wrist so the technician can work freely, without too much interference in their range of motion, but also attached as close to workpieces as possible to bleed off any charge that may develop.

Besides grounding straps on the wrist, there are also grounding “cleats” that can be attached to a shoe or work boot. A small strap wraps around the ankle, or simply attaches to the back of the shoe. From there, a conductive strap lies over the back of the shoe to a metal or conductive nodule that rests under the heel of the shoe. This helps ground the technician as they walk through the facility.

Electronic static protection

There are numerous electrical designs that can minimize static damage in electronics and filter out static pops due to EMI/RFI. Capacitors placed between devices and ground can shunt static charges to ground. Because a static charge can act like an impulse signal, which can be thought of as a short duration, high frequency signal, the charge is shunted through the capacitor instead of through the device. Series-connected inductors can filter out a static discharge as well (as inductors block high frequency signals), but often the added inductance can cause other problems.

Zener diodes and transient voltage spike (TVS) diodes can be used to limit voltage differences, thus reducing the chances of ESD. Some combination of diodes, capacitors and inductors are used to protect the pins of many dual-inline package chips that are routinely handled by technicians. Without these mechanisms, the technician could cause damage without even feeling the spark.

Printed circuit boards (PCBs) can be designed to minimize static as well. Traces can be placed without sharp corners, where charges tend to collect. Large, conductive areas, such as thin sections of copper, can be placed on the PCB to serve as a place to drain charges.

Static dissipative materials

There are some circumstances where grounding is not desirable. Consider a large production facility with metal flooring and workers who are equipped with grounding straps. Suddenly, a forklift in the next room snags a wire and breaks it. The wire now puts a potential on the floor, which was supposed to act like a ground. Not only can this generate an electrical spark, it may electrocute the worker who is now at the same potential as the floor.

To mitigate this effect, there are static dissipative materials. Shoe soles, cart wheels, paints and other components are electrically resistive, but not totally resistive. These moderately conductive composites will prevent electrocution, but will allow for the slow bleed-off of static charge.

Static dissipative materials are like balloons. A balloon holds helium for a while, but over time, the helium escapes through the mouth and permeates through the balloon itself. The same is true with static dissipative materials; they can be charged, but the more conductive path, or the preferred flow of electrons is still through the material to ground instead of jumping across an air gap and sparking. They are a way to bleed off a charge without having a direct conductor tied to the ground.

Gloves, floor mats, benchtops and other commonly touched surfaces can be made from these static dissipative materials. Typically, they are a composite made from a polymeric material and the addition of a conductive phase, such as carbon black in relatively low volumetric percentage. There are other conductive polymers, and conductive polymer research is a hot topic in the materials engineering world.

Anti-static mat with a grounding mat for working on computers. Source: Wikipedia Commons

Final thoughts

Static charge happens, and there is nothing that can be done to prevent it from occurring entirely. Instead, the best approach is to find ways to mitigate the accidental discharge into sensitive objects or through flammable vapors. Through the use of grounding systems and static dissipative materials, the danger and the quality threat of static electricity can be greatly reduced.