Shock from static electricity can be more than a brief, unpleasant sensation; it can ignite flammable vapors, damage sensitive electronics and may even trick some electromagnetic sensors.

While most people think of static as simply the little blue “spark” they see on a cold winter day between their hand and a metal doorknob, plenty of children have repeated this experiment by scuffing their feet on the carpet and shocking their friends. As it turns out, this “spark” has a whole lot more potential (pun intended) than most people realize. It is something that can become a major safety and quality concern in manufacturing, chemical and materials processing.

What is static shock?

Static charges are electrical energy that is stored up on two surfaces. As one material moves past another, such as scuffed feet against carpet, a few of the electrons on one surface might move to the other surface. This creates an electrical charge imbalance. If enough of them move from one surface to another, the electrical potential between the two surfaces can be large.

In the case of the scuffed feet on the carpet, the person’s body becomes charged, and when they reach for a doorknob, or some other object at lower potential, the voltage is high enough to cause electrical current to flow through the air to the doorknob.

The child slid down the slide and built up a charge. Now her hair stands up to try to spread the like charges out as far as possible. As soon as she discharges the static, her hair will return to normal. Source: Wikipedia Commons

In order for this jump through air to occur, voltage differences between surfaces must be large. Air is not a very good conductor, so there needs to be a lot of electromotive force to push the electrons through it. Once they do, the electrons flow quickly and dissipate the charge imbalance. Roughly, the voltage is typically on the order of tens of thousands of volts, but the current is only in the microampere range. This is why the jolt is felt, but they are not electrocuted: high voltage and extremely low current.

Ignition source

Perhaps the most dangerous aspect of static shock is not from the shock itself. That little spark can be an ignition source for starting fires and explosions. Consider the manufacturing of flash powder or fireworks — a brief discharge through the air and its extremely hot core channel of current flow can easily ignite the explosive.

However, it’s not just the manufacture of explosives where static discharge is an issue. Any flammable substance can ignite for this same reason. Flammable vapors, such as those found in refineries, drilling operations, fueling stations, chemical manufacturing, pharmaceutical manufacturing and even some food and beverage production lines can emit flammable vapors that can be ignited by static electricity.

Besides actual explosives and flammable vapors, many dust explosions are traced to a static discharge. Grinding operations that reduce material to small particle sizes that can be dispersed in the air are particularly vulnerable to dust explosions. Therefore, grain milling, sawmills, sanding, grit blasting, powder feeding, grinding and polishing, and many other common industrial processes generate flammable dust, most of which is sensitive to static shock ignition.

Food manufacturing facilities, such as flour, sugar and corn processing, are particularly vulnerable, as the threat is often underestimated. It can be difficult teaching technicians that flour — the same substance that is in their lunch sandwich is a potential hazard. When dealing with flammable liquids, there is fair respect given to the potential hazard, but common food products are often overlooked.

Damage to electronics

Besides being a safety hazard, static electricity discharges can cause damage to sensitive electronics. Microelectronics that are used to seeing a few volts at most may be exposed to a potential difference of tens of thousands of volts. At the microscopic level, this jolt is no different than an arc-flash hazard, which can melt or even vaporize material.

The resulting discharge blasts rapidly expanding vapor clouds and molten material across the surface of the microelectronic device, physically and chemically deforming it. Electrostatic discharge (ESD) damage can range from punching holes in dielectrics, removing fine traces, melting solder connections and causing them to open circuit or short circuit against another conductor, and numerous other failure mechanisms.

A capacitor that suffered ESD damage. Source: NASA Electronics Parts and Packaging Program

What is particularly troublesome with ESD damage in electronics is that the static shock does not have to be felt. Recall that a “felt” shock may be on the order of 30,000 V. It is possible to have a potential difference of hundreds or even thousands of volts without physically feeling the shock. The charge is simply transferred and wreaks havoc on the microelectronics.

False sensor readings

In the study of signals and systems, there is the concept of an “impulse” signal. This signal is infinitely brief in time, yet transmits a signal on all frequencies. While static electricity is not infinitely brief, it emits plenty of radio frequency interference (RFI) across many frequencies. It’s not a perfect impulse signal, but it is close.

The RFI can cause sensor readings to jump, become unpredictable and give false readings. Many devices cannot tell the difference between a nearby static discharge and the true signal, and so the RFI from the static is interpreted with the signal.

Playing with static

A simple holiday light tester can be used to demonstrate the buildup of static electricity. These devices are typically used to find the bulb that has burned out, (the old ‘if one light goes out, they all go out’ series connections). Where there is a potential difference, the LED on the device lights.

As an experiment, this device can be rubbed on carpet, clothing, even bare skin. The friction causes a charge difference which then dissipates, but it is typically below the detection threshold of the skin. The end result is the LED on the device lights up briefly as the charge builds, but then goes out as it is discharged. It looks like a flashing light.

Holiday light tester can help determine which light (connected in series) is burned out, or as a tool to learn about static electricity. Source: EveryMarket.com.

Also, consider that a lightning storm is the ultimate display of static electricity. To demonstrate the impulse-like response and RFI from static, turn on an AM radio during a lightning storm. Each bolt of lightning will likely produce a static crash on the radio.

Final thoughts

Static charge will continue to be a constant source of hazard in virtually any industry or manufacturing facility. It is both a safety and a quality hazard, yet it is hard to predict, model or calculate. The best approach is to find ways to mitigate the buildup of static and prevent current from jumping to unwanted places, such as through flammable vapors or onto sensitive equipment.