Some science behind the scenes

Static electricity, energy and voltages

The energy released in a static electricity discharge may vary over a wide range and is dependent on the capacitance (C) of the object and the static potential V in volts (V); it is expressed by the formula E= ½CV2. The capacitance of us obviously varies a lot. 

But the maximum potential is limited to about 35–40 kV, due to corona discharge dissipating the charge at higher potentials. Potentials below 300 volts are not typically detectable by humans. The maximum potential commonly achieved on the human body can range between 1 and 10 kV, though in some conditions as high as 20–25 kV can be reached. Low relative humidity increases the charge buildup; walking 20 feet (6.1 m) on vinyl floor at 15% relative humidity causes buildup of voltage up to 12 kilovolts, while at 80% humidity the voltage is only 1.5 kV.

Current

Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points, and inversely proportional to the resistance between them. The mathematical equation that describes this relationship is:

where I is the current through the conductor in units of amperes, V is the potential difference measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms. 

If we thus summarise, the intensity can vary considerably depending on how big we are [capacitance], the voltages that have built up [which will depend on the humidity and the length of time the charge has been allowed to build up] and the resistance of the particular bit of us that gets the shock. Overall we coud end up with a big shock or only a little shock – which is what we know from experience. 

To keep the intensity down it helps to be in high humidity conditions and not let the charge build up too much by prolonged exposure. 

A very funny story over the page will help to show this. It shows the effects of humidity and the prolonged build up of charge.

The Final Frontier – the ESD Journal -
Paraphrased and dramatized by Cynthia Waters

In August of 1980 in the southeast United States, problems were being experienced at a polypropylene plant. Strange things were happening around a film slitting machine. David Swenson of 3M Electrical Specialties Division in Austin, Texas was called in to investigate. Little did he know that he was about to enter the final frontier.

When Dave asked what type of problem they wanted him to look at, he was told that he would have to come to the plant. They could not explain it over the telephone. They did tell him that they were experiencing problems with contamination of wide web film as it was being run at high speed, converted (split) into "film jumbos" with a width of 3' x 5' for coating with adhesive to make tape. 

The Polypropylene web was 21 feet across - almost the width of three lanes of traffic on the interstate. ?The film ran from one roller up 20 feet to another roller; across 15 feet to a third roller; down 20 feet to a splitter; and was then wound onto two rolls. It formed a huge dynamic "tent". On the morning that Dave intended to measure the static electricity inside the web tent , the temperature was 80°F with a relative humidity of 75 to 80%.

As he walked toward the web with his field meter in hand, the machine operator said "I wouldn't do that if I were you. Strange things happen inside the web tent this time of day." Dave paused and thought about what the man had said but he was there to do a job and that was what he intended to do. You see, Dave is a very brave guy. Besides, it was just film running on a splitting machine. He had seen that many times and nothing seemed particularly unusual that morning. That is, until he looked down at his static field meter.

With a 200kV @ 12" measurement device, the voltage measurements were off the scale and he was still 20 feet away from the web! For the meter to be pegged at that distance, the amount of static electricity had to be in the Megavolt range.

Since he had come this far, Dave decided to enter the web anyway. As he did so, the hair on his head, arms and entire body began to stand on end. Even worse, he began to feel like sparklers were hitting him all over his body. It felt like burning sensations at hundreds of points all over his being. At exactly one-half way into the web, David slammed into a wall that was both invisible and impenetrable. It felt like he had hit a force field like he had seen on Star Trek. 

Although he knew this was Coulomb's Forces - the law of static attraction and repulsion - he was held in place for what seemed like ages as he tried to grasp what was actually happening to him. It's one thing to know about Coulomb forces, but it is quite another to feel them with such gusto. When he finally came to his senses, he had to "peel" himself off the invisible wall. 

As he backed out of the web, he saw a fly drawn right up into the film and wondered if conditions ever existed inside that web that could actually draw a person up there as well. Dave hurried to talk with the Plant Manager and excitedly explained what had just happened to him. The Plant Manager agreed to go to the web to see for himself what was going on. By the time he was able to go, however, it was later in the day and the temperature and relative humidity had risen. He was able to walk into the web and from one side to the other without experiencing anything at all unusual.

When the machine operator corroborated Dave's story, the Plant Manager agreed to meet early the next morning to see if the same conditions would exist as they had experienced earlier in the day. They met bright and early the next day. As the Plant Manager entered the web, his curly grey hair immediately straightened. As he strutted toward the centre of the web, his confident gait suddenly came to a complete stop as he hit the wall of invisibility.

Once he peeled himself off the force field wall, a shaken Plant Manager came out of the web and exclaimed ...  that he "didn't know whether they should fix it or sell tickets".

They decided to fix it.