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A shocking experience December 19, 2010

Posted by mareserinitatis in electromagnetics, engineering, physics.
Tags: electrostatics, ESD, ground, triboelectric
41 comments

Shortly after I began doing some research on electrostatic discharge (ESD), I was having a discussion with some engineers about my work and how I was doing it. I got a couple of surprising comments.

The first engineer was actually a chemical engineer, and when I explained that part of my work involved monitoring static buildup on manufacturing equipment, this person asked me what this had to do with ESD.

The second question was frustrating: how can you have ESD unless you have a path to ground? The engineer in this case was an electrical engineer…and this is an example of a little knowledge being a dangerous thing.

I explained a bit before about triboelectric charging. Basically, when you rub two objects together, one will tend to want keep electrons and the other may be more likely to give them up. The result is that after you’re done rubbing these two objects, one will have given up electrons and become positively charged while the other took as many electrons as it could handle, resulting in that object being negatively charged.

If your electrons stay on an insulator, it’s probably not a huge problem. They aren’t terribly mobile that way. The real problem comes when this charge is somehow transferred to a conducting object. A really simple example is taking off your polyester-lined winter jacket while wearing a nice wool sweater. If that doesn’t generate charge, I can’t tell you what will. Anyway, it’d be nice if that charge just stayed on your sweater. It doesn’t: it gets on your body, which will behave like a conductor. When you reach your hand out to touch the closet door, *ZAP!*.

So the first engineer was partially right: a buildup of static electricity does not imply with 100% certainty that ESD can or will occur. However, if you prevent charge from building up on an object, even if it’s an insulator, then you’ll not have the opportunity to fry any sensitive electronics should it somehow get transferred elsewhere.

In the example above, you have an ESD event because you touched a door, which was presumably grounded. Interestingly enough, that doesn’t have to be the case. What you really need to cause an ESD event is for one object to be at a higher electrical potential than the other object. For instance, sometimes people get shocked after getting out of their cars in the winter. The car, you’ll notice, is not grounded: it is insulated from the ground by the tires, so electricity can’t flow to ground. However, a car is large enough that it can spread any charge it acquires fairly thin, leaving it at a very low potential. When you get out of the car, you touch the side, and you feel a shock. However, it’s not because the car is grounded: you transferred static electricity from your body to the car, which is at a lower potential.

The electrons on a conductor are going to repel each other as much as possible, and as soon as they see that they can go someplace else that has less electrons, they’re going to blow the joint. If the potential between the two objects is very great, then you get a large current, and the resulting discharge may be visible and possibly painful. If the potential is small, then the current may be minimal, smaller than can be felt or seen by the human body. However, when you’re dealing with electronics, even a tiny discharge can be damaging.

I’ve not worried about the first engineer’s question too much because I assumed his expertise relied on different knowledge. I assumed the second engineer should have known better. On the other hand, I realize that most engineers, when introduced to the concept of ESD on a technical level, are shown discharge models that almost always indicate that the current flows from a person (or machine) to the electronics and out to ground. I really wish they’d change those diagrams because it isn’t always true.

I always wished I’d had the opportunity to go up behind the second engineer with an ESD gun and zap him with it. That way, when he turned around and got mad at me for shocking him, I could tell him that it wasn’t possible because he wasn’t grounded (in more ways than one). Not that I’d ever really do it, but the thought crossed my mind more than once.