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As the gentle pitter patter of rain begins, it often evolves to the flashing of lightning followed with the boom of thunder. This lightning, damages electronic instrumentation everyday. Electronic equipment is typically damaged when transient over-voltages cause a breakdown in the insulating layers within an IC. Transient damage happens even with the very small charge a human body stores on its surface (in which case we call it ESD or ElectroStatic Discharge). Both the small charge transferred in ESD and the enormous charge lightning transfers can damage electronics in transducers and instrumentation.
Perhaps surprisingly, the cost of diagnosing and correcting a transient induced failure can be many times the cost of the equipment. A transducer may cost a few hundred dollars, but by the time you put a technician on a plane, fly him to the site, house him while he waits for replacement parts, the cost of the transducer is insignificant. Still worse, closing down a plant operation may pale the just mentioned losses. Whenever a business is losing money, the responsible engineer is in a stressful situation. Thankfully, by understanding the difference between common mode and differential mode transients you can increase your 'luck' and reduce damage to transducers and associated controls caused by lightning.
Correlation and anecdotal evidence are twisted into cause-and-effect in both preventing elephants and transients. Salesmen blur the distinction between conductive and anti-static plastic when dealing with ESD. Extra 'crowbar' protectors are installed, even when there are no signs of power mains over-voltage. Why are lightning and ESD two problems, that like elephants, seem so difficult to get one's arms around?
First of all, there is no easy way to test lightning suppression. There are ways to produce modest discharges to test our assumptions but this takes time and money that is rarely available. Confusing the problem are a flood of anecdotal stories and 'expert' salesmen all offering unqualified help. Also, no engineer will ever get credit for taking the time to scientifically analyze the problem and implement a cure to reduce lightning damage; the reduction is always assigned to luck! I know you have heard it. Remember, it goes something like this:
"There sure was a big storm last night. Hubert saw lightning hit the utility pole. We sure were lucky that it didn't take out any equipment"
As is the fate of engineers, we do our job knowing that the result of the work we perform may not be fully understood or appreciated.
It turns out that the amount of lightning striking the earth varies greatly depending on where you live. "Thunderstorm-days per year" is a number (besides being a tongue-twister) that we define for any given location, as the total number of days on which we hear thunder at least once during that day, per year. In the US the number can vary from 5 to 90. A number between .05 and .8 multiplied by the thunderstorm-days per year gives you the number of lightning strikes per square mile per year. According to this calculation, here in Kansas with 60 thunderstorm-days per year we have between 3 to 48 lighting strikes per square mile per year. That is about average for the US -- in the tropics there are many places with 100 to 200 thunderstorm-days per year.
Not all lightning is created equal. About 30% of all lightning strikes have a peak current of over 10 kA, while about 10% of all lightning strikes have a destructive current of over 50 kA (yes, that exceeds the rating of the protector you just bought!) A percent or two of strikes get over 100 kA! Strikes have even been recorded with current peaks well over 200 kA! Because power goes up with the square of the current ("Twinkle, twinkle little star/ power equals I squared R") we are dealing with quite a range of power!
Of course this current is shunted to ground by the protectors on the utility entrance - or is it? A typical lightning rod (8' X 1/4") has an impedance of about 50 ohms in good soil (dry climates may be much higher) Dusting off ohm's law, and scratching out the numbers, we see that 50 ohms times 10 kA is 500,000 volts! Or is it? Well, we did neglect the inductive reactance of the ground wire at the 100Khz to 100 MHz associated with lightning. You might notice by now that it isn't a pretty picture even using gentle numbers.
The fact of the matter is that lightning will find a network of paths to ground by arcing over to whatever is at a lower potential. Your power line protector will do a fair job at suppressing the differential voltage that becomes superimposed over the normal power line voltage.
Understanding the difference between differential and common mode transients is key to analyzing susceptibility to lightning damage. Protectors (in the form of MOV's, gas discharge tubes, zener diodes, etc.) connected across the power line suppress differential transients. Protectors connected from line-to-ground 'tend' to suppress common mode transients. (I use the word 'tend' because the suppression is severely limited by the impedance of the ground connection.) Another way to distinguish differential and common mode transients, is to think of differential mode as a transient injected with a parallel connection and common mode as a series connection.
The local ground connection may not be at ground potential. A little thought experiment may provide some enlightenment.
Imagine, if we could freeze time at the instant a lightning strike connects, we would see that the local ground has a large voltage at the center of the strike due to the limits of the earth's (as in dirt) impedance. As we move away from the strikes center there is a voltage gradient.
Imagine further, concentric circles with the lightning stroke's connection with ground at the center. With your feet about shoulder width apart, do you want to be standing parallel or perpendicular the voltage gradient? The unfortunate sole who makes the wrong choice could be killed by the strike with his two feet at vastly different potentials, while someone next to him standing with both feet on the some voltage gradient line can walk away unscathed.
While holding this visualization for just a bit more, we can see that for any direction one faces it is possible for lightning to strike at a point that exposes our feet to ground connections of different potentials.
Finally, bringing the above story back to equipment damage prevention, we need to assume that any two ground sources entering a building can have a large voltage difference when lightning strikes nearby. The common mode transient, as defined here, is a transient voltage difference between two local ground sources.
Common mode transients, can be of great concern when running wires from instrumentation at one ground source to another. If the electric utility ground connection was the only ground entering your building, there would be little problem because if everything would go up and down in voltage together, there will be no electric discharge.
Ah, but life is never so simple. There is a ground source from the plumbing entrance, another ground comes with the telephone lines, one more with cable. Bonding them together with heavy, low impedance cable at a ground pavilion helps. For example, the phone company will try to run the phone line near the electric service entrance and run a wire that bonds the telephone ground to the electric power ground.
Other ground sources are more troublesome. Plumbing is a very low impedance ground source and often enters on a different side of the building than the electric utility ground source. The building frame and concrete flooring also offer a low impedance to ground.
Failure to understand common mode transients is the number one cause of improper installations! It appears that the understanding of ground pavilions, why and when to use them is not something that gets taught in school.
Suppose we take the example of a pressure transducer screwed into a pipe at the bottom of a water tower. Imagine next a lightning strike connecting with the power line's shield ground wire (ground source 1). The utility ground ends up with a large voltage compared to the ground of the water pipe (ground source 2).
What if a transducer and instrumentation form a current path connecting the two grounds? The water pipes here may be the lowest impedance ground for the power grid. In this case it is not uncommon to see damage to both transducer and instrumentation.
The first 'fix' usually is to install is a ground wire running from the instrumentation cabinet to the transducer body. While a noble attempt, examination of the voltage drop due to the impedance of the ground wire will show that the case of the transducer can be 10's of thousands volts above the signal wire. This high voltage can destroy transducers and instrumentation input circuitry. (To demonstrate this impedance in lightning seminars, I connect a ground wire between the case of a transducer and the ground of an equipment box. I also connect a flash bulb in parallel with this ground connection. After showing and explaining this setup, I take bets on whether the bulb will flash or not when I apply a little man-made lightning to the transducer case. I always win the bets!)
A much better remedy is to isolate the transducer body from the water pipe with several feet of nylon tubing to provide a high impedance water channel. Also, place the transducer body in a short length of PVC pipe to insulate it from other 'ground sources'. Remember that high voltage will 'arc over' (about 10,000 V per inch) so you can have a ground connection without a physical connection. The most important thing to remember in applying transducers is to avoid introducing ground-sources that can be at quite different voltages than the ground of yout instrumentaion panel.
My favorite example of a transducer body causing problems, is the submersible pressure transducer. It typically has a stainless steel body that forms an excellent electrode. In a water tank it will often form a connection with ground. Used in media that is highly conductive (such as sewage) things get even more interesting. Just like the best ground rods that secrete conductive salts to lower their ground impedance, a metal transducer body will also form an excellent ground connection in the presence of salts. While a few manufactures make insulated body submersibles I have yet to see such transducers promoted for their resistance to lightning transients.
You may have noticed that, over the years, stereos and other consumer electronics have evolved from metal cabinets towards all enclosure surfaces being insulated. While a metal enclosure may divert ESD, the ESD can still cause damage via magnetic and near field coupling. The answer is to eliminate the discharge all together by using all insulating material. Eliminating your stereo as a ground source for voltage emmiting fingers prevents the discharge that causes damage. The future belongs to sensor manufacturers who understand that this effect applies to lightning borne discharges in transducers as well.
1 Uman, Martin A., All About Lightning, Dover Publications, Inc.,New York, 1971,1986, pp56-57
2 Uman, Martin A., Lightning, Dover Publications, Inc.,New York, 1969, pp127
3 Morrison, Ralph, Grounding and Shielding: Circuits and Interference, second edition, John Wiley & Sons, Inc., 1977, pp138-139
Pressure Transducer Primer
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