Lightning Failures in Transducers

This article was originally published in the January 1997 issue of Sensors Magazine.

So what does all this have to do with transducers?

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 a 'ground sources'

Isolate metal bodied transducers!

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.

Why are audio and electronic cabinets made of all plastic or insulated metal?

You may have noticed that, over the years, stereos and other consumer electronics have evolved 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 prevents the discharge that causes damage. The future belongs to sensor manufacturers who understand that this effect applies to lightning borne discharges and transducers as well.

How to get lucky (or a check list to prevent lightning damage)

Put your power line transient suppressors at the mains service entrance if at all possible. If your instrumentation box has a transient suppressor, see that the ground goes only to the breaker box and not via the safety ground. Make sure any protector diverted transient is not daisy chained through other equipment. Without this precaution, installation of a transient suppressor will often just move the point of damage!
Use transient suppressors that use high voltage inductive filters. This type helps keep common mode transients out of equipment and reduces transient induced breaker trips.
Identify ground sources and isolate sensors from them. Use insulated-bodied sensors where possible or try to improvise isolation.
Use ground pavilions. Bond service grounds together at a common entrance point. This is not always feasible for economic and political reasons, but makes a huge difference in equipment losses and reliability.
Tie your pavilion to a lightning rods if permissible. Whenever lightning is diverted through a known circuit it is less likely to damage instrumentation. Special low impedance ground rods can make a big difference or the use of multiple dispersed ground rods - and be sure to consider the non conductive soils in arid climates. There are special meters to measure the resistance and inductance to ground.

Bibliography

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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