Protecting Control Systems From Lightning
Fear of lightning has been with mankind from the dawn of history. However, in the computer age, this topic takes on a new relevance.
Why Worry about Lightning?
As the nation implements greater automation, distributed computer processing architectures are becoming ubiquitous. They're in the office, factory, schools, and homes. Unfortunately, the computers, computer-related products, and process control equipment found in these architectures can be damaged by high-voltage surges and spikes. In particular, it should be emphasized that, in the modern manufacturing environment, such interference can affect the operation of the Program Logic Controllers (PLCs) as well as the data communications which often occur between sensors and PLCs, and between PLCs and the host computer.
Such power surges and spikes are most often caused by lightning strikes. However, there are occasions when surges and spikes may result from any one of a variety of causes. The causes may include direct contact with power/lighting circuits, static buildup on cables and components, high energy transients coupled into equipment from cables in close proximity, potential differences between grounds to which different equipments are connected, miswired systems, and even human equipment users who have accumulated large static electricity charge build-ups on their clothing. In fact, electrostatic discharges from a person can produce peak voltages up to 15 kV with currents of tens of Amperes in less than 20 microseconds.
The factory environment with process control is particularly susceptible to such surges because of the presence of motors and other high voltage equipment. The essential point to remember is, the effects of surges due to these other sources are no different then those due to lightning. Hence, protection from one will also protect from another.
When a lightning-induced power surge is coupled into your computer equipment, any one of a number of harmful events may occur.
Semiconductors are prevalent in such equipment. A lightning-induced surge will almost always surpass the voltage rating of these devices causing them to fail. Specifically, lighting induced surges usually alter the electrical characteristics of semiconductor devices so that they no longer function effectively. In a few cases, a surge may destroy the semiconductor device. These are called "hard failures." Computer equipment having a hard failure will no longer function at all. It must be repaired with the resulting expense of "downtime" or the expense of a standby unit to take it's place.
In several instances, a lightning-derived surge may destroy the printed traces on the printed circuit boards of the computer equipment also resulting in hard failures.
Along with the voltage source, lightning can cause a current surge and a resultant induced magnetic field. If the computer contains a magnetic disk, this interfering magnetic field might be responsible for a "disk crash."
Finally, there's the effect of lightning on those PLCs found in the manufacturing environment. Many of these PLCs use programs stored in ROMs. A lightning-induced surge can alter the contents of the ROM causing aberrant operation by the PLC.
These are some of the unhappy things which happen when a computer experiences lightning. People may never, or rarely, experience direct lightning strikes on exposed, in-building cable feeding into their equipment. However, its not uncommon to find computer equipment being fed by buried cable. In this environment, a lightning strike, even several miles away, can induce voltage/current surges which travel through the ground and induce surges along the cable, ultimately causing equipment failure. The equipment user is undoubtedly aware of these failures but usually does not relate them to the occurrence of lightning during thunderstorm activity since the user did not experience a direct strike.
Figure 1: Mean Annual Days of Thunderstorm Activity
Where Should You Worry?
The question primarily relates to the geographical location of computer and end-users. Life is unfair for the computer users in certain regions of this country as is shown on the map, (Figure 1) Produced by the Electric Power Research Institute, the map denotes mean annual days of thunderstorm activity. Upon examination, the map shows the high point of thunderstorm activity as being in Western Florida. It also shows a ripple effect out from this focus of lightning activity. In addition, it shows several areas outside of the southeast where intense thunderstorm activity perturbs the "dying ripple." Notice, for example, the intense areas in Colorado, New Mexico, and Arizona. Also observe that the high level of the southeast extends all the way west into Texas.
There are a number of other maps of thunderstorm activity which the reader may find of interest, including:
National Weather Service map showing average number of annual thunderstorm days by regions.
IEEE Working Group Report indicating thunderstorm-Hour Data in various regions.
Annual Lightning flash contours.
Examining the map and the other referenced maps pretty much answers the original question. If you're in Florida, lightning protection absolutely must be a concern. In fact, it should be throughout the whole southeast US extending west to Texas and northwest to Missouri. The map shows where other intense pockets of activity occur. Apparently, only the Pacific coast can be "light-hearted" about lightning as a problem.
When Should You Worry?
This question deals with two separate issues when during the year should you worry about lightning and when during a thunderstorm.
The first of these issues can be effectively dealt with by observing Figure 2. This is based upon over 13.4 million cloud-to-ground lightning flashes as recorded by the National Lightning Detection Network. Notice the slow rise in the lightning rate in the Spring and the relatively fast decrease in the Fall during 1989.This seems to be the characteristic feature of lightning rate variation in the United States. The lightning season extends over six months, beginning in April and ending in early October.
Figure 2: Seasonal Variations of Lightning in 1989
Typically, thunderstorms are characterized as intense individual rain cells or showers embedded in a broad area of light rain. These intense cells are only over a fixed location for a few minutes. They're on average, several miles in each dimension.
Thunderstorm cells move from west to east along a squall line (Figure 3) about 12-30 miles in width and up to 1,250 miles long. The speed at which the thunderstorm cell moves is generally 30 knots (approximately 34.4 statute miles per hour).
Figure 3: Thunderstorm Cell Movement
Coming right down to it, a lot can be done as far as protection is concerned. However, it's best to begin by describing the magnitude of the threat from which you need protection.
The first stroke of lightning during a thunderstorm can produce peak currents ranging from 1,000 to 100,000 Amperes with rise times of 1 microsecond. It is hard to conceive of, let alone protect against, such enormous magnitudes. Fortunately, such threats only apply to direct hits on overhead lines-- hopefully a rare phenomenon.
More common is the induced surge on a buried line. In one test, lightning-induced voltages caused by strokes in ground flashes at distances of about 5km were measured at both ends of a 448 meter long, unenergized power distribution line. Figure 4 is typical of the results. Notice that the maximum-induced surge exceeds 80 V, peak-to-peak. This is more than enough to destroy semiconductor devices and computer related equipment. Yet, 80 V is well within the range of affordable protection.
Figure 4: Measured Induced Voltage
Providing affordable protection
Conceptually, lightning protection devices are switches to ground. Once a threatening surge is detected, a lightning protection device grounds the incoming signal connection point of the equipment being protected. Thus, redirecting the threatening surge on a path-of-least-resistance (impedance) to ground where it's absorbed.
Any lightning protection device must be comprised of two "subsystems," a switch which is essentially some type of switching circuitry, and a good ground connection-- to allow dissipation of the surge energy. The switch, of course, dominates the design and the cost. Yet, the need for a good ground connection cannot be emphasized enough. Much computer equipment was damaged by lightning, not because of the absence of a protection device, but because inadequate attention was paid to grounding the device properly.
The basic elements used as protective switches are: gas tubes, metal oxide varistors, and silicon avalanche diodes (transorbs). Each has certain advantages and disadvantages.
Because they can withstand many kilovolts and hundreds of Amperes, gas tubes have traditionally been used to suppress lightning surges on telecommunications lines. This is just what is needed to protect against a direct strike. Because gas tubes have a relatively slow response time, this slowness lets enough energy to pass to destroy typical solid state circuits.
Metal oxide varistors (MOVs) provide an improvement over the response time problem of gas tubes. But, operational life is the drawback. MOV's protection characteristic decays and fails completely when subjected to prolonged overvoltages.
Silicon avalanche diodes have proven to be the most effective means of protecting computer equipment against over voltage transients. Silicon avalanche diodes are able to withstand thousands of high voltage, high current, transient surges without failure. While they cannot deal with the surge peaks that gas tubes can, silicon avalanche diodes do provide the fastest response time.
Thus, depending upon the principal threat being protected against, devices can be found employing gas tubes, MOVs, or silicon avalanche diodes. This may be awkward, since the protection device selected should be robust, using a combination of the basic circuit breaker elements. The architecture of such a device is shown in Figure 5. This indicates triple stage protection and incorporates gas tubes, MOVs, and silicon avalanche diodes as well as various coupling components and a good ground.
Figure 5: Robust Protection Device
With the architecture shown in Figure 5 a lightning strike surge will travel, along the line until it reaches a gas tube. The gas tube dumps extremely high amounts of surge energy directly to earth ground. However, the surge rises very rapidly, and the gas tube needs approximately one microsecond to fire.
As a consequence, a delay element is used to slow the propagation of the leading edge wavefront, thereby maximizing the effect of the gas tube. For a 90 Volt gas tube, the rapid rise of the surge will result in its firing at about 650 Volts, approximately 650 ns. The delayed surge pulse, now of reduced amplitude, is impressed on the avalanche diode which responds in about one nanosecond or less and can dissipate 1,500 Watts while limiting the voltage to 18 Volts for EIA-232 circuits. This 18 Volt level is then resistively coupled to the MOV which clamps at 27 Volts. The MOV is additional protection if the avalanche diode capability is exceeded.
As was previously mentioned, the connection to earth ground cannot be overemphasized. The best earth ground is undoubtedly a cold water pipe. However, other pipes and building power grounds may also be used.
The robust structure shown in Figure 5 is embodied in Telebyte's Model 22 (Figure 6). The Model 22 is designed to protect 4-wire lines as used in short haul modems operating up to speeds of 38.4 KBaud.
Figure 6: Telebytes Models 22, 29 and 22NX
Determining the final choice
When presented with the imperative need to buy a lightning protection device, the equipment user is often confused by a plethora of offerings in the marketplace: some are appropriate for the application of interest, others are not. Here's a simple step-by-step procedure for determining which product fits the application's needs. In each step, the answer to the question results in the recommendation of the proper products for the application.
Step 1-- What is the interface on a computer or computer related equipment?
Is it EIA-232C, RS-422, TELCO etc.? Whichever it is, you must look for protection devices for this interface. In particular be aware of the mechanical aspect of the interface, e.g., is EIA-232C used with a DB 25 or a DB 9 connector?
Step 2-- How many interface lines need to be protected and which ones?
For EIA-232C is it just TD/RD or others? you must have similar answers for other interfaces. You must then look for a device which protects the appropriate lines or pins.
Step 3-- What are the speeds on the lines which need to be protected?
A minor but important point. Make sure that the available products support your application speeds during quiescent conditions.
Step 4-- What is your cable routing? Is it inside the building or outside?
If inside the building, you may only need a single stage lightning protector with just one type of switching element, e.g. silicon avalanche diodes. If the cabling is outside the building, your equipment may be more vulnerable to a direct strike. You may need the type of two- or three-stage protection discussed.
Step 5-- What is your ground connection?
Specifically, if you have EIA-232C interface equipment to be protected, is Pin 1 to the chassis ground supported? If not, you need a protection product with a separate ground stud or you must bring Pin 1 to the chassis ground yourself.
Many vendors, including Telebyte, aid their customers in this step-by-step procedure providing either application engineers or self-help charts.
If after surveying the market, you still cannot find an appropriate device, do not give up. Many vendors offer services to design and manufacture any type of protection device.
Orville, Richard E., "Lightning Ground Flash Density in the Contiguous U.S.-- 1989," Monthly Weather Review, Vol. 119, No. 2 Feb. 1992, pp. 573-577.
Crane, Robert K., "Predictions of the Effects of Rain on Satellite Communication Systems," IEEE Proceedings, Vol. 65, No. 63, March 1977, p. 466.
McGraw Hill Encyclopedia of Science and Technology, Vol. 17, 7th Ed., p.289.
Georgiadis, N., et. al., "lightning-Induced Voltages at Both Ends of a 448-m Power-distribution Line," IEEE Transactions on Electromagnetic Comparability, Vol. 34, No. 4, Nov. 1992, pp. 451-459.