Lightning in the Computer Age
How it affects technology-
What it is-
How to protect against it-
Can it help us?
Lightning and the Modern Technical World
From the dawn of prehistory, perhaps even before, mankind has stood in awe and fear of lightning. In the western world witness the mythological gods in Greek and Norse mythology associated with this natural phenomenon. Witness the occurrence of lightning within the Old Testament in the exodus from Egypt. Unfortunately, I plead ignorance of eastern and African religions however, I would expect to find lightning as occupying a similar position in their theological works.
The electrical age began well over a century ago. The deleterious effects of lightning on electrical advances were recognized with respect to power distribution, telegraphy and telephony. Much significant effort has been expended to ameliorate its consequences Figure 1 shows two legends of the early electrical age, Edison and Steinmetz, discussing its effects.
Figure 1: Legends of the early electrical age, Edison and Steinmetz, discussing its effects.
In the present computer age problems with lightning continue. As the global village implements greater and greater automation, distributed computation is found everywhere - factories, homes, educational facilities and of course office buildings. Yet, the data equipments, the computers, computer related devices and industrial process control devices found in these distributed architectures can be damaged by high voltage surges and spikes arising from cloud-to-ground lightning strikes. This is a particular problem in the industrial environment, right on the production line. Lightning induced surges can affect the operation of Program Logic Controllers (PLC's). They can also affect the data communications between sensors and PLC's, between PLC's and a host computer and between a myriad of tools and the PC's which control them.
When lightning strikes and a power surge is coupled into computer equipment any one of several different events may happen.
Semiconductors are prevalent in such equipment. A lightning surge may surpass the voltage rating of these devices causing them to fail. They may alter their electrical characteristics so that they no longer function effectively. The lightning surge may cause defects on the printed circuit board tracings. It may induce a magnetic field that may destroy and/or alter data stored on a magnetic disk. It may activate relays in an aberrant manner. The lightning induced surge may even alter programs stored in ROMs associated with PLC's.
What is Lightning?
Above, a good case is made for why we should still be interested in lightning in the modern technical world. Now it will be worthwhile going to basics and describing exactly what lightning is. To be specific it will be worthwhile describing what a typical cloud-to-ground lightning flash is - such flashes are often referred to as streaked or forked lightning. Why care about what it is? We are at an advantage in dealing with a problem like lightning if we truly understand how the phenomenon comes about.
It is documented by experimental observation that thunderstorms contain very strong electrical charges. The very process by which such storms originate is still very much a subject of considerable research, discussion and speculation. Nonetheless, there is common agreement within the scientific community concerning several elements. Worldwide over 90% of the cloud-to ground flashes are estimated to be what are termed Category 1 lightning. It this type of lightning which I shall describe in detail.
With Category 1 lightning flashes, the Earth, at some local focus, concentration point, acquires a considerable excess of positive electrical charge. Simultaneously, the bottom of a thunderstorm cloud, a cumulonimbus, cloud, acquires a significant excess of negative charge- while the top of the cloud may have an excess of positive charge. These charge concentrations are generally of the order of ten's (10's) of Coulombs. The cloud-to-Earth distance, the separation of these charge concentrations is of the order of several miles. Putting this in perspective you can think of a thunderstorm cloud as setting up more than hundreds of billion of billions of electrons and protons separated by a few miles. This is a pretty strong electric field.
How do such excess charge concentrations form during thunderstorms? There are a number of theories postulated. The excess charge concentrations may be a result of precipitation or it may be a result of convection.
In precipitation theories, heavy falling precipitation particles interact with lighter particles which are carried in updrafts. The interaction between the two types of particles serves to charge the heavy particles negatively and the lighter particles positively. After this gravity and updrafts separate the opposite charges to form the positive cloud-earth dipole. Charge transfer in these interactions can be by collision in which two initially uncharged precipitation particles become oppositely charged after collision. This is the case for collisions between hail and ice crystals.
In the convective theories charge that has been accumulated near the Earth's surface or across regions of varying air and cloud conductivity is moved in one fell swoop, a bulk transfer, to the concentration centers on cloud and Earth by the airflow associated with a thunderstorm.
It is most likely that the charge accumulation is a result of a combination of the precipitation and convection mechanisms.
In any case, a result of the charge concentration is a tremendous electric field building between cloud and Earth. When this field becomes large enough then the air corridor between the cloud and Earth becomes susceptible to electrical breakdown. You can view this as the cloud and the earth constituting the plates of a giant capacitor with an air dielectric. A 'too large' battery is placed across the capacitor and this causes the dielectric to breakdown.
But, what does this mean-breakdown of the air dielectric? Under usual circumstances air is not a good conductor of electric current. However, when breakdown happens the rules of the game change. It becomes a much better conductor. Why? We have to look to the atomic level. As a result of breakdown the atoms constituting the mixture, air, become ionized. This ionization causes the constituent elements of air to become an extension of the negative charge in the cloud. This ionization continues as kind of a propagating charge wave. It begins at the bottom of the thunderstorm cloud and moves down to the Earth. The entire electrodynamic process here is somewhat complex, but actually quite beautiful. The air actually ionizes in narrow paths that split apart and branch down as the chain reaction induced by the propagating charge wave reaches toward the Earth. This chain moves down in discrete steps rather than in a steady continuous flow. It is almost in a sequence of straight lines reminiscent of the connect the dots puzzles popular with children. These configurations of Earth directed moving paths of ionized air are called the 'stepped leader.'
The stepped leader actually illuminates every time that a new straight line is appended to the downward moving sequence. However, this is usually not evident to the human observer. This is a result of the stepped leader moving so rapidly and the fact that the true bright lightning we observe occurs at such a small delay after the stepped leader.
Well the stepped leader can be thought of as the propagation of a concentration of negative charge beginning at the cloud and going towards the positive charge concentration on the Earth. This is illustrated in Figure 2. When the stepped leader gets about 0.1 miles or so from the Earth then a second electrical breakdown is initiated. However, this proceeds from the Earth on up towards the cloud. The positive charge concentration on the Earth propagates upwards in paths called leaders. However, these do not have the straight line property of the stepped leader. When the negative charge concentration of the stepped leader finally comes in contact with the positive charge concentration of the upward bound leader then the whole giant capacitor attempts to reach electrical equilibrium. The capacitor essentially discharges. In the resultant neutralization of the electrical field the potential energy of the field is converted into the energy of light, sound and heat. The massive flow of current which effects the electrical neutralization, measures on average, 100-to-1,000 Amps and may have peak values from 1,000- to-100,000 Amps.
This total discharge is usually referred to as the lightning 'flash.' It has a time duration of about 0.5 seconds. The flash is made up of several distinct discharge components. Each component, itself, is composed of three or four high-current pulses called 'strokes.'
This as I said is Category 1 lightning, by far the most commonly occurring cloud-to-ground lightning. There are other forms and it is worth mentioning them in passing. Categories 2 and 4 lightning evolve from leaders which move upward from the Earth. Sometimes they are termed ground-to-cloud discharges. Category 3 lightning is similar to Category 1. It evolves from a downward moving leader from the cloud. However, the leader is positively charged.
Figure 2: Category 1 Cloud-to-Ground Lightning
Protecting Against Lightning
When we talk about protecting equipment against lightning in the computer age we are essentially talking about protecting the data interfaces- the debarkation points where data enters and exits the equipment- where it is transmitted and received. We are really talking about protecting things like the EIA-232 interface and others similar to it.
Conceptually, lightning protection devices clamp voltage and direct the excess energy to ground. Once a threatening surge is detected, a lightning protection device grounds the incoming signal connection point of the equipment being protected. It redirects the threatening surge to a path of least resistance to ground- where it is absorbed.
Any type of lightning protection device must be composed of two functional parts, a switch- or some type of switching circuitry, and a good ground connection-to allow dissipation of the surge energy. The switch dominates the design. However, the need for a good ground connection can not be over emphasized. Data equipment has been destroyed by lightning, not because of the absence of a protection device, but because inadequate attention was paid to grounding the device properly.
What makes the design and choice of a lightning protection device a challenge is the fact that the nature of the event itself does not allow us to predict in advance the size of the surge threatening our data equipment. The magnitude of the strike may vary. Furthermore, the strike may hit the data equipment directly or it may hit the Earth miles away generating ground currents which result in the threatening surge. A threatening surge may arrive with a wide variation in energy magnitude and a wide variation in temporal behavior- peak width duration etc. Of course, the appropriate choice for the switching element depends upon what the actual surge properties are.
The basic elements used as protective switches are; gas tubes, metal oxide varistors and silicon avalanche diodes (tranzorbs). Each has pros and cons.
Because they can withstand many kiloVolts and hundreds of Amps gas tubes have traditionally been used to suppress high energy lightning surges resulting from direct strikes on telecommunication lines. This is just what is needed to protect against a direct strike. However, because gas tubes have a relatively slow response time, this slowness lets enough energy pass to destroy the typical solid state circuits so prevalent in data equipment. A gas tube can knock down the big pulses, but can not insure against threatening leakage which gets through before it can act.
Metal oxide varistors (MOVs) provide an improvement over the response time problem of gas tubes. However, operational life is a drawback. A 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 data equipment against overvoltage transients. They are able to withstand thousands of high voltage, high current, and transient surges without failure. While they can not deal with the surge peaks that gas tubes can, tranzorbs do provide the fastest response time.
Putting these facts together, depending upon the principal threat being protected against, devices can be found employing gas tubes, MOVs and tranzorbs. This may be awkward since the threat is never really known in advance. Ideally, the protection device selected should be robust, using all three basic circuit breaker elements. The architecture of such a device is illustrated in Figure 3. This architecture has triple stage protection. It incorporates gas tubes, MOVs and tranzorbs as well as various coupling components and a good ground.
With the architecture shown in Figure 3 a lightning strike will generate a surge which will travel along the line until it reaches the 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 several microseconds to fire.
Figure 3: Architecture of a robust protection device with all three circuit breaker elements. This design protects against lightning-induced voltage.
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. The delayed surge pulse, now of reduced amplitude, is impressed on the silicon avalanche diode, the tranzorb, which responds in about 1 nanosecond or less. It can dissipate 1,500 Watts while limiting the voltage to 18 Volts. This 18 Volt level is then resistively coupled to the MOV which clamps at 27 Volts. The MOV is additional protection if the tranzorb capability is exceeded.
The robust structure that is provided by this architecture is embodied in Telebyte's Model 22. This is shown in Figure 4. This unit is designed to protect 4-wire lines as used in short haul modems to interconnnect data equipments. The short haul modems can operate at speeds up to 38.4 KBPS and still be protected by the Model 22.
Figure 4: The Telebyte Model 22 is an actual implementation of the robust
three device protection architecture shown Figure 3
Remember not to forget the importance of that good ground to the overall protection device. The best Earth ground is undoubtedly a cold water pipe. However, other pipes and building power grounds can be used. While cold water pipes are good candidates you should be careful here. A plumber may replace sections of corroded metal pipe with plastic. This would render the pipe useless as a ground. Also avoid using chemical or natural gas pipes. The ground connection should be kept as short as possible to avoid potential differences. Heavy gauge wire should be employed, at least 10AWG.
Can lightning Help Us?
I would like to finish this article on a positive note if possible. To this point the energy presented by a lightning flash has been emphasized in terms of the damage that it can cause to data equipment. What about the opposite side of the coin? Is there a possible way of using this energy for constructive purposes?
Of course a few ready examples of the constructive use of lightning come to mind. It was lightning which brought Frankenstein to life- although this was in the Hollywood version not the Mary Shelley original. It was lightning which powered the flux capacitor and gave us a happy ending in Back to the Future. However, putting aside this humor it appears that while lightning presents a tremendous source of energy, it is not really available for constructive use. There are two reasons for this. First, lightning just occurs too sporadically. There is no way to plan for it and thus to sink this source energy efficiently. The sun, wind, ocean tides and moving water all represent natural phenomena which are tremendous sources of energy which can be harnessed. However, they all have a sense of regularity which lightning does not. Secondly, there appears to be no practical and efficient way to store the energy of lightning. If any readers have ideas along these lines please contact me at firstname.lastname@example.org.
On a parting note you might consider that even if you could constructively harness the energy of lightning you might not want to do this. Any taming of such a vast energy source has to affect the ecological balance. The hidden costs probably far outweigh the benefits.
1. Uman, Martin A., 'The Lightning Discharge, International Geophysics Series Vol. 39, Academic Press 1987.
2. Schneider, Kenneth S., 'Protecting Control Systems From Lightning,' Control Engineering, Cahners Publishing Company, June 1993.
3. Schneider, Kenneth S., 'Primer on Premises Data Communications- 2nd Edition,' Telebyte Technology, Inc. 1995.
4. Internet Web site wvit.wvnet.edu, 'How Lightning Works- A Typical Cloud-to-Ground Lightning Flash,' 1996.