Don't Let Lightning Catch You Off Guard

Although there are a number of lighting protection systems available, the decision of which one's right for your facility basically comes down to what you're trying to protect. Just look at the odds. Who needs lightning protection, right? Electrical installations often exist for years before actually experiencing a lightning strike. Thus, the demands for this protection in older facilities are less

Although there are a number of lighting protection systems available, the decision of which one's right for your facility basically comes down to what you're trying to protect.

Just look at the odds. Who needs lightning protection, right? Electrical installations often exist for years before actually experiencing a lightning strike. Thus, the demands for this protection in older facilities are less vocal. You've all heard the statement: The electrical system works, so why bother with it? Because lightning strikes can wreak havoc on your otherwise sensitive electronic equipment and result in costly downtime and lost production. They also occur more frequently than you may think.

Lightning research indicates several cloud-to-earth lightning strikes occur every minute across the earth. The isokeraunic level, or the quantity of thunderstorm days per year in a certain location, gives you an idea of the potential for lightning activity at that location. To illustrate the frequency of annual lightning strikes, flat portions of central Florida receive about three dozen cloud-to-earth lightning strikes per square mile per year. When a strike occurs near a facility, several events occur:

• Charge neutralization occurs to equalize the voltage between the earth and the bottom of the overhead cloud. This involves current flow through both the air and earth toward the strike location.

• An electromagnetic pulse in the form of an induced voltage develops in all mutually coupled wiring, pipelines, or other conductors in the vicinity of the strike.

• This creates an electrostatic pulse in the form of an induced transient waveform that travels from any conductor above the charged earth down to the earth through a connecting conductor.

Although some of the technology you can use to defend your facility against lightning-induced problems is so new that codes and standards are only recently beginning to catch up, there are four basic approaches to protect your site against lightning strikes, including:

Do nothing. Plan to replace failed components; betting the problem will be infrequent. This is a good plan for locations where there are no critical loads, and you only have a few thunderstorm days per year.

"Harden" electrical equipment. This practice improves your chances of "riding through" strikes. Many use this approach on transmission and distribution lines; even where installers place the equipment-grounding conductor (static line) above the phase conductors within the theoretical "30 degree cone of protection."

Most electric utilities even over-insulate by a factor of 2 to 10 to increase the flashover distance of the insulator string. They also purchase equipment, like transformers, with much higher breakdown insulation level (BIL) values than the system voltage would need itself. Electrical professionals also "harden" the power and communications equipment by installing lightning and surge arresters to help divert the strike electron flow to earth instead of through electrical equipment.

Install the old standby "diverter" type of lightning protection system. With this system, the lightning strike is induced and attracted to a high point that connects to earth by "down" conductors, and under which (within the theoretical "30 degree cone of protection") you should not subject the equipment to a direct strike. Note: This method also requires you to harden the equipment, as discussed above, to be of much value in typical facilities.

Install a capital-intensive lightning avoidance system. This approach is necessary at locations where you cannot allow lightning strikes of any kind -- due to the explosive nature of process chemicals or critical nature of electronic devices. Lightning avoidance systems equalize the earth-to-cloud charge continuously over a long time, instead of permitting it to build up to the point where a stroke of lightning occurs. The technology of lightning avoidance systems is to make use of the old "point discharge" concept, in which a sharp point in a strong electrostatic field will emit electrons into the air by ionizing the adjacent air molecules. This methodology provides for a potential at the sharp point that exceeds the normal air dielectric strength of 10kV to 15kV per mil. This permits low ampacities of electron flow (instead of a high-current stroke) to continually emit into the atmosphere, wherein they are free to travel up to the cloud, working to equalize the charge there.

After weather creates a potential between a cloud and the earth, the air between the two is essentially a "leaky" capacitor dielectric where the cloud is one capacitor plate and the earth is the other. The sharp points connected to the earth literally permit electrons to flow from the earth into the air in a continuous stream that is often visible as a glow known as St. Elmo's Fire.

Lightning avoidance systems simply provide a very good connection to the earth (low-grounding electrode resistance to "remote" earth) and multiple sharp points located above the facility installation. These systems literally move the "earth plate" to above the elevated sharp points. This action prevents the electron difference between the cloud and earth from being great enough to form a potential that can exceed the breakdown voltage (conduction voltage) of the air, thus preventing a strike at the protected facility. Remember, this does not prevent a strike from a nearby location. You're still required to use lightning arresters and Class C surge limiters to protect sensitive equipment from induced traveling waves entering the facility through power and communication conductors.

When designing a system to protect against lightning strikes, consider the following options:

• Class C surge protection from each phase and neutral to ground and from each phase to neutral.

• Class B surge protection at circuit breaker enclosures.

• Class A surge protection at individual power outlets.

• Dataline protectors between incoming telecommunications lines and each of the lines and ground. These protectors should have a fast rise time of about 5 nanoseconds and be rated for approximately 500 joules. The leads between these protectors and the surge limiters listed above must be straight, not in metal conduit, and as short as possible on their way to the grounding electrode system -- where you must mechanically connect them first and then thermo-weld, silver-solder, or braze.

• Sharp-point ionizers in dissipation arrays to literally bleed off electrons into the air to equalize the cloud-to-earth charge and prevent strikes from occurring.

• Very-low-resistance grounding electrodes filled with conductive salts that physically flow from reservoirs in the electrodes into the soil, maintaining a low-resistivity soil surrounding the electrode.

• Electrode backfill mixtures containing clay that can hold moisture, thus working to maintain a low-resistivity soil surrounding the electrodes.

Although you can't control Mother Nature, the good news is the surge limiters of today are a quantum leap ahead of those available just a few years ago. The old generation of metal oxide varistors (MOVs) literally sacrificed themselves (blew up) while conducting away transient currents. The new generation of surge limiters are rugged enough to do the job repeatedly without the need for replacement. Thus, when protected by these "little jewels" (rated in joules), your delicate electronic equipment will continue to operate as if nothing ever occurred—despite the reality of lightning strikes.

Paschal is a Sr. Engineering Specialist, Bechtel Corp., Houston.

Sidebar: Cost of Downtime

You may find the old saying "pay me now or pay me later" often holds true. You can spend money on the prevention of a strike, or pay to rebuild the equipment later. However, there's one important change to remember today. You must consider the cost of business "downtime" while replacing damaged equipment after a lightning strike. Typically, this downtime frequently dwarfs the cost of the damaged equipment. Therefore, strike prevention is increasingly becoming the protection method of choice.

TAGS: Design
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