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Quick Checkpoints To Prevent Lightning-induced Damage

May 1, 1998 12:00 PM, Ken Michaels

Be sure to make an annual visual inspection of your facility and check connections throughout the grounding system. And, don't forget to take bonding resistance measurements between grounding electrodes.

Beware: The dreaded lightning season is near. Yes, between May and September, electronic equipment will be at the mercy of this unpredictable yet devastating force of nature. Sure, it's almost impossible to predict where or when lightning will strike. But based on extensive field work, this author believes you can prevent much of the equipment damage caused by lightning-if you make periodic visual inspections of your premises wiring system to locate trouble areas. Let's look at some items you should check prior to lightning season.

Checkpoint 1: The grounding electrode system. Make sure you inspect this system at the main service entrance and look for anything that may contribute to lightning damage. For instance, check all grounding electrode system connections. If surge currents are present on the grounding electrode system, loose or corroded connections can increase the overall impedance of the lightning dissipation path. This correlates into a higher voltage with respect to ground and can result in equipment damage.

Checkpoint 2: Bonding between grounding electrodes. If you use more than one electrode to reference your premises wiring system, make sure you effectively bond them together. If you don't, any lightning surge current will create enormous voltages between the ground rod and the building's grounding electrode system. We know this voltage as transferred earth potential. Transferred earth potentials often create damage to the input/output (I/O) printed circuit boards within electronic equipment. To determine if two electrodes are effectively bonded together, you should make two-point bonding resistance measurements (also known as Direct Method testing) between electrodes. The bonding resistance between any two electrodes should be less than 0.1 ohm.

Checkpoint 3: Separately driven electrodes. In the field, this author recalls seeing lightning damage where separately driven electrodes exist at equipment locations. Often, the maintenance electrician does this to satisfy an equipment manufacturer's mistaken interpretation of required or recommended grounding practices. There are two places in a facility where you may find separately driven electrodes: at the base of industrial machine tools in manufacturing plants and in the telecommunication/data equipment room. If either of these locations requires a supplemental equipment grounding connection to enhance the equipment grounding system, you should bond the grounding planes for both to structural steel (assuming the structural steel bonds to the main service entrance ground). But remember: Separately driven electrodes at the telecommunication equipment are transparent to the end-user because of their installation without the end-user's knowledge or consent. As such, don't just rely on the end user's information; make the inspection at the telecommunications equipment yourself.

Checkpoint 4: Connections between buildings. Transferred earth potentials can also develop between buildings when lightning strikes a structure. Lightning damage at I/O boards is an indication this condition exists. Conductive connections provided by power and data wiring run between buildings can create a path for surge energy. Yes, fiber-optic (FO) cabling is one method to eliminate this problem, but be careful. Some FO cabling contains a conductive wire that's run in or around the cable's outer jacket for support. If you terminate this wire to equipment at both buildings, you may negate the benefit of installing an FO cable.

Checkpoint 5: Surge protection. Lightning damage can occur at the AC input to electronic equipment. For this reason, you may have to install surge protection devices at various points within the electrical distribution system. At the very least, you should provide surge protection at the main service entrance panelboards to prevent AC power line surges from traveling downstream to any equipment.

Some end-users also prefer to install additional surge protection at outlets that feed hardware critical to their operation. As an example, a LAN server may require an uninterrupted power supply (UPS) that has surge protection provided at its input.

If you're installing surge protection at the equipment location, test the branch circuits for correct wiring. Verify the equipment grounding conductor impedances are at the lowest practical value. (IEEE Std. 1100-1992 recommends 0.25 ohms or less.) This ensures the equipment grounding system is adequate for providing a low impedance path to dissipate surge currents.

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