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Lightning Protection of Roof Top Equipment

Questions:

Q1. The recommended protection diagram shows roof-mounted air-condition equipment to be bonded to the lightning protection system. Does this not serve to create an internal ground loop into the building? The building ground ties to the internal grounding network and thus to the air handling equipment and ductwork. The metal ductwork extends to the roof lightning system and back to another ground point on the lightning system ground.

Q2. A ground stroke in the area of the building will be conducted into the building envelope by the multiple ground paths in the multi-point grounding. Should there not be just one point of ground where lightning current can pass outside the building to ground? That way there are no incidental ground loops for surge current to take.

Q3. Should the lightning conductor be insulated from the building structure at all places? Recently a stroke hit our City Hall roof lightning system. The roofing material behind the metal flashing melted and burned. The conductor was laying against the flashing in that area.

Answers:

A transient voltage rise occurs within the lightning protection system during the flow of a lightning current through it. Nearby metallic objects, including the air handling system, remain at ground potential. If the separation between these two systems is within a certain value, a side flash will occur. This could cause fire and/or other damage. That is why they must be bonded together. In other words, the duct system and the wiring of the air handling system will end sharing the lightning current regardless of whether they are bonded together or not. By intentionally bonding them, you insure that the discharge of the current will be done in a safe manner.

The case of the down wires lying against the flashing is the ultimate example of the proximity discussed above. They should have been bonded together and this would have prevented the damage. In the absence of bonding, arcing occurred at that spot. The resulting high temperature burned the roofing material beneath the flashing.

Lightning is a high frequency phenomenon. As such, most of the current tends to flow in the "cage" formed by the multiple down wires. Only a small fraction of the current will flow in wires, ducts, gas pipes, plumbing pipes that exist in the building. This happens all the time and it poses no problems.

As the above indicates, there is no need for insulating the down wires. The system with bare copper wires has functioned well for over 250 years. Actually, the bare wires make it easier to inspect the system. This should be done about once per year.

Abdul M. Mousa, Ph.D., P. Eng., Fellow IEEE

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