Conductors
The conductors are next on the path from air terminal to ground. They are typically made from braided metal cables; metal rods and straps can be used for special conditions. Conductors must be attached to the building with approved mechanical anchors or adhesives at intervals prescribed by NFPA 780.
Installing down conductors on the outside surface of exterior walls can be economical, simplify trade coordination, and facilitate inspection and system modifications. If a building site is not secure, it may be necessary to cover the conductors, especially copper, to deter theft or damage.
Even when exposed to view by the public, conductors can be placed with sensitivity to the architectural design. For example, conductors can run down the ‘back’ side of chimneys, be installed along building edges, or in corners, and be located away from main entrances.
Conductors (but not air terminals) can also be painted to match adjacent materials. Several of these techniques were employed recently when the lightning protection at Thomas Jefferson’s Monticello was updated—flat-strap conductors were attached to the back of balusters around the rooftop and painted to match the woodwork, with down conductors tucked behind downspouts.
Conductors can also be concealed within the building. The conductors are sized to carry the momentary surge of power without generating enough heat to cause fires. This means they can be located beneath roof decks and in attics, in wall cavities and chases, cast into concrete, run through conduit, and installed in grooves routed into other material. Further, structural steel members can be used in lieu of conductors if they provide electrical continuity.
Concealing conductors is the obvious choice for new construction. However, they can also be concealed in remodeling projects. For example, the Virginia Museum of Fine Arts in Richmond was recently retrofitted for lightning protection without using exposed conductors. Rooftop conductors entered the building through abandoned roof vents and were bonded to existing structural steel columns that connected to ground electrodes installed in the basement of the building.
Penetrations
Locations where conductors pass through roofs, walls, structural members, and other building materials require the A/E’s attention to ensure the function of the penetrated construction is not compromised. The integrity of fire-rated assemblies and water-resistant barriers, for example, must be maintained. A preinstallation meeting can help with trade coordination and scheduling.
Roof penetrations can be made with various types of boots, pitch pockets, and flashings—the selection and installation must be coordinated with the roofing supplier to make certain the roof warranty is not voided. It may be possible to avoid roof penetrations altogether. In one example, the owner of a computer server farm prohibited roof penetrations as a way to reduce the likelihood of roof leaks. Since the building’s walls were tilt-up concrete panels, down conductors were located in the gaps between panels and became concealed when the joints were sealed.
Through-structure assemblies for wall penetrations are typically made with metal rods that can be cast or built into the walls or installed through drilled holes or in conduit.
Bonding
Lightning does not ‘care’ what path it takes between sky and ground; it will side-flash (arc) from components of the lightning protection system to parts of the building not designed to handle the current. The lightning protection system must, therefore, be connected (bonded) to grounded metal structural elements, piping, ductwork, wiring, equipment, antennae, and other equipment/building components within about 2 m (6 ft) of a conductor.
In this regard, judgment should be exercised before specifying corrugated stainless steel tubing (CSST) in buildings to be protected against lightning. Used to distribute liquid petroleum gas (propane), CSST is more economical to install than black iron pipe because it is flexible and requires fewer joints. While CSST can be bonded to the lightning protection system, its thin walls are still susceptible to perforation when exposed to a lightning side flash, allowing ignition of escaping fuel.
A research report states:
The underlying issue…is whether or [not] CSST is as safe as conventional black pipe. In this regard, reported fire losses indicate it is not as safe as black pipe in regards to the issue of lightning. While we cannot state black pipe will never fail from lightning, we have yet to see such a fire.
