It takes a lightning protection system

by Jennifer Wilson | February 29, 2016 4:07 pm

by Jennifer A. Morgan, CSI, and Michael Chusid, RA, FCSI, CCS
In the March 2016 issue of Construction Canada[1], these authors explore lightning protection and the five concepts one needs to understand when installing this type of system. 

Metal
Copper and aluminum are the most commonly used metals in a lightning protection system (LPS). Metals must be high grade to minimize electrical resistance to powerful lightning surges.

Copper weathers to blend into dark-coloured surfaces and can be treated to accelerate patina. It can be used in contact with earth but should not be used where runoff contacts steel or aluminum. Bronze castings should be used for compatible fittings.

Aluminum may be more economical and blends well with light-coloured surfaces. It should be used with aluminum roofing but should not be embedded in concrete or used within 460 mm (18 in.) of grade.

Tin-plated copper provides a dull grey metal appearance and is recommended for increased corrosion resistance in coastal areas. Stainless steel can be considered in highly corrosive environments.

Air terminals
Air terminals, formerly known as ‘lightning rods,’ are located at the highest points on a structure and at spacings necessary to provide protection for the entire building. For example, the National Fire Protection Association (NFPA) 780, Standard for the Installation of Lightning Protection Systems requires air terminals at regular intervals of no more than 6 m (20 ft) on centre (o.c.) along a roof ridge and around the roof perimeter, and within 0.6 m (2 ft) of outside corners. However, an architect may want closer spacings to align with architectural features or the modular spacing of elements on the exterior wall.

To minimize visibility from the ground, the air terminals can be mounted on the backside of parapets. While of less concern from a visual standpoint, air terminals are also required within the fieldof a large roof and on top of rooftop equipment. Their location will be determined by using ‘rolling sphere’ calculations.

The simplest air terminals are metal rods. An air terminal can be as narrow as 9.5 mm (3/8 in.) in diameter and extend as little as 255 mm (10 in.) above the element on which it is mounted. However, project conditions may dictate larger and longer rods to provide sufficient conductivity and coverage.*

While tapered rods may be preferred to match historic styles, blunt-tip rods offer better performance and greater safety for personnel. Additional safety can be achieved by mounting air terminals on springs, which can also reduce the potential for damage due to movement of roof-mounted window-washing equipment.

These authors caution against the use of so-called ‘early streamer emissions,’ ‘dissipation array,’ and ‘charge transfer’ air terminal devices. Claims these devices ‘attract’ or ‘repel’ lightning to reduce the quantity of air terminals to protect a building have been debunked by NFPA[2], court rulings[3], and international studies[4].

Conductors
These are next on the path from air terminal to ground. They are typically made from multi-strand metal cables, although metal rods and straps can be used for special conditions. Conductors must be attached to the building at intervals prescribed by NFPA 780.

Conductors can be concealed within a building. They are sized to carry the momentary surge of power without generating enough heat to cause fire. This means uninsulated conductors can run alongside wood, be located beneath roof decks, in attics and wall cavities, cast into concrete, and installed in grooves routed into other material. Further, structural steel members can be used in lieu of conductors if they provide electrical continuity. Covering conductors should be considered if they run on the building exterior—especially copper, to deter theft or damage.

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. For example, the integrity of fire-rated assemblies, water-resistant barriers, and air barriers must be maintained.** A preinstallation meeting can help with trade co-ordination and scheduling.

Roof penetrations can be made with various types of boots, pitch pockets, and flashings—the selection and installation must be co-ordinated with the roofing supplier to make certain the roof warranty is not voided. It may be possible to avoid roof penetrations altogether. For 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 floor and roof decks or installed through drilled holes or in conduit.

Fittings and connectors
Hundreds of configurations of fittings and connectors are required to satisfy myriad construction conditions.

Whether fabricated from stamped metal or solid castings, they must be selected for galvanic compatibility with adjacent materials; special bi-metallic parts are available for when transitions between aluminum and copper conductors are necessary. Fittings and connectors are mounted with approved mechanical fasteners or construction-grade adhesives.

Bonding
Lightning ‘does not care’ what path it takes between sky and ground, and will sideflash (arc) from components of the lightning protection system to other building components not designed to handle the current. Therefore, the lightning protection system must be connected (bonded) to grounded metal structural elements, piping, ductwork, wiring, equipment, antennae, and other equipment and building components within approximately 2 m (6 ft) of a conductor.

The importance of bonding is demonstrated in the following case study by the BC Safety Authority.

Lightning struck the ground near an underground natural gas distribution line, and energized the tracer wire used to locate underground gas lines. This created an electrical arc that shorted from the tracer wire (serving the individual gas service to the home) to the gas service riser. This arc created a hole in corrugated stainless steel tubing (CSST). The resulting gas leak and sources of ignition started a fire in the home. During the investigation, it was found the home’s gas lines were not bonded as required by CSA B149-10 Natural gas and Propane installation and Handling Code (CSA B149-10) and the CSST manufacturer’s certified installation instructions[5].

However, even with bonding the thin walls of CSST are still susceptible to perforation when exposed to a lightning side flash. 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[6].

Ground electrode
The conductivity of soil at a building site affects its suitability as a ground for lightning protection. Wet clay may not be desirable from a structural perspective, yet it is highly conductive and performs well as a ground. Dry sand, gravel, and rock have more resistance and will require more extensive methods to ground. If an owner obtains a soil investigation report, it can be made available to lightning system designers and installers; contractual provisions, however, should spell out what happens if conditions onsite differ from those in the report.

 

In conductive soil, a copper rod, located at least 0.1 m (2 ft) outside the building perimeter and driven 3 m (10 ft) vertically into the earth, may provide sufficient ground. In non-conductive soils or where rocky conditions, such as the Canadian Shield, make it difficult to drive a ground rod, a shallow ground plate or ground ring will help to distribute charges over a wider area. A ground ring, also known as a counterpoise, can be more economical than installing separate ground rods at each down conductor and is required at tall buildings.

A test well is recommended to simplify inspection of a ground. The well selected should have a cover suitable for traffic loads that may be applied.

 

Surge protector
Any wire entering a building is a potential path for lightning. In addition to power lines, contemporary buildings can be connected with wires for:

• telephone and cable TV services;
• satellite dishes and antennas;
• irrigation controls;
• electric gates;
• wells;
• fire pumps;
• surveillance cameras and security systems;
• site lighting and remote power receptacles;
• building-mounted lighting and signage; and
• photovoltaic (PV) collectors and generators.

In addition to protecting against lightning, these surge protectors resist transient voltage from other external sources. However, since they do not protect against surges that originate within a building, individual pieces of equipment may still require their own surge protectors.

Surge protectors can burn out due to lightning strikes or other surges; they should be equipped with indicator lights or connected to a monitoring system to facilitate inspection.

Surge-protective devices are usually furnished as part of the lightning protection work as their selection is integral to a complete lightning protection system. However, installation is typically performed by an electrical contractor because few lightning protection installers have the electrician license necessary to install surge-protective devices.

Site work
Tall trees next to buildings can present a problem when stuck by lightning, either by falling on the structure or by causing the lightning to side-flash and strike building walls unprotected by air terminals. NFPA 780, therefore, recommends installation of lightning protection in trees with trunks within 3 m (10 ft) of a building or that overtop a building. Consideration should also be given to protecting valuable specimen trees and other items onsite such as pole-mounted lights required for security concerns.

Items installed in open areas, such as pieces of equipment or small temporary structures, can be protected by a mast-mounted air terminal. Large areas—such as docks and military encampments—in which a multitude of masts would not be practical, can be protected by conductors draped between widely spaced poles. This overhead shielding approach is called ‘catenary lightning protection’ after the shape assumed by the cables. It has been proposed as a means to protect arenas and other large outdoor venues where it is impractical to evacuate a crowd to safety when lightning approaches[7]. The conductors can also be used to support lighting fixtures for a dual-purpose catenary lighting and lightning system.

*Deep snow accumulation is unlikely at the exposed locations where air terminals are generally required. One should consider tall, braced terminals if required. At large sloped roofs, especially metal or other slippery roofs, sliding snow can damage lightning protection components installed at the eaves or mid-roof; consider locating conductors on fascia or soffits instead of on top of roof and providing snow retention devices to protect roof-mounted components. While winter thundersnows are rare, a video of one in Montréal can be viewed here[8] .

** Damage due to condensation on lightning protections system penetrations through a building’s thermal envelope is rare, but should be considered if high interior humidity and extremely cold exterior temperature is expected.

Jennifer A. Morgan, CSI, is an officer of East Coast Lightning Equipment Inc., a UL-listed manufacturer of lightning protection components, and an officer of the Lightning Safety Alliance. She can be reached via www.ecle.biz[9].

 

Michael Chusid, RA, FCSI, CCS, is an authority on building materials and a consultant to building product manufacturers specializing in product innovation and marketing. He can be reached via www.BuildingProduct.guru[10].

 

Source URL: https://www.constructioncanada.net/it-takes-a-lightning-protection-system/

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