Traditionally, when we plan to do anything, we should ask ourselves three questions:What?Why?How? Let’s apply this approach to the subject at hand.
Seriously, as electrical designers, installers and regulators, what do we know about a need for a lightning protection system? Except for a few references, the Canadian Electrical Code is silent on this subject (and we’ll get back to the Code references later). With proliferation of metal flagpoles, spires, chimneys, tall construction cranes, cell towers and other communication installations on rooftops of the buildings, this question becomes more and more relevant.
Even if we know about this system of protection against lightning strikes, why would we install it on certain buildings and structures and why would other buildings remain without such protection? Is the criterion for an answer transparent to everyone?
And if we have already decided to install such a protection system, do we know how to install it in a safe manner, so it could perform as intended (i.e., to perform so as to provide the desired protection)?
So, let’s discuss the nature of lightning and the fundamental principles of lightning protection. Lightning is the result of saturation of an electrical charge on a cloud. When saturation of this charge reaches a sufficient level, a lightning stroke to earth may occur, which acts as a conduit for the electric charge stored in the cloud. During this stroke a current of a magnitude of thousands of amperes flows between the cloud and earth via ionized air. Dynamic and thermal effects of this high ampacity current result in injuries, loss of life and tremendous damage to the property.
So, what is the main principle in the protection of a person and property against lightning? It is—to provide adequate means by which a lightning discharge current will enter the earth without resulting injury, loss of life or property damage. It is a mechanism of creating the lowest impedance path to the lightning current (in comparison to other possible paths provided by the building materials). When lightning current follows the higher impedance paths, heat and mechanical forces may generate sufficient damage. Therefore, a continuous conductive path with the lowest possible impedance must be providedto carry the lightning discharge current between the air terminal and the grounding electrode. A typical lightning protection system usually consists of the following components: combination of air terminals and intercepting conductors located on a top of a structure or a building; system of grounding electrodes and grounding (down) conductor or conductors that connect the air terminals and the grounding electrodes. OK. So far, so good. But what parts of a building or a structure are most likely to be struck by lightning? These are the parts that are usually elevated above a building, and they include elevator machine rooms, rooftop air conditioners, chimneys, skylights, flagpoles, ridges, tanks, masts and terminals of rooftop communication equipment. Therefore, all components of a lightning protection system (the lowest impedance path) should be isolated from the metal components of a building structure, unless metal parts of the building are completely insulated from the system used for bonding to ground of electrical equipment installed in the building. In this latter case, in some situations, metal parts of a structure may be used as components of the lightning protection system.
Let’s examine why some buildings and structures should be provided with a lightning protection system.
The criteria for determination of a need for lightning protection may include (but is not limited) to the following aspects:
1. Relative exposure. This aspect takes into account a possibility of hazard to each building in closely built up areas versus hazard to a structure in the open rural areas, where farm buildings may become the most prominent targets for lightning. Buildings located on high ground in areas with hills and mountains are usually subjected to a higher degree of exposure to lightning strike than the structures located in a valley or in an otherwise sheltered area.
2. Frequency of thunderstorm/lightning activity and severity of such activity. The need for protection based on this criterion varies, but it is not necessarily directly proportional to a sheer frequency of thunderstorm. Rather, a few severe lightning storms during a season may make the need for protection greater than a relatively high number of thunderstorms with lighter intensity.
3. Safety of occupants. This aspect has become a very important factor in light of the liability for safety of the building occupants. A type of a building use and a type of building occupancy should be considered in evaluating lightning safety.
4. Nature of a building and the contents in it.This aspect should take into consideration building design, type of construction and available fire protection means, replacement value of the structure and building contents, historical value of the building and its susceptibility to lightning loss or damage.
5. Indirect losses. This aspect should take into account losses that relate to interruption of business or operation in respect to safety, comfort or well-being of people.
There are various risk assessment guides offered by the industry standards in respect to determining the risk of loss due to lightning strike.
These industry standards also provide installation requirements for all components of a lightning protection system. So, for the purpose of our discussion, these standards will answer the question how to install the lightning protection system.
It is interesting to note that although the Canadian Electrical Code provides criteria for mandatory installation of such devices as lightning arresters and prescribes specific installation rules for lightning arresters and low-voltage surge protectors, it does not specifically mandate installation of a lightning protection system. However, some Rules of the CE Code allude to such installation.
For example, Rule 12-016 mandates clearances between lightning rod conductors and conductors of a building wiring system. Rule 10-702 states that interconnection between the grounding electrodes of a lightning protection system and the grounding electrodes of wiring systems in a building (grounding electrodes of power, communication and community antenna distribution systems)must be made only at or below ground level. Rule 10-706 forbids use of a lightning protection system for a purpose of grounding the building electrical wiring systems and for a purpose of bonding to ground the building electrical equipment. All these requirements of the CE Code are intended to prevent safety hazards that might arise from directing the lightning discharge current to the grounding electrodes via building wiring systems.
It is also interesting to observe that Explanatory Note in Appendix B on Rule 10-706 clarifies that practices for installation of a lightning protection system are provided in the CSA standard CAN/CSA B72 and other national and international standards. Some jurisdictions specifically mandate use of the CSA B72, when a lightning protection is installed in a building.
For example, city of Vancouver has published Bulletins 2000-040-EL and 2000-046-EL (see http://vancouver.ca/commsvcs/LICANDINSP/bulletins/index.htm). These Bulletins explain the difference between grounding of a communication and community distribution equipment and systems that are installed on rooftops, and between installation of a lightning protection in a building or a structure. These Bulletins intend to clarify the hazards associated with inadvertent use of grounded rooftop communication equipment as a lightning rod. The Bulletin 2000-040-EL also requires a declaration from every electrical contractor installing a lightning protection system that the system is installed in compliance with B72.
B72 is also referenced in the National Building Code of Canada as a mandatory standard to which installation of a lightning protection system must conform when it is providedon chimneys and venting equipment.
So, hopefully this article helped to clarify the essence of a lightning protection system (what question at the top of the article), desirable criteria of its installations (why question) and installation methods (how question).
However, as usual, authorities with jurisdictional power for regulating this subject matter must be consulted prior to the installation of a lightning protection system.
Read more by Ark Tsisserev