Multi-Level Car Park Lightning Safety
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Wed Aug 6, 2008
A question has been raised by a member from Australia regarding lightning safety in the multi-level buildings used for parking cars. Such buildings are common in the commercial districts in many large cities. The building is usually made of steel reinforced concrete. Of course, the top floor is open space (no roof). Hence it is open to direct lightning strikes.
To protect against the risk of falling, the roof is usually equipped with a balustrade (metal railing supported on metallic elements). The roof is illuminated via steel light poles. In the quoted example, the poles are 6 meter high and are arranged in a grid.
To protect the building against lightning, the designer proposed the following typical measures:
a) Bonding the light poles to the steel reinforcing steel bars of the concrete roof by welding their cast-in bolts during construction, and;
b) Bonding the balustrade to the reinforcing steel bars.
It is neither practical nor desirable to block access to the roof during thunderstorms, as users would at least need to retrieve the cars which may have been parked there before start of the thunderstorm. Hence the question whether the exposed metals surfaces of the light poles and the balustrade pose a risk of electrocution in case of contact of a person with these at the instant of a lightning strike. This issue is addressed hereafter. As an introduction to this, I will first discuss the issue of safety of electric utility workers as it raises similar questions that have been addressed by the industry many decades ago.
The maintenance and operation of power lines and substations necessitates contact with grounded surfaces which are subject to transient ground potential rise (GPR) at unforeseen times. The GPR may be caused by a 50/60 Hertz short circuit current, induction from nearby energized objects, or flow of lightning currents. In connection with the above, it should be noted that lightning can strike a power line at a point far from the work site and hence thunder would not be heard by the workers. When that occurs, a "travelling wave" propagates over the line and reaches the work site. When working on a de-energized grounded power line, the travelling wave then gets discharged into the ground and it generate a GPR.
Safety of workers in the above case is accomplished by creating the so-called "equi-potential zone". Basically, this is done by bonding together all the points which are likely to be contacted by different points of the body of the worker (hands, feet, etc.). In case of a worker standing on the ground, e.g. when operating an overhead disconnect switch at a tap of a power line, the equi-potential zone for his feet is created by placing a wire mesh or metallic plate beneath the soil where he will be standing. This "mat" is then bonded to the handle of the switch which he will be operating.
From the electrical point of view, the situation at the roof of the car parkade is similar to the above. First, the re-enforcing steel bars within the concrete floor form a grid having a small mesh size (separation between the bars). The steel light post or the balustrade which the user of the parkade may be contacting is bonded to the above grid. Hence these together form an equi-potential zone. It follows that the person would be safe if a lightning discharge occurred at the instant of the contact. Similarly, his feet would be within an equi-potential zone if he was just walking on the roof without touching any of the above metallic surfaces.
The above situation is different from being inside a home where, in the general case, the floors are made of insulating materials. Hence a large voltage difference can arise at the instant of the lightning strike between the feet and hand of a person if he contacted some metallic component (a plumbing fixture, chassis of an electronic device, metallic frame of a window, etc.). That is why persons taking shelter within buildings are advised to avoid contact with such objects during thunderstorms.
Returning to the case of the parkade, the above does not address the risk of a direct lightning strike to a person on the roof. However, an adequate level of safety can be accomplished by designing the shielding system for a higher interception efficiency. For example, Australian Standard AS/NZS 1768 lists four levels of which Level I provides a 99% interception efficiency and requires a Rolling Sphere radius of 20 meters, and a level II provides a 97% interception efficiency and requires a Rolling Sphere radius of 30 meters. Comparable levels exist in IEC Standard 1024-1 but the numerical values may be slightly different. Either of Levels I or II would provide adequate protection for the roof of a car parkade.
Depending on the separations between the steel light poles, they might form an adequate shielding systems, perhaps subject to the minor modification of a adding a lightning rod to each pole. If the separation is too large to provide adequate protection, the situation may be improved by using overhead shield wires instead of masts. The light poles can still be used as the support points for some or all of those shield wires.
Finally, it should be noted that the bonding requirement described above is a must to protect the building itself regardless of whether lightning safety was an issue or not. (It would not be an issue if access to the roof were blocked during thunderstorms.) This is because the current of a lightning strike to a light post or to the balustrade would end flowing into the reinforced steel of the concrete. In the absence of bonding, arcs will jump across the corresponding gaps within the concrete. This would then damage the concrete. This arcing phenomenon is well known to competent building professionals world wide.
Abdul Mousa, Ph.D., P. Eng., fellow IEEE
Lightning protection consultant
Vancouver, Canada
Abdul_mousa@hotmail.com
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