Rolling Sphere Method
The Rolling Sphere Method is a popular and acceptable method of implementing lighting protection on structures. In the Philippine Electrical Code, the rolling sphere method is mentioned in Articles 8.10.2.5 and 8.20.4.1. However, the articles refer to NFPA 780 for additional information on the rolling sphere method.
The Rolling Sphere Method is an aid in identifying the areas of a building or structure that needs protection using air termination rods. This method also takes into account the possibility of strikes to the side of buildings.
Lightning Protection System
Lightning Protection System (LPS) is defined as
- Lightning Protection System (NFPA 780)
- A complete system of strike termination devices, conductors (which could include conductive structural members), grounding electrodes, interconnecting conductors, surge protective devices, and other connectors and fittings required to complete the system.
- Lightning Protection System (IEC 62305)
- A complete system used to reduce physical damage due to lightning flashes to a structure.
Philippine Electrical Code 2017 Edition
The Philippine Electrical Code 2017 edition, rolling sphere method of lightning protection are mentioned in:
8.10.2.5 Grounding. Masts and metal structures supporting antennas shall be grounded in accordance with 8.10.2.11, unless the antenna and its related supporting mast or structure are within a zone of protection defined by a 45 m radius rolling sphere.
FPN: See 4.8.3.1 of NFPA 780-2014, Standard for the Installation of Lightning Protection Systems, for the application of the term rolling sphere.
8.20.4.1 Cable Bonding and Grounding. The shield of the coaxial cable shall be bonded or grounded as specified in 8.20.4.1(A) through (E).
Exception: For communications systems using coaxial cable completely contained within the building (i.e., they do not exit the building) or the exterior zone of protection defined by a 46 m (150 ft) radius rolling sphere and isolated from outside cable plant, the shield shall be permitted to be grounded by a connection to an equipment grounding conductor as described in 250.118. Connecting to an equipment grounding conductor through a grounded receptacle using a dedicated bonding jumper and a permanently connected listed device shall be permitted. Use of a cord and plug for the connection to an equipment grounding conductor shall not be permitted.
FPN: See 4.8.3.1 of NFPA 780-2014, Standard for the Installation of Lightning Protection Systems, for the theory of the term rolling sphere.
NFPA 780 - Standard for the Installation of Lightning Protection Systems
The article on lightning protection systems in the Philippine Electrical Code 2017 referred to NPFA 780 which was prepared by the NFPA Technical Committee on Lightning Protection and approved as an American National Standard.
NPFA 780 provides a rolling sphere diameter of 45 meters as provided in the following illustration.
IEC 62305 - Protection Against Lightning
The IEC 62305 has four (4) recommended rolling sphere radii depending on the Lightning Protection Level (LPL).
From the above table, the NFPA 780 is equivalent to IEC 62305 LPL III.
As only a single rolling sphere radius is provided in NFPA 780, a risk assessment is not required. On the contrary, IEC 62305 requires assessment to determine the LPL required for a particular structure. In normal practice, however, only LPL III is commonly used which is equivalent to the NFPA 780 recommendation. IEC 62305 LPL I or II are mostly used in explosive atmospheres and hospitals.
Beware of Uproven Lightning Protection Methods
The rolling sphere method mentioned in the Philippinie Electrical Code is included in the National Fire Protection Association standard NFPA 780 - Standard for the Installation of Lightning Protection Systems. This standard is based on extensive, peer-reviewed science and decades of field observations. Some companies providing lightning protection services, however, claim their "early streamer emission" air terminals are more effective than the distances embodied in Standard NFPA 780. Some even claim they can protect an entire building with a single air terminal regardless of the configuration of the structure. Other companies claim their "charge dissipation array" systems can prevent a lightning event altogether. Be cautious, these claims of exaggerated performance are not substantiated. In fact, courts, governmental agencies, and scientific committees have rejected the claims made by these fringe companies.