LPI Technical Bulletin 

Lightning Protection Systems & Bonding  
"An electrical connection between an electrically conductive object and a component of a lightning protection system that is intended to significantly reduce potential differences created by lightning currents." 

General Definition of Bonding, as provided by the NFPA 780 Standard for the Installation of Lightning Protection Systems .  

Examples of lightning protection system rooftop bonding requirements for structures and their appurtenances

The contractor who installed lightning protection at our airport facility bonded the system to our fire sprinkler system. On the Lightning Protection Institute website, I see mentions of bonding but don't see any detailed information. Can you provide information about bonding requirements for fire sprinkler systems as part of the lightning protection installation process?  

In the U.S. we follow the requirements of the National Fire Protection Association (NFPA 780 Standard for the Installation of Lightning Protection Systems). The current Standard edition is dated 2017. A complete lightning protection system for any structure includes strike termination devices, interconnecting conductors, a grounding system, interconnection or bonding to other building grounded systems, and surge suppression on incoming service lines. 
The standards require the interconnection of all incoming metallic piping systems that are grounded (led into earth ground) to the lightning protection system (LPS), which potentially includes water, gas lines, and sprinkler systems.  The interconnection is devised to create a common ground potential on all internal building grounded systems with the LPS. If this bonding is not accomplished, then lightning could potentially leave the lightning protection system to seek a "better" path to ground along an alternate path or piping system. 
Lightning side-flashing within a protected structure can cause significant damage to internal systems not designed to carry the lightning current--or worse can damage building materials en route, causing a fire or explosion.  When these systems are joined in equipotential bonding, the lightning current stays on the specified network provided by the LPS, rather than "seeking" an undesirable or alternate path to ground.
It's important to note that the required bonds are for the purpose of potential equalization and are not employed to utilize the piping as a lightning protection ground. The removal of such bonds eliminates a critical function of the LPS and could compromise the integrity of the system, resulting in fire, damage or an unsafe situation. 

A lightning protection bonding calculation table is available on the LPI  website. Copies of LPI 175 and NFPA 780 are also available for purchase and download on the LPI website store.
The Lightning Protection Institute (LPI) is a not-for-profit, nationwide group founded in 1955 to promote lightning safety, awareness and education. LPI is a leading resource for lightning protection and safety standard-compliant system requirements. Visit the LPI website at  www.lightning.org to locate a LPI-certified specialist in your area.