I recently encountered a lightning protection system on a water treatment plant that was certified by UL. What is a "Letter of Findings"?

Question: Letter of findings

I recently encountered a lightning protection system on a water treatment plant that was certified by UL with what was referred to as a ‘Letter of Findings.’ The only type of lightning protection certification I’m familiar with is the UL Master Label. What is a ‘Letter of Findings,’ and what other types of certifications currently are available for lightning protection systems?


A Letter of Findings is one of the three basic types of certifications that Underwriters Laboratories Inc. provides for lightning protection systems. The others are the more recognized Master Label and Reconditioned Master Label certifications. Below is a description of each of these programs.

Master Label — The Master Label is a Listing Mark that covers a lightning protection system installed in accordance with UL 96A, Installation Requirements for Lightning Protection Systems. With any certification based on UL 96A, there are limitations regarding the types of buildings that may qualify for the protection system. Buildings that do not qualify for a Master Label include those that handle, process or store flammable or explosive materials and structures with incomplete lightning protection systems. A partially protected structure may include a church where only the steeple is protected or a school where only the chimney is protected. Other examples of partial protection include a series of interconnected buildings where only one building is protected and the others are not, or interconnected buildings where both buildings have systems, but only one system complies with the current Standard UL 96A.

Reconditioned Master Label – The Reconditioned Master Label is a Listing Mark that covers a lightning protection system that has been refurbished or expanded to comply with UL 96A. This type of certification is identical in scope with the Master Label, except that it applies only to refurbished systems.

Letters of Findings – UL’s Letters of Findings indicate that an isolated roof section or an entire building complies with either UL 96A, NFPA 780 or Army Material Command (AMC)  Regulation 385-100. The requirement regarding the size of an isolated roof section states that firewalls or similar means of building separation must isolate the protected roof section from other sections of the roof.

The lightning protection system must also be complete within itself (i.e., have the proper number of down conductors and air terminals for its size, etc.).

The following Standards include the noted limitations:

UL 96A, Installation Requirements for Lightning Protection Systems – This Standard does not cover buildings that  store or handle explosive materials, flammable liquids, or flammable gases. NFPA 780, Lightning Protection Code – This standard does not cover buildings that house explosive materials.

AMC-R 385-100, Safety Manual — This standard does not include limitations.

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