Lightning Protection Application Guide

At its core, lightning is a massive electrical spark between either the cloud and ground, ground and cloud, cloud and cloud, or cloud and upper atmosphere. The science behind understanding how lightning forms, propagates and dissipates is constantly evolving. The current theory is that particles such as water vapor or ice particles are in a constant state of movement via the convection cycle (warm air rises, cools and falls) which creates positively or negatively charged particles. The building energy in these particles eventually reaches a point where it must equalize its electrical polarity.

All objects can generate streamers of oppositely charged energy which assist in creating a pathway for the resulting energy to flow (in our instance) from the cloud to the ground. Massive amounts of energy begin to travel through the pathway once the streamer from the ground and the stepped leader from the cloud unite. A typical lightning stroke carries current of 5 kA - 20 kA and a voltage of approximately 100 kV. The equalization-of-energy event is over in less than 30 microseconds or 1/10,000th the amount of time it takes to blink your eyes. This process repeats around the globe approximately 50 times a second or 1.4 billion times a year.

Did you know?

• A typical lightning stroke will heat up the surrounding air to a staggering temperature of 53,000°F (29,730°C). To put that temperature into perspective, the surface of the Sun is 10,340°F (5,730°C).
• Thunder is caused by the rapid expansion of the air within and surrounding the path of a lightning stroke due to the massive temperature spike associated with lightning.
• A typical lightning bolt has a diameter of 1" (25.4 mm) but it looks much larger due to the intense flash of light.
• Lightning can and will strike the same place multiple times. For instance, the Willis Tower in Chicago, IL is struck 50-100 times a year.

This guide is intended to assist code authorities, designers and installers in the development and installation of complete lightning protection systems to mitigate potential damage caused by a direct or indirect lightning event.

The safe dissipation of a lightning stroke requires proper system design and installation in accordance with an applicable standard such as UL 96A, NFPA 780, or IEC 62305, including common bonding to grounded building services such as electrical and communications. This installation guide covers installations used on virtually all types of structures designed and built today.

This application guide is not a substitute for the correct use of standards that are published or which could be adopted by a jurisdiction. The intent of this application guide is to help the user better understand the concept of developing a correct protection scheme for a structure.

System components that are described in this application guide are evaluated in accordance with standards such as UL 96 or IEC 62561 and are Listed by UL Solutions under an appropriate product category. A four-letter code (shown in parenthesis) following every category title in this guide is the UL Solutions product category code designation. A list of lightning protection equipment and related product categories evaluated by UL Solutions, along with the applicable standard(s), can be found in Appendix A.

Every UL Solutions product category code provides a direct link to guide information for the particular product category. The guide information includes the scope of the products covered, information relating to limitations or special conditions applying to the product, the requirements used for the investigation of the products, installation and use information, and information on product markings and the UL Mark to be used on the product. Guide information is available on UL Product iQ at www.ul.com/piq.

The product markings identified in this Guide do not include every possible marking that could be provided either on a product or in its installation or operation instructions. The purpose of this Guide is to provide you with an indication of the type of text and location of markings that address features which may be critical in determining if a product is certified and / or if it is installed correctly. Refer to the specific Guide information for the product category for additional marking information.

The numbering for referenced sections used in this document may change as the specific standard or code is updated. A list of model codes and standards applicable for each product can be found in Appendix B.

Upon completion of the installation, an inspection is performed against a specified standard and a Master Label Certificate^® or Letter of Findings is issued to compliant systems. The provided certification comes standard with a five-year expiration date and after this expiration date the system should be inspected and a new certificate issued. This five-year re-inspection duration takes into consideration any possible damage from weather related events, any intentional or unintentional damage by maintenance personnel or civilians, modifications to the building envelope, and updates to the originally specified installation standard.

Most codes and regulations require the certification of this equipment to applicable safety-related standards. They also may require this equipment to be certified to energy performance standards. Products that are certified to safety-related standards have been evaluated with regard to all reasonably foreseeable safety-related hazards, including fire, electrical shock and mechanical hazards. Such products are termed “UL Listed.” Products that are certified to a limited range of hazards, or for use under specific conditions, are termed “UL Classified.” Alternatively, any of these products can be “UL Certified” and bear the UL Certification Mark. Whether Listed, Classified or Certified, UL Solutions is required to include the model designations of UL Listed, Classified and Certified products in the product certification directory, known as UL Product iQ.

It is important to distinguish the difference between “UL Listed” and “UL Classified” and the relation these terms have with the term “listed,” as used in various codes. The term “listed” in the codes generally indicates that the product is required to be evaluated in accordance with the appropriate standard(s) by an independent third-party certification organization such as UL Solutions. The term “listed” in the codes should not be confused with the term “UL Listed,” as explained above. It is important to recognize that not all certification agencies make this distinction in their certification services.

There are several types of UL Marks that can be found on lightning protection related equipment. General information on each of these Marks is provided below. Each has its own specific meaning and significance. The only way to determine if a product has been certified by UL Solutions is to look for the UL Mark on the product itself.

The UL Mark on a product means that UL Solutions has tested and evaluated representative samples of that product and determined that they meet the requirements in the applicable standard(s). Under a variety of UL Solutions programs, certified products are periodically checked by UL Solutions at the manufacturing facility to determine that they continue to comply with the standard(s).

UL Marks may only be used on, or in connection with products certified by UL Solutions, and under the terms of a written agreement between the manufacturer and UL Solutions.

AIR TERMINAL – A type of strike termination device intentionally installed for the purpose of intercepting lightning flashes. These items are sometimes referred to as lightning rods.

BONDING – An electrical connection between an electrically conductive object and a component of a lightning protection system with secondary conductor that is intended to significantly reduce potential differences created by lightning currents.

BUILDINGS:
a) Ordinary Building – A building of common or conventional construction used for ordinary purposes, whether commercial, farm, industrial, institutional or residential.
b) Class I Ordinary Building – A building that is not more than 75 feet (22.9 m) high.
c) Class II Ordinary Building – A building that is more than 75 feet (22.9 m) high.
d) Metal-Clad Building – A building with either sides or roof made of or covered with sheet metal.
e) Metal-Framed Building – A building with electrically continuous framing of sufficient size and conductivity to be used as part of the lightning protection system.

CHIMNEY – A smoke or vent stack not meeting the requirements of a heavy-duty stack.

CONDUCTOR – The portion of a lightning protection system intended to transfer lightning discharge currents between strike termination devices and ground or to provide potential equalization between conductive bodies in/on the structure.
a) Main Conductor – A conductor intended to conduct primary lightning currents that interconnects strike termination devices with grounding electrodes.
b) Secondary Conductor – A conductor that connects metal bodies within the zone of protection to the lightning protection system to eliminate electrical potential that may create arcing.

EARTH – Finished grade level around a structure.

FASTENER – An attachment to secure a conductor to a structure.

GROUND GRID – A system of grounding electrodes consisting of interconnected bare cables buried in the earth to provide a common ground.

GROUNDING ELECTRODE – That portion of a lightning protection system extending into the earth, such as a ground rod, ground plate or conductor, serving to bring the lightning protection system into electrical contact with the earth.

GROUNDED – Connected to earth, or to a conductive material that is connected to earth, so that electric charges are distributed freely to the earth.

LIGHTNING PROTECTION SYSTEM INSTALLER -- Installers of Lightning Protection Systems shall be Listed in UL Product iQ under CCN “OWAY” as installer for Lightning Protection System installations. The issuance of a Master Label^® Certificate is evidence that the installation of the lightning protection system (1) has been made by a Listed company, (2) employs materials subject to factory inspection service and bears the UL Mark, and (3) is subject to a field inspection program covering proper installation of the system.

LIGHTNING PROTECTION SYSTEM – A complete system of strike termination devices, conductors, grounding electrodes, interconnecting conductors, surge protective devices, connectors or fittings.

LISTED - Equipment, materials or services included in a list published by an organization that is acceptable to the authority having jurisdiction and concerned with evaluation of products or services, that maintains periodic inspection of production of listed equipment or materials or periodic evaluation of services, and whose listing states that either the equipment, material or service meets appropriate designated standards or has been tested and found suitable for a specified purpose.

LOOP CONDUCTOR – A conductor:
a) That encircles a structure; and
b) That is used to interconnect grounding electrodes, main conductors or other grounded bodies.

METAL BODY:
a) Grounded (Non-Isolated) Metal Body – A metal body having a ground path independent of the lightning protection system.
b) Isolated (Ungrounded) Metal Body – A metal body having no ground path.

ROOF, FLAT OR GENTLY SLOPING – A roof having a span of 40 feet (12.2 m) or less and a pitch of less than 1/8, or a roof having a span exceeding 40 feet (12.2 m) and a pitch less than 1/4. Roof pitch is determined by the method illustrated in Figure 5.1.

SECONDARY ROOF AREA – A secondary roof is less than 10 percent of the total roof area of the protected building and lower than adjacent roofs.

STACK, HEAVY-DUTY – A smoke or vent stack more than 75 feet (22.9 m) high, and in which the cross-sectional area of the flue is more than 500 square inches (0.3 m2).

STRIKE TERMINATION DEVICE – A metallic component of a lightning protection system that intercepts lightning flashes and connects them to a path to ground.

STRIKING DISTANCE – The distance over which final breakdown of the initial strike to ground or to a grounded object occurs.

SURGE PROTECTIVE DEVICE (SPD) – A device composed of at least one non-linear component and intended for limiting surge voltages on equipment by diverting or limiting surge current and is capable of repeating these functions as specified.

ZONE OF PROTECTION – The space adjacent to a grounded air terminal or mast or overhead ground wire that is substantially immune to direct strokes of lightning.

Determining the correct methodology for designing a system of protection is critical to its efficiency and functionality.

Protection techniques may be straightforward and easy to design for an average size structure with a simple roof type, one roof elevation and no uneven vertical changes. However, if the building structure becomes complex with changes in roof elevations such as a multi-story section, defined shapes such as dormer projections or tall objects such as stacks, it will require different considerations for design to afford proper protection of the entire structure.

These are dedicated and intentionally located objects on a structure designed to receive direct lightning strikes. Some common names for strike termination devices include air terminals, lightning rods and Franklin rods, but in some instances objects such as roof mounted flag poles, handrails or structural metal screen walls can be utilized as strike termination devices.

It is important to note that on a traditional lightning protection system, the presence of strike termination devices does not attract nor increase the likelihood of the structure being struck by lightning.

As an integral component of a lightning protection system, conductors carry and conduct the harmful currents associated with a lightning strike to grounding electrodes buried within the earth.

These conductors are quite small, typically between 3/8" and 5/8" in diameter, but can effectively carry thousands or tens of thousands of amps for a short duration without exceeding their current carrying capacity. Compare this to your typical #4/0 AWG bare copper conductor which when subjected to a continuous current of 3.2 kA begins to fuse.

Although it seems impossible for such a small conductor to carry such significant current, it is important to remember that an average lightning event lasts less than 30 microseconds. As an example, 1 electrical cycle measured in Hertz (Hz) on a 60Hz electrical system is approximately 16.67 milliseconds or 16,670 microseconds. During an electrical system fault, the overcurrent fuse or circuit breaker will trip or open in less than 3 electrical cycles and in most cases faster than that. The conductors in the electrical system will heat up but, in most cases, will not be damaged. The rapid nature of a lightning event allows the conductor to remain cool and typically undamaged after conducting upwards of 20 kA or more.

An effective lightning protection system must include a connection to earth and successfully dissipate hazardous currents into the earth.

A grounding electrode can be one or a combination of the following: a driven ground rod, concrete-encased electrode (Ufer), ground ring, radial or ground plate.

The most common grounding electrode is a driven copper-clad ground rod. Other materials, such as solid copper and stainless steel, are also available. However, conditions such as sandy soil, bed rock or corrosive compounds within the soil may require grounding electrodes beyond the industry standard.

As lightning currents travel from the strike termination devices to the grounding electrodes, differences in potential can occur on and within a structure. Often overlooked or improperly addressed, these potential differences can cause sideflash or arcing from the lightning protection system to nearby conductive metal bodies such as HVAC units, ducting, piping or electrical conduit.

For example, assume a particular portion of the lightning protection system has 20 kA worth of current on it and has been routed 12 inches away from an electrically motorized roof hatch. There is a zero-volt electrical potential difference at a reference point where the lightning protection system was connected to the electrical system at grade or considered a ground reference point. Back on the roof there is approximately 150 feet of electrical conduit from the ground reference point to the motorized roof hatch and 75 feet of lightning protection conductor to the same ground reference point.

The through-air dielectric breakdown per foot is about 914 kV and, with tens of millions of volts potential difference in the average lightning strike from cloud to ground, the spacing of the lightning protection system to objects on a roof can be critical.

The sideflash between the lightning protection system and grounded metal objects is based on the current flow and potential difference. The more paths for current flow, the lower the potential difference for each path and thus the shorter the distance the lightning energy will seek other sources of grounding for an alternate path to dissipate in earth.

The current path will follow the lowest resistance or, more accurately, the lowest impedance path since the lightning event is a high frequency occurrence. For the purposes of this example, we will use resistance as the measurement to keep the math simple and assume a run of electrical metallic tubing (EMT) is used as the conduit. This EMT has an end-to-end resistance of 0.201Ω and the end-to-end run of lightning protection conductor has a resistance of 0.00737025Ω.

The resistance in the EMT is about 186% higher than the resistance in the lightning protection conductor and the lightning energy will follow the path of least resistance; however, it may still flashover to the motorized roof hatch. Damage can be mitigated or reduced by either creating another ground reference point by bonding the motorized roof hatch to the lightning protection system at the roof level or increasing the separation distance from the motorized roof hatch to the lightning protection system.

The interconnection will not cause the lightning energy to use the EMT as a path to ground. Rather, the higher resistive path will raise the voltage potential above zero for the duration of the lightning strike event and allow the energy to dissipate through the lightning protection system without causing a sideflash.

Based on information available through the Insurance Information Institute (III), more damage occurs to structures through indirect lightning strikes hitting the electrical utility than a direct lightning strike to the structure. This makes sense as there may be miles of overhead or buried electrical lines feeding the structure which make a perfect attachment point for lightning.

An SPD (previously known as a transient voltage surge suppressor or TVSS) provides that front-line protection where electrical and conductive communications utilities enter the structure. The operation of an SPD is rather simple in that, under normal operation, it operates within a high impedance state waiting for excessive voltages.

Upon sensing voltages above a threshold (known as voltage protection rating or VPR), the SPD enters a low-impedance state and provides a means to shunt excess voltage to ground. The SPD automatically enters a high-impedance state once the surge voltage is cleared until the next surge of voltage is detected.

The use of a single continuous conductor in a lightning protection system is rare. Connectors and fittings allow the joining of two or more conductors and are required at all end-to-end, tee, “Y” or parallel splice locations. Connectors can either be a crimp, mechanical (bolted), high compression, brazed or welded type.

While not a requirement of leading standards or codes but rather good installation practice, each type of connector has its benefits and drawbacks and really should only be used where their benefits outweigh the drawbacks. For instance, you wouldn't want to install a crimp or mechanical (bolted) connection in a concealed or buried environment as they require routine inspections to verify proper clamping and torque. Conversely, a high compression, brazed or welded style of connection offers excellent longevity with little to no maintenance. For these reasons, you would not want to install a high-compression, brazed or welded style of connection on a roof where removal of connectors is often required for the maintenance of various rooftop mechanical units.

Stacks above 75 feet in height require some special consideration in both design and material used for the system. All components shall be Class II and Class II modified (Class III). The components in the upper 25 feet shall be copper, copper alloy, bronze or stainless steel. The top 25 feet can be a high corrosion zone and extra protection of a min of 1/16 in coating of lead is required for terminals, mounting brackets and conductors. Aluminum components are prohibited in this installation.

Class II air terminals on stacks shall be solid copper, copper alloy, stainless steel, titanium or Monel^®. The installation shall be evenly distributed around the top of a round stack at intervals not to exceed 8 feet. If the stack is square, the location shall be not more than 2 feet from corners and spaced not more than 8 feet apart.

Each Class II air terminal shall be not less than 18 inches in height and not more than 30 inches in height. The diameter shall be not less than 5/8 inch. All terminals shall be interconnected and there shall be at least 2 down conductors located on opposite sides of the stack. Reinforcing steel in concrete stacks shall be electrically continuous and shall be bonded to the lightning protection system at its upper and lower ends at down-lead locations.

A lightning protection system that has been designed and installed in compliance with a recognized standard can provide a reasonable expectation or means to mitigate damage caused by a lightning stroke. Proper maintenance of the installed system is essential to assuring the long-term viability of the protection level afforded.

Visual inspections of the installed system should occur yearly or after any known severe weather event. While no visible damage is typically observable immediately following a direct lightning event, other weather-related events such as high winds during a severe storm can directly or indirectly cause immediate damage to the system.

Any issues found during a maintenance inspection should be remedied as soon as practical by a qualified individual.

Certification allows for independent third-party verification that the lightning protection system was designed and installed in compliance with a specified standard. Beware of “self-certification” by the installation company. Approximately 85% of installations inspected by UL Solutions require some sort of repair or modification to comply with the specified standard. These findings are often instances such as loose cable fasteners or improperly protected down conductors which, if not properly repaired, could lead to further or more excessive damage over time. Unfortunately, higher risk situations have been found related to issues such as improper application of zone of protection or down conductor frequency.

Utilize an International Accreditation Service (IAS) accredited inspection agency in accordance with ISO/IEC Standard 17020:2012, Conformity assessment — Requirements for the operation of various types of bodies performing inspection that will provide an unbiased review and inspection of the installed system.

Once the system is completed, UL Solutions Field Staff will perform an on-site inspection at the request of the UL Listed Installation contractor. The system inspection is not limited to, but begins with an overall visual inspection for neat and professional installation of the following:

• Air terminal layout and placement position with at least 2 directional paths to ground based in structure elevation, roof style and projections as specified within the Zone of Protection.
• Air terminal tips shall be a minimum of 10 inches in height above the structure spaced not more than 20 feet apart for perimeter protection.
• When terminals greater than 24 inches in height are used, they shall be supported at a point at least half the height of the rod and spaced more than 20 but not less than 25 feet apart for perimeter protection.
• Any object elevated above the normal roof height and with a surface thinner than 3/16 inch shall be protected. This includes objects such as exhaust fan housings and camera masts.
• All lightning protection system components shall be listed, except components such as hardware screws, bolts and adhesives.
• Main size conductors shall be used for bonding the lightning protection system to other systems, including, but not limited to metallic water systems, steam or water heating systems, electric services, telephone systems, antenna grounds, and other large grounded metallic masses. See section 10 of UL96A for details.
• The grounding terminal rod shall be at least 8 feet in length and not less than 1/2 inch diameter, buried not less than 10 feet. Grounding plates, concrete encased electrodes, ground rings and trenched electrode systems are also permitted.
• Copper components are not mounted on aluminum or external galvanized surfaces.
• Cable bend radii throughout the installed system have a minimum 8-inch radius and 90 degree bend.
• Cable securement attachments for both main size and secondary size conductors shall not be more than 3 feet apart.
• Aluminum components shall not be installed in direct contact of copper roof material or below any runoff from copper surfaces.
• Aluminum components shall not be in direct contact with earth.
• Aluminum down conductors shall be connected to copper conductors using bimetallic fittings above earth level.
• Stacks above 75 feet in height require Class II components.

This document demonstrates that the system installed was found to be fully compliant for the protection of the entire structure.

Although similar to a Master Label Certificate^®, a Letter of Findings is issued in instances of limited scope or a partial installation. Such instances could include a new surgical wing at an existing medical center. During the bidding phase, the project scope for the lightning protection system is typically limited to the new portions of the structure only. In these instances, a Master Label Certificate^® might not be a practicable deliverable as the condition of the remainder of the system might not be known, or inspection and/or maintenance of the existing system is not in the budget scope of the project. A Letter of Findings can provide evidence that the newly installed lightning protection system is compliant with the standards specified.

An Engineering Inspection Report may be issued for inspections where the standard specified is not a nationally adopted standard or industry accepted standard for lightning protection system installations; when criteria for the inspection is based on manufacturer’s installation guidelines; when inspection criteria require additional information to be included in the report such as ground resistance test results, design drawings, risk analysis reports or installation photographs; when a UL customer is not also a UL Listed Installer under the product category code (OWAY).

The Engineering Inspection Report is an informational document. The document is provided directly to the customer requesting the inspection and is not typically published in the public domain on UL Product iQ (www.ul.com/piq).

When we look at a Lightning Protection System in its most elementary form, it is quite simple. Air terminals intercept a lightning strike, a low impedance conducting cable connects the air terminals to the earth using a conducting electrode and provides a pathway to dissipate the energy into the earth.

However, while the overall concept may appear simplistic in what it takes to get a lightning strike grounded into the earth, there has been a great amount of science and theoretical investigation over the last two plus centuries. Today’s modern products, such as air terminals, mounting and support hardware, and main and secondary conductors conducting the path to earth along with grounding electrodes, have undergone testing and evaluation to achieve product listing and compliance with the Standards for installation. The physical science has been studied and modern theories have evolved to help give a structure better protection by maximizing air terminal placement on roof surfaces.

The UL Lightning Protection Master Label Certificate is NRTL (Nationally Recognized Testing Laboratory) evidence that the system has been installed and inspected to a specified standard. This certification has a 5-year lifespan and can demonstrates that the building has been afforded an adequate means to mitigate the potential damage caused by a lightning event.

Installers of Lightning Protection Systems shall be Listed in UL Product iQ under CCN “OWAY” as installer for Lightning Protection System installations. The issuance of a Master Label^® Certificate is evidence that the installation of the lightning protection system (1) has been made by a Listed company, (2) employs materials subject to factory inspection service and bears the UL Mark, and (3) is subject to a field inspection program covering proper installation of the system.