Structural lightning protection

Structural lightning protection (lightning rods), grounding, bonding, and surge protection are all important factors when considering the development of a reliable lightning protection system. However, as modern processes become more reliant on electronic automation and electrical equipment to increase productivity, advances in surge protection design have provided what has become the most critical element to any successful equipment lightning protection system. Lightning strikes and the dangerous surges and transients induced by lightning, as well as surges caused by switching and regulation problems, can lead to catastrophic equipment losses, operational downtime, production and revenue losses, and ultimately will lead to significantly increased equipment repair and electrical maintenance costs. During a lightning strike, voltage differences of tens of thousands of volts can arise between different parts of even the best grounding network. If the connecting wires do not pass through a Surge Protection Device (SPD), then the voltage differences are carried directly into the equipment, and can cause severe damage.


Grounding, Bonding, and Surge Protection

Many equipment installers work under the misconception that the grounding connection to the building ground system is what caused the equipment to fail in the lightning event. They assume that to protect their equipment, they will need to remove the building ground connection and/or add an isolated ground rod for their equipment. This is in error. The National Electric Code (NEC) specifically forbids the use of isolated ground rods unless the ground rods are properly bonded to the building ground system. Isolated grounding can lead to ground resistance readings at a single un-bonded ground rod in the hundreds or even thousands of ohms. The problem is, the equipment bonded to this isolated ground rod is also connected to the AC power ground and this usually presents a ‘differential’ in ground potential that will lead to a severe voltage rise at the equipment. This differential in ground potential and the resulting voltage rise will often lead to catastrophic equipment failure in a lightning event.

Equipment mounted outside a building is particularly vulnerable to damage. Rooftop equipment (HVAC, elevators, fire alarms, etc…), parking lot light poles, perimeter security, remote pumps and controls, etc… are potentially high energy entry points during a lightning strike. The higher risk of a direct lightning strike coupled with the distance from the electric service ground make these the most likely failure points in most buildings. Compressors, well pumps, and other outdoor electrical equipment are frequently mounted and bonded to concrete pads. In most cases, this pad can be a more effective ground than the connected, yet untested ground electrode or building ground system. Additional isolated grounding will not help this situation and while bonding these two references together will help, bonding alone will not prevent the loss of equipment. Only an appropriate SPD, mounted at the equipment, bonding all line wires, neutral, and the ground, can prevent this type of damage.

Copper conductors for communications and data can become ‘bonding jumpers’ between structures of different ground potentials and can carry destructive surge currents to sensitive electronic and electrical equipment. These ‘jumpers’ are attempting to bond the structures together and will carry destructive energy directly to the connected equipment. The only reliable defense for this situation is to install an appropriate Surge Protection Device (SPD) at each point closest to the equipment to be protected or where the conductor enters and leaves the structure. All inbound copper for power, signal, and communications should be protected at both ends of the ‘jumper’ that connects the structures to each other.

Choosing the Right Surge Protective Device

On power distribution systems, the SPD must be robust and capable of withstanding repetitive extreme lightning induced peak surge currents. The primary factors in choosing the appropriate SPD should be based on life expectancy and the effective clamping levels throughout its design life. Specification information provided by some manufacturers can be misleading and not very useful when comparing life expectancy and performance capabilities of respective SPDs. UL tests SPD products for safety and does not make performance or durability comparisons. Numerous independent test labs can provide more accurate performance comparisons of SPD products.

Multiple ‘layers’ of coordinated protection have long proven to be the most effective approach to preventing lightning damage to equipment. On the AC power, it is important to have a hardwired SPD at the service entrance, at all branch panels, and again, as close as possible to each critical load. IEEE recommends a ‘cascade’ of the power protection at three or more levels to remove any of the SPD “let-through-voltage” from the high peak surge current of the lightning event. The basic rule is: start at the load and work your way back to the main. In some cases, the three most effective layers of protection may not even involve the main service.

The only way to completely protect your equipment from surge related failure is to suppress every power and signal circuit into the equipment. Appropriate signal protectors are available for most connections, but they must be carefully selected and matched to the application.

Peak Surge Current Ratings

The Peak Surge Current rating is normally the largest single surge current (kA) that a device can withstand without damage. This rating should exceed the largest single surge that the SPD would experience in its lifetime. Installations in high-lightning areas should use an SPD with a higher surge current rating, in the range of 50 kA (50,000 amps) to 200 kA (200,000 amps), in order to provide a longer service life and higher reliability.

Many manufacturers publish Peak Surge Current ratings that are aggregate totals of individual components within the SPD. Actual peak performance may be only a fraction of this total. Many SPD comparisons begin with the peak surge rating and we have found that this can sometimes be very misleading information. For example, published Peak Surge Current Ratings in excess of 200,000 amps were not ever tested to these levels. The average lightning strike will vary from 20,000 to 50,000 amps. Extreme lightning strikes may exceed 100,000 amps. SPD ratings that exceed 100,000 amps are usually the sum total of the parts in the device and while the higher peak rating numbers will provide more durability, any event that exceeds the weakest link of the device will still cause it to fail.

Over-current Protection

Circuit breakers, fuses, and relays are not fast enough to protect sensitive electronic and electrical equipment from damage due to lightning surges. However, to reduce the severity of the SPD damage in overload situations and to protect from the risk of fire, and to protect the integrity of the power distribution system, the AC power SPD should be installed with the appropriate over-current protection (circuit breaker or fuses). It is very important that some means of disconnection to the power be close by the installed SPD to safely service the device.


Effectiveness of an SPD is strongly affected by the installation. The primary issues that need to be addressed are grounding, lead length, and over-current protection. Only a qualified electrician should install an SPD on an electrical system. Proper testing of the grounding system at the time of the SPD installation is very important. However, even with the best grounding and bonding practices, differentials in ground potential will still exist, especially with multiple structure sites. This fact makes the proper application of the SPD at both ends of data and communication wiring combined with cascaded power protection even more critical to the protection system.

To achieve optimum peak surge current and ‘let-through-voltage’ protection, the connecting leads between the panel unit SPD and the panel or protected equipment should be as short as possible and without sharp 90-degree bends. The proper operation of the SPD will depend on the proper installation, so if there is any doubt, manufacturers assistance should be requested to assure proper installation. For complex systems, the installer should consult with the protector manufacturer or with installers who specialize in surge protection for complex systems. There have been many problem calls generated by installers who tried to use the wrong protector for the given application. This is especially true for low voltage data and communication circuits.

Temporary Over-voltage (TOV)

Short term (swells) and longer-term Temporary over-voltage (TOV) conditions can lead to rapid degradation and even complete failure of SPD components. This (not lightning) is the leading cause of SPD failure. TOVs are typically the result of loose or open neutral conductors, voltage regulator problems, or the inadvertent contact of higher voltage to the system. If the TOV voltage exceeds the Maximum Continuous Operating Voltage rating (MCOV) of the SPD, the Device will attempt to suppress the over-voltage and will begin to conduct current. This can lead to significant component level over heating and eventual destruction of the SPD.

Patented frequency based power suppressors have circuitry that allows them to combine a very low surge limiting voltage (let through voltage) with the capability to withstand TOVs. They are not as vulnerable to most open-neutral faults and subsequently survive in the field for a significantly longer operating life while protecting at the lowest possible clamping levels.

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Signal Protection

Isolated grounding remains the biggest problem with most communications systems. It is critical that the Telco system be bonded to the AC Power to eliminate any potential differentials. Most telephone systems have “primary protectors”(spark gap protectors, based on either carbon or gas discharge tubes) provided by the phone company. The combination of poor grounding and this low grade suppression will place many phones systems and the connected equipment at risk in a lightning event. The addition of secondary signal surge protection and bonding protection for the system is the only way to protect these types of circuits.
Complete protection can be obtained only if every power and signal circuit entering into the specific equipment passes through a protector. Appropriate signal protectors are available for most connections, but they must be carefully selected and matched to the application. For communications or data lines that do not leave the structure, we recommend a minimum of 500 amp peak surge capability. For communications and data lines that do leave the structure, we recommend a minimum of 10,000 amp peak surge capability.

Other Factors, Myths, and Misrepresentations

The installation of lightning rods will not protect electrical and electronic equipment from surges and lightning damage. Lightning rods sole function is to protect buildings from structural damage and potential fire as a result of a direct strike.

Surge Protective Devices are incapable of saving energy. Conclusive testing has determined that the claims by some manufacturers of saving power with the addition of an SPD to your power panels are complete misrepresentations.
SPD warranty claims that involve connected load guarantees are extremely conditional and virtually impossible to collect. The fine print of the connected load guarantee usually specifies that the burden of proof is on the consumer that all circuits have been properly protected and grounded and, in addition, that the SPD was damaged during and not prior to the event. Connected load guarantees are marketing driven and much like extended warranties, are not designed to benefit the consumer.
When SPD protection is added to an operation that has been unprotected for any length of time, there will already be existing damage to processors, circuit boards, wiring insulation, and connections that took place prior to the installation of the SPD. The SPD will not ‘heal’ this damage and losses may continue for some time while the damaged equipment cycles out.

SPD product warranties that exceed five or even ten years (ie: 20 years to lifetime) are marketing tools that fail to recognize the normal life expectancy of the components within the SPD. Lifetime and long term warranties generally do not accurately reflect SPD durability or performance capabilities. By their very nature, the SPD is a sacrificial device designed to take excess energy to protect your equipment.

Due to the difficulty in comparing Joule ratings, many companies no longer publish this number. Most standards written recently in the surge protection industry either warn of the possible misuse of Joule ratings or, by omission, do not recommend the use of Joule ratings when comparing an SPD.

In Conclusion

We have found that good grounding, bonding, and accurate ground testing is fundamental, yet remains a complex problem and is not always possible. Even with the best grounding and bonding system that money can buy, electronics and electrical equipment will still suffer damage from lightning and surges without the addition of a quality surge protection on the power, communication, and the data circuits.