What a surge is
A surge, or transient overvoltage, is a very brief spike in voltage, often lasting only microseconds to milliseconds. Even a short spike can be enough to damage electronic equipment, degrade insulation or confuse a controller. Modern plants are full of sensitive electronics, which is why surges that once passed unnoticed now cause real problems.
This article is a general overview. Lightning protection systems and surge protective device selection are specialist work that follows local codes and standards and should be handled by qualified people.
Where surges come from
External sources. Lightning is the best-known one. A strike does not need to hit a building to cause trouble. A nearby strike can induce voltage in overhead lines, buried cables, data lines and metal piping, and raise the potential of the ground itself. Utility switching and faults on the network also create transients.
Internal sources. In practice, many surges start inside the plant. Switching large inductive loads such as motors and transformers, operating capacitor banks, and clearing faults can all create transients. Variable speed drives and welding equipment add electrical noise of their own. Because internal sources are frequent, equipment can suffer gradual wear from repeated small surges, not just damage from a single big event.
Direct strikes versus surge protection
Two things are often confused. A lightning protection system, with air terminals, down conductors and grounding, is meant to give a direct strike a controlled path to earth and reduce fire and structural damage. Surge protective devices, usually shortened to SPDs, limit the voltage that enters the electrical and signal wiring. A building can have one without the other, and they work best together, with consistent bonding between them.
Whether a facility needs a lightning protection system depends on its exposure, contents, height, location and local rules, so this is normally assessed through a formal risk evaluation rather than guesswork.
What a surge protective device does
An SPD contains components that stay passive at normal voltage and become conductive when voltage rises above a threshold. During a surge they divert the excess current away from the protected equipment, usually toward the grounding connection, and then return to their normal state. Common internal components include metal oxide varistors and gas discharge tubes, sometimes combined.
An SPD does not stop lightning. It reduces the voltage that reaches downstream equipment, and has a limited capacity to absorb energy. Repeated or large surges gradually wear it out, and many devices include an indicator to show when they have reached the end of life.
Equipment that is most exposed
Control electronics, programmable logic controllers, instruments and industrial networking gear are usually more vulnerable than heavy electrical equipment. Standby generators and their controls, which sit outdoors in many plants, can also be exposed.
Layered protection
No single device handles everything. A common concept is to protect in stages:
- At the service entrance or main distribution, where larger devices deal with the biggest surges coming in from outside.
- At sub-distribution panels and feeders, where further devices reduce what remains.
- At sensitive equipment, such as control cabinets, instrumentation and drives, where smaller devices provide a final level of protection.
- On data, communication and signal lines, which can carry surges into equipment even when the power wiring is well protected.
Devices at different stages need adequate wire length and coordination between them, as the manufacturer instructs.
The role of grounding and bonding
An SPD is only as good as its connection. Long or poorly routed ground leads add inductance, which allows voltage to build up during a surge and weakens protection. A low-impedance connection to a properly bonded grounding system is essential. Differences in potential between separate grounding points can also stress equipment connected between them, such as a sensor in one building wired to a controller in another, which is why bonding between structures is a design topic.
Typical sites at higher risk
Remote or outdoor equipment, long cable runs between buildings, tall structures, pump stations, tank farms, wind turbines and areas with frequent thunderstorms tend to have higher exposure. Facilities with expensive electronics, or where downtime is costly, usually have more to lose and so give protection more attention. A power outage or downtime can also lead to losses well beyond the damaged parts.
Inspection and maintenance
SPDs are not install-and-forget; they belong in a preventive maintenance routine and in routine checks of electrical equipment. Typical practice includes checking status indicators, inspecting connections and leads, looking for signs of overheating, and replacing devices after major surges or at intervals recommended by the manufacturer. After a thunderstorm season, some sites review their protective devices. A device that has failed may leave equipment unprotected without any obvious sign.
Limits and safety
Surge protection reduces risk but does not eliminate it. Some damage can still occur, particularly from a direct strike. During thunderstorms, outdoor work and work on exposed lines should follow site rules, and electrical work on SPDs should be done only by qualified personnel following manufacturer instructions, lockout tagout procedures and local regulations.