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The usual mental model of lightning damage is a bolt hitting the house and frying everything plugged in. That happens, but it is the exception. The typical case is quieter: a strike hundreds of feet away, a surge that arrives through the cable TV line, and a dead modem and TV with a perfectly fine surge strip sitting between them and the wall.
Understanding how a surge gets in and where it does its damage is what makes protection effective.
Three ways lightning reaches your equipment
Direct strike to the building
Lightning attaches to the structure, a chimney or an antenna mast. Current seeks ground through every conductive path available, including wiring, plumbing and the building itself. Voltages are high enough to arc across inches of air. Damage is often widespread and can include fire.
Strike to the utility lines
Lightning hits a power line, phone line or cable plant outside. Most of the energy is diverted by utility arresters and dissipates into the ground along the way, but a fraction travels down the line into nearby services. Homes near the strike see the largest surges.
Induced surges from nearby strikes
A strike to a tree or the ground nearby creates a rapidly changing magnetic field and a large rise in local earth potential. Every long conductor in the area picks up a voltage: overhead lines, buried cables, and even long runs of Ethernet inside walls. This is the most common mechanism of lightning damage to electronics, and it is the one that surge protection is designed for.
| Event | Relative frequency | What helps | Realistic outcome with good protection |
|---|---|---|---|
| Direct strike to structure | Rare | Lightning protection system (NFPA 780), bonding, SPDs | Structure protected; some electronics may still be lost |
| Strike to lines near the home | Occasional | Service-entrance SPD, bonded coax and phone protectors, point-of-use SPDs | Most equipment survives |
| Induced surge from nearby strike | Most common | Same layers plus single-point bonding | Damage largely preventable |
| Internal switching surges | Very frequent, small | Point-of-use SPDs | Normally harmless |
The four entry paths
Every metallic conductor that crosses from outside to inside is a door. Count yours.
| Path | What it connects to | Typical protection | Common mistake |
|---|---|---|---|
| AC power | Everything | Type 1 or Type 2 SPD at the panel, Type 3 at outlets | Assuming that is the only path |
| Coax (cable TV, cable internet, satellite) | Modem, TV, set-top boxes | Grounding block bonded to the electrical grounding system; coax surge protector | Grounding block missing, or bonded to its own separate rod far from the panel |
| Copper phone or DSL | DSL modem, alarm panel, phones | Telco protector at the network interface device, bonded to the electrical ground | Old protector that is no longer bonded after renovations |
| Ethernet and low-voltage runs to outside | Switches, PoE cameras, access points, detached buildings | Grounded Ethernet surge protectors at both ends; fiber between buildings | Running bare copper Ethernet between two buildings with separate grounds |
Outdoor cameras and access points deserve special attention, since they put a long copper run in exactly the place induced surges are strongest. See UPS for security cameras and NVRs.
Why the damage happens between ports
A surge protector does not "absorb" a surge in isolation. It limits the voltage between the conductors it is connected to, typically by diverting current to the protector's ground. What your device experiences is the difference in voltage between every pair of its connections.
Consider a TV plugged into a surge strip, with a coax cable from the wall. During a nearby strike, the power wiring's ground and the coax shield may rise to very different voltages if the coax is grounded at a separate point, or not at all. The surge strip faithfully clamps line to ground on the power side. The coax side is at another potential entirely. The TV's tuner and power supply bridge the two, and current flows through the TV's circuitry to equalize them.
Worked example: two grounds, one TV
Here is an illustrative calculation, not a measured case. Suppose a cable company grounded its coax to a separate ground rod at the far side of the house, with no bond to the electrical service ground, and the earth between the two rods has a resistance on the order of 25 ohms during the event. If a nearby strike drives just 1,000 A of current through the earth between those two rods:
Even if the real numbers are an order of magnitude smaller, that is thousands of volts across the TV's coax input and power input at the same time, and no amount of joule rating on the power strip changes it. Bond the coax ground to the electrical grounding system with a short conductor, and the two sides rise and fall together. The absolute voltage relative to distant earth may still be large, but the difference across your equipment is small.
Our analysis: bonding beats joules
When people lose equipment to lightning, the post-mortem conversation is usually about whether the surge protector was "big enough." In practice, the most common weak point is not the protector's capacity but an unbonded or separately grounded coax, phone or Ethernet path. A cheap grounding block bonded at the service entrance can do more than doubling a strip's joule rating. If you only check one thing after reading this page, trace where your coax and phone lines enter and confirm they are bonded to the same grounding system as the electrical panel.
The single-point ground concept
The goal is that every service entering the building (power, coax, phone) enters close together and bonds to one grounding electrode system at that point. The NEC requires communications and coaxial cable grounding to connect to the building's grounding electrode system, typically at an intersystem bonding termination near the service equipment, and limits the bonding conductor length in common residential cases. Your local code and installer practices govern the details.
When services enter at opposite ends of a house, a single point is not physically possible. Then the priority is to bond each service's ground back to the electrical grounding system with conductors that are as short and direct as practical, and to avoid creating separate, unbonded ground rods. Installing an extra ground rod that is not bonded to the rest of the system can make surge damage worse, because it creates exactly the potential difference described above. This is electrician territory; see site wiring faults and grounding.
Surges you make yourself
Not every transient comes from the sky. Inside a building, inductive loads generate surges every time they switch off: the magnetic field in a motor winding, relay coil or transformer collapses and produces a short, sharp voltage spike. Common sources include:
- Air conditioner and heat pump compressors, refrigerators and freezers
- Well pumps and sump pumps
- Garage door openers, shop tools, vacuum cleaners
- Laser printers (heater switching) and older fluorescent fixtures
These events are usually a few hundred volts and very short. They rarely destroy equipment directly. Their significance is cumulative: a metal oxide varistor degrades a little each time it conducts heavily, so a strip on a circuit shared with a compressor may age faster. See surge protector ratings for wear-out and indicators.
A realistic protection plan
Layered protection reduces a surge in stages, with each layer handling what the previous one let through.
- Service entrance: a Type 1 or Type 2 SPD at the main panel, installed by an electrician, diverts large external surges before they spread through the house. See whole-house surge protection.
- Bond every service: confirm coax, phone and any metallic entry are bonded to the electrical grounding system near the service.
- Point of use: a quality surge strip or UPS at each cluster of electronics. Route the coax and Ethernet through the same device's data protection jacks when it has them, so all ports share one reference.
- Outdoor runs: grounded Ethernet surge protectors where cables leave the building; fiber links between buildings.
- Structure: in high-lightning areas or for exposed buildings, consider a lightning protection system designed to NFPA 780 by a qualified installer.
- Maintenance: replace point-of-use protectors whose protection indicator has gone out, and after any known nearby strike.
Unplugging: when it is worth it
| Situation | Recommendation |
|---|---|
| Storm forecast, you are home, no lightning yet | Unplugging irreplaceable or expensive gear is reasonable. Disconnect coax and Ethernet too. |
| Leaving home or traveling during storm season | Unplug what does not need to run. Leave a UPS-protected network and security system if you need them. |
| Thunder audible or lightning visible nearby | Do not handle cords, wired equipment or plumbing. Wait it out. |
| Equipment that must stay on (NAS, sump pump, medical device) | Rely on layered protection and a UPS; plan for graceful shutdown. See UPS for a NAS. |
Pulling the power plug but leaving the coax connected protects much less than people assume, because the coax path is one of the most common ways in.
Personal safety first
The National Weather Service advises avoiding corded phones, wired electronics and plumbing during thunderstorms, because current from a strike can travel through these systems. If you can hear thunder, lightning is close enough to be a risk.
After a strike: checking for damage
- Check the protectors first. A dark protection light, scorch marks or a tripped internal breaker means the device sacrificed itself or is spent; replace it.
- Test network ports individually. Lightning damage often kills a single Ethernet port or a modem's coax tuner while the rest seems fine.
- Check the UPS. Run a self-test and confirm the battery and site wiring indicators look normal. See UPS self-test and runtime calibration.
- Look for heat or smell. Any burning odor near outlets or the panel means call an electrician before restoring power to that circuit.
- Document for insurance and warranties. Some protector makers offer connected equipment warranties with specific conditions. See UPS warranties and equipment guarantees.
Frequently asked questions
Can a surge protector stop a direct lightning strike?
No consumer device can. A direct strike can carry tens of thousands of amperes and will arc across gaps of inches to feet, including across the open contacts of switches and the internals of a protector. A properly installed lightning protection system (air terminals, down conductors, grounding) protects the structure; surge protectors reduce damage to electronics from the much more common nearby and induced events.
Should I unplug my electronics during a thunderstorm?
Unplugging is the only complete protection, and it is sensible before a storm arrives or when you are leaving home. Unplug the network and coax connections too, not just the power cord. Do not do it once lightning is close: handling cords and touching wired equipment during a nearby strike is itself a shock risk. Corded phones and plumbing carry the same risk.
Why did my router and TV die but not the power strip?
Usually because the surge came in on, or traveled between, the coax or phone line and the power wiring. The strip clamped the power side relative to its ground, but the coax shield was at a different potential, and the voltage difference drove current through the device between those two ports. Bonding the coax at the same ground point, or using a protector that handles both, addresses this.
Is fiber internet safer from lightning?
The fiber itself is nonconductive, so it does not carry surges into the house. The ONT's power supply still plugs into your wiring, and some fiber drop cables include a metallic strength member or tracer wire that should be grounded per the installer's practice. Overall, fiber removes one major entry path compared with coax or copper phone lines.
Do surges from inside the house really matter?
Individually they are small, typically hundreds of volts at most, and rarely destroy anything on their own. They are frequent, though, and each one that exceeds a protector's clamping threshold uses a little of its capacity. Over years they contribute to the gradual degradation of metal oxide varistors in surge strips and UPS units.
Does a UPS protect against lightning?
A UPS includes surge suppression similar to a decent surge strip, and battery isolation can help on online models, but it is not lightning-proof. Many units also have data line protection jacks that only help if you route the coax or Ethernet through them. Treat a UPS as one layer, not the whole plan.
Sources and further reading
- National Weather Service: Lightning Safety
- NFPA 780, Standard for the Installation of Lightning Protection Systems
- NFPA 70, National Electrical Code (Article 250 bonding and grounding; Articles 800 and 820 for communications and coaxial cable)
- IEEE C62.41.1, Guide on the Surge Environment in Low-Voltage AC Power Circuits
- IEEE Std 1100 (Emerald Book), Recommended Practice for Powering and Grounding Electronic Equipment