Lightning Protection
No system described in this lesson makes a building “lightning proof.” A direct strike carries tens of thousands of amps in microseconds and can damage almost anything in its path. What earthing, bonding and surge protection do is far more common and far more useful: they protect equipment and wiring from the surges that a nearby strike, or a fault on the power network, induces in mains, telephone, data and aerial cables — which is how most lightning damage to electronics actually happens.
How Lightning Damages Electronics
A lightning strike within a few hundred metres raises the voltage of the ground near it by thousands of volts and creates intense magnetic fields. Long cables — the mains supply, a telephone line, an aerial or satellite cable, a network cable to an outbuilding — pick up a surge. The damage typically appears where two different cable systems meet in the same piece of equipment: a TV connected to both mains and an aerial, a router connected to both mains and a telephone line, a computer connected to both mains and a network cable. The surge arrives on one system and leaves through the other, through the equipment.
Earthing and Bonding
The building's main earth (the earth electrode or the supplier's earth, connected at the consumer unit) is the reference point for the whole installation. Bonding connects metalwork and the outer conductors of incoming cables to that same earth, so that during a surge they all rise and fall together and no large voltage appears between them.
- Aerial and satellite cable screens should be bonded to the building's earthing system where they enter the building.
- Any additional earth rod (for example at an outbuilding or a mast) must be bonded to the main earth, not left isolated. Two separate earths can be thousands of volts apart during a strike.
- Keep bonding conductors short, straight and heavy. Surge current is fast and sharp bends add impedance.
- Electrical earthing and bonding must comply with local wiring regulations; changes to the mains installation are a job for a qualified electrician.
Mains Surge Protection
Surge protective devices (SPDs) contain metal-oxide varistors (MOVs) or gas discharge tubes that conduct only when the voltage rises far above normal, diverting the surge to earth. Protection works best in layers:
| Layer | What it does | Limitation |
|---|---|---|
| SPD at the consumer unit | Diverts large surges entering on the mains supply before they reach the house wiring | Installed by an electrician; does not protect against surges entering on other cables |
| Surge-protected power strip | Clamps residual surges close to sensitive equipment | MOVs wear out with each large surge; replace strips after a major event or when the indicator goes out |
| Combined mains-and-data protector | Protects mains and the data, telephone or aerial cable at the same point | Only works if every cable to the equipment passes through it |
| Physical disconnection | Creates an air gap with no conductive path into the equipment | Only protects equipment that is unplugged before the storm arrives |
Data, Telephone and Aerial Cables
Because damage happens where cable systems meet, every cable entering a piece of equipment needs the same treatment as its mains supply. Surge protectors for telephone lines, Ethernet, coaxial aerial and satellite cables are inexpensive. Route cables from outside the building into the house close to where the mains enters and bond their protectors to the same earth point, so that all the cables share one reference.
Protecting Outdoor Equipment
Cameras, weather stations, solar panels and wireless bridges are mounted high and connected to indoor equipment by long cables, which makes them both exposed and effective at bringing surges inside. Bond their metal mounting poles to earth, fit surge protection on the data and power cables where they enter the building, and keep outdoor cable runs as short as practical. For network cables between buildings, consider fibre-optic links, which carry no electrical surge at all.
During a Storm
Surge protectors and bonding protect equipment that has to stay connected; disconnection protects everything else. A sensible routine combines both, and after a storm, check the status indicators on surge-protected strips and replace any that show their protection has been used up.
Frequently Asked Questions
Will a surge-protected power strip protect my computer from lightning?
It helps against surges arriving on the mains, which are common, but not against surges arriving on the network, telephone or aerial cable, and not against a direct strike. For real protection, use a protector that covers every cable to the equipment, and unplug the equipment when a storm is forecast.
Do surge protectors wear out?
Yes. The varistors inside degrade a little with each surge they absorb, and a large surge can use them up completely. Many strips have a “protected” indicator that goes out when the protection has failed; the strip still passes power, so it is easy to miss. Replace strips after a major storm or every few years.
Should I add my own earth rod for outdoor equipment?
Only if it is bonded to the building's main earth. An isolated earth rod can sit thousands of volts away from the house earth during a nearby strike, driving surge current through any equipment connected to both. Earthing changes should follow local wiring regulations and are best done by an electrician.
Test Your Knowledge
Answer the questions below to check your understanding of this lesson. Every answer can be found in the lesson above.