Should You Unplug Appliances When Thunder Rumbles?
― Even a Strike 1 km Away Can Put Tens of Thousands of Volts on Power Lines
The answer is: unplug before the thunder starts. Lightning can wreck your appliances without ever hitting your house. The electricity from a distant strike travels along the wires to your home. But power lines are not the only route it can take.
It's a summer evening, and thunder is rumbling in the distance. A flash lights up the window, and a moment later comes a deep boom. Nothing seems to have hit your house.
The next morning, the TV won't turn on. The box that connects your computer to the internet has no lights on. A neighbour says a telephone at their house died the same day.
The lightning never struck the house. So why did the machines inside it break?
Lightning's Electricity Has Two Ways Into Your Home
A lightning current can reach tens of thousands of amperes in an instant. When the magnetic field around it changes suddenly, it creates a voltage in wires far away. The longer the wire, the bigger that voltage.
Current that flows into the ground spreads outward. While it does, the ground near the strike point briefly sits at a high voltage. Electricity can then enter through other lines tied to the ground, such as an aerial or a phone line.
In both cases, the current flows for only a few tens of thousandths of a second. Even so, that is more than long enough to harm the tiny parts inside appliances. This sudden burst of high voltage entering a home is called a "lightning surge".
A Sudden Change in a Magnetic Field Creates Voltage in Distant Wires
When a current flows, it creates an invisible magnetic field around it. This is the force field that moves a compass needle. A lightning current leaps to tens of thousands of amperes in just a few millionths of a second. The magnetic field around it grows at a tremendous rate too.
The change in the field is what matters. When the magnetic field passing through the area enclosed by a wire changes, a voltage appears in the wire. This is called "electromagnetic induction". It is the same principle behind a bicycle's dynamo light and the wireless charging pad for your phone.
Look at the left side of Figure 1. The dotted rings show the magnetic field around the lightning. The wire running to the right forms a large area between itself and the ground. A line stretching hundreds of metres between poles encloses a very large area. That means it catches a much larger change in the field.
The Most Vulnerable Appliances Have Two Entrances
A TV is connected to both the mains cable and the wire from the rooftop aerial. Computers and internet boxes also have two connections, one to the mains and one to a data line. The base unit of a landline phone is the same.
During a strike, the voltages on these two lines do not rise together by the same amount. One line carries a voltage picked up from a distant pole, and the other carries a different one from the roof or ground. The difference between the two then punches through the appliance. A large current briefly flows along a path that normally carries none.
So unplugging only the mains cable is not enough. Disconnect the aerial cable and the data or phone lines as well, and the entrances are truly shut.
The gap between the contacts inside a breaker, where the current is cut, is said to be only a few millimetres. A lightning surge of tens of thousands of volts can jump that gap as a spark. Besides, aerial and data lines don't pass through the breaker at all. The only sure method is to pull the appliance's plug itself.
Strips with a part that diverts surges into the ground are said to work well against weak surges. But they can't fully stop a strong strike nearby. The part wears down each time it absorbs a surge, so one used for years may have lost much of its strength.
So What Should You Do?
- Unplug your important appliances before you hear any thunderIf the forecast says "unstable atmosphere", do it that same day.
- Disconnect the aerial cable and data lines tooEspecially for TVs, computers and landline phones, which have two entrances.
- Once thunder starts overhead, keep away from cords and phonesGive up on unplugging and wait in the middle of the room.
When thunder is right overhead, do not touch sockets, metal pipes or corded phones. Wires and pipes can carry a lightning surge. Anyone outdoors should get into a building or a car at once. A broken appliance can be replaced, but a body cannot. If you find someone struck by lightning, call emergency services (119 in Japan) immediately.
Summary
Lightning's electricity can reach your home without a direct hit. One route is a suddenly changing magnetic field creating a voltage in a long power line. The other is the ground's own voltage rising and entering through aerial or data lines. Appliances with two entrances break most easily, so the sure way is to remove the cables as well as the plug.
Lightning can reach you through the wires from 1 km away.
So unplug before the thunder starts, not after.
How lightning jumps to trees and people is explained in "Why Shouldn't You Shelter Under a Tree During Lightning?", and how big the energy of one strike is, in "How Much of a Home's Electricity Could One Lightning Strike Supply?".
- Tune a small battery-powered radio to an AM frequency with no station, just static.
- On a day when thunder is rumbling far away, listen: each flash brings a crackle of noise. This shows that every change in the lightning current creates a voltage in the radio's antenna.
- Even on a day without lightning, put the radio near a light switch and turn the light on and off. At the moment the current changes suddenly, you may hear similar noise.
If you test this on a thunderstorm day, always listen indoors, away from windows. Never go outside, and never extend the aerial and stand by a window.
For Those Who Want More ― Terms, Formulas, and Links to TextbooksShows clearly which level each topic belongs to, from middle school science to university courses
- Middle schoolCovered in middle school science
- High schoolCovered in high school physics
- High school+Advanced high school material, or a textbook sidebar
- UniversityUniversity-level courses not taught in high school (electromagnetism, power engineering)
- ResearchNot taught as settled fact even at university; researchers are still studying it
Middle schoolTerms: This Phenomenon Has Names
- Electromagnetic induction: when the magnetic field passing through the area enclosed by a coil or wire changes, a voltage appears in the wire. It is the very principle of a generator.
- Lightning surge: a very brief high voltage or current that enters power or data lines because of lightning.
- Induced lightning: a surge produced in wires by, for example, the electromagnetic induction of a nearby strike, rather than by a direct hit. Most appliance damage is said to come from this.
Middle schoolHigh schoolChecking With a Formula: How Many Volts Does a Strike 1 km Away Create in a Power Line?
We estimate how sharply a straight lightning current changes the magnetic field passing through the area below a wire 1 km away. Take the wire to be 1 km long and 10 m above the ground, with the field passing through the rectangle between them.
| In symbols | V ＝ ( μ0 ÷ 2π ) × ( S ÷ r ) × ( ΔI ÷ Δt ) |
| In words | Voltage produced ＝ magnetic constant × (area enclosed by the wire ÷ distance to the lightning) × (rate at which the current changes) |
| Where it comes from | It combines the strength of the field made by a straight current (inversely proportional to distance) with Faraday's law of induction: the rate of change of the field through an area becomes the voltage |
| Symbol μ0 ÷ 2π (magnetic constant, in volt-seconds ÷ ampere ÷ metre) | 0.0000002 |
| Symbol ΔI: rise of the lightning current (a commonly cited size) | 30000 amperes |
| Symbol Δt: time taken to reach that current | 0.000003 seconds (3 millionths of a second) |
| Length and height of the wire | 1000 m and 10 m |
| Symbol r: distance to the lightning | 1000 m |
| Area S enclosed by the wire | 1000 × 10 ＝ 10000 square metres |
| Area ÷ distance | 10000 ÷ 1000 ＝ 10 metres |
| Rate of current change (per second) | 30000 ÷ 0.000003 ＝ 10000000000 amperes |
| (Area ÷ distance) × rate of change | 10 × 10000000000 ＝ 100000000000 |
| Multiply by the magnetic constant to get voltage V | 0.0000002 × 100000000000 ＝ 20000 volts |
| How many times a household socket (100 volts)? | 20000 ÷ 100 ＝ 200 times |
The sum says that even a strike 1 km away briefly creates about 20,000 volts in a long wire. Real induced lightning is also said to reach thousands to tens of thousands of volts. By contrast, wiring inside the house alone (enclosing about 10 square metres) has 1/1000 the area, so the voltage stays at about 20 volts. The voltage that breaks appliances is not made inside the house. It is gathered by the long wires outside.
High schoolHigh school+What Sets the Voltage Is the "Speed of Change", Not the "Size of the Current"
High schoolIn Faraday's law of induction, the voltage produced depends on how fast the magnetic flux (the amount of field passing through an area) changes. The same 30,000 amperes, if built up over one second, would give less than a millionth of the voltage. What makes lightning special is the speed of its rise even more than the size of its current.
High school+The ground voltage of route ② can also be estimated simply. If the current spreads out in a hemisphere, the ground voltage at distance r from the strike point is ρI ÷ 2πr (ρ is the soil's electrical resistivity). Voltage is inversely proportional to distance, so if the aerial's earthing point and the pole's earthing point are far apart, the difference lands on the appliance.
UniversityReal Wires Are Not "Exactly One Loop"
Real calculations of induced lightning cut the wire into small segments and solve for the voltage in each, together with the waves travelling along the wire. The electromagnetic field from the lightning channel is computed with an extension of the Biot–Savart law, and coupling to the wire is handled by methods called the Agrawal model and the Rusck model. Waves along the wire are solved with the "transmission line equations" of distributed-parameter circuits.
📖 Derivations and further reading: Electromagnetic induction (Wikipedia) ／ Biot–Savart law (Wikipedia)
ResearchWhat Isn't Yet Clear
- The "shape" of lightning current differs every time. The speed of rise is said to differ greatly between a first stroke and later strokes that follow the same path. Which shape to assume when protecting equipment is still being revised as observations accumulate.
- Winter lightning differs in character from summer lightning. Winter lightning on the Sea of Japan coast often has discharges that grow upward, and the current can last longer. How such lightning affects power lines is still under study.
- The current's path inside a house. In a real house, power, aerial, data and water pipes are tangled together. Even specialists find it hard to work out afterwards where the current came in and where it left.
In other words, this article too is "an explanation of what is known so far". The calculation above is a rough estimate for grasping the mechanism.
Links to Textbooks (by Level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Year 8 science: "Current and magnetic fields", "Electromagnetic induction" | The magnetic field around a current; a changing field creating voltage |
| High school | Physics: "Electromagnetic induction", "Magnetic field of a straight current" | Checking with a formula; the speed of change sets the voltage |
| High school+ | Physics (advanced): electric potential and resistivity | Estimating the ground voltage |
| University | Electromagnetism and power engineering (lightning surges, transmission lines) | How real induced lightning is calculated |
| Research | Lightning observation, and lightning protection of power and telecom equipment | Current shape, winter lightning, paths inside a house |
| ― | Link to daily life | Deciding to remove plugs and aerial and data cables "before the thunder" |
- Japan Meteorological Agency (気象庁), "How to Protect Yourself from Lightning" (leaflet and explanatory page)
- The Institute of Electrical Engineers of Japan (電気学会), ed., 『雷と高度情報化社会』 (Lightning and the Advanced Information Society)
- V. A. Rakov, M. A. Uman, Lightning: Physics and Effects, Cambridge University Press
- Wikipedia, "Electromagnetic induction" (電磁誘導, Japanese)
※This article is a general-audience science explainer. The figures given are rough guides for understanding the mechanism. When lightning approaches, follow the Japan Meteorological Agency's lightning advisories and instructions from fire services and local authorities.