During thunderstorm everyone unplugs PC power cord from the socket. Is it necessary to unplug LAN cable?
Can a PC be damaged over LAN cable when thunder strikes router?
Your title is a bit hard to follow. I assume you mean lightning, not thunder. You've
strikes router which could be taken to mean you are asking a theoretical question
about a direct hit, but I assume what you mean is whether a computer is at risk from power surges
(caused by lighting) over a lan cable. Rather than just answer the assumption I'll try to answer each of
the possible questions, though the quality is bound to be poor for some as I don't imagine
there is much research available on the subject of thunder-and-electronics to draw from, let alone
research specific to transference of such energy from a router over lan to a computer.
Indirect To Router, Lightning:
Your computer doesn't need to be plugged into a wall outlet to be at risk during a lighting storm; wire is wire. So yes, if your computer is connected to a router via lan, your computer is still at risk of damage from any power surge that affects the router. Some surge-protection-power-bars have support for lan cables, but surge protectors offer limited protection; it's best to unplug your equipment.
Telephones, modems, computers and other electronic devices can be damaged by lightning, as harmful overcurrent can reach them through the phone jack, Ethernet cable, or electricity outlet
Direct To Router, Lightning:
Since your computer is at risk from indirect strikes to the router, a direct strike to the router would be even more dangerous for to your computer. If your router were directly struck by lighting it would pass on that electrical energy through any conductor it finds, and lan cables are wire, wire is an excellent conductor, which is why it's used in electronics. You router would be destroyed by the millions if not billions of volts striking it , and your computer would be at risk. It's worth noting that cable's have a voltage limit, if exceeded they will melt from the heat. The voltage of the lightning would exceed that of the cable, and melt it, but the computer might be destroyed first. Even if it didn't damage your computer, the mass amounts of electromagnetism being sent to your system might wipe your hard drive's data, since hard drives are magnetic storage devices.
Cables are the primary means of transferring EU (Energy Units) from one device to another. There are several different tiers of cables, depending on how much Voltage they can tolerate. If too much Voltage is applied to a cable, it will instantly melt
Lightning can have 100 million to 1 billion volts, and contains billions of watts.
Where the lightning current path passes through rock, soil, or metal these materials can become permanently magnetized
Indirect To Router, Thunder:
Thunder is a sonic shockwave, but the distance between you and it reduces it's power greatly; by the time it reaches you it's usually only around 120db. Only so much of that sound wave's physical force could pass over a cable, because unlike electricity, pushing works best against items that are highly resistant. A cable can bend sideways discarding that energy unlike a pole. Cars horns are about 120db, and they don't damage the electronics in the vehicle. However some really bad thunder claps have been know to cause physical damage to homes, like breaking windows and such. I suppose if you did get a thunder clap powerful enough to knock over the router, since your computer is attached, the cable could get yanked out in a way that damages the outlet, or if the cable doesn't come out the could computer get pulled over, and suffer damage from the fall. So yes it's possible, but would require a lot of fate to be involved, not to mention the perfect storm.
At very close range, thunder is capable of causing property damage. The shock wave, pressure, and propagation of thunder may cause exterior and interior damage to structures. Popping of nail-supported drywall away from horizontal and vertical wooden studs inside houses has been documented. Glass windows have been broken by the concussion of thunder.
A clap of thunder typically registers at about 120 dB in close proximity to the ground stroke. This is 10 times louder than a garbage truck or pneumatic jackhammer drill. By comparison, sitting in front of speakers at a rock concert can expose you to a continuous 120+ dB level.
Thunder is rated at 120 decibels, compared to a chainsaw at 125 and a car horn at 110. It is the same decibel as an ambulance siren, rated also at 120 decibels.
Well, according to the Oklahoma University Meteorology records, the loudest thunder clap ever recorded, was recorded in 2003, with the decibel meter hitting an astonishing 712.7! It was said to be as close as 8 meters away
Thunder, thus, starts as a shock wave moving at speeds in excess of the speed of sound. This initial shock wave rapidly loses its energy to the surrounding air.[...] Although less than one percent of the total energy in the initial shock wave is transformed into the acoustic wave (the remaining 99 percent is dissipated into heating the surrounding air), the total energy available for that sound wave is still extremely large. Therefore, thunder is one of nature's loudest sounds.
Direct To Router, Thunder:
The answer would be the same as an indirect strike to the router, yes it could cause damage to your computer if you fall prey to freak accident caused by a massive clap of thunder. Personally I would be more concerned with my ears, as such a loud clap would have potential for immediate, and permenant hearing damage.
A clap of thunder from a nearby storm (120 dB) or a gunshot (140-190 dB, depending on weapon), can both cause immediate damage.
long or repeated exposure to sounds at or above 85 decibels can cause hearing loss. The louder the sound, the shorter the amount of time it takes for NIHL to happen.
When one gets things disconnected, a surge has probably already done damage. Anything that requires human intervention is unreliable. Many at risk devices cannot be disconnected. And worse, humans are only available one third the time. Disconnecting is unreliable.
A telco's $multi-million switching computer connects to building all over town. Therefore suffers about 100 surge with each storm. How often is your town without phone service for four days while they replace their computers? How often does the telco disconnect when thunderstorms approach? Never? Exactly. Because a direct lightning strike without damage is routine. For a homeowner, this proven solution costs about $1 per protected appliance.
Most consumers are only educated by hearsay and advertising. Few know anything about effective protection. For example, one mistakenly assumed voltage destroys wires. Nonsense. Current - not voltage - is the relevant parameter. High voltage only exists when one foolishly tries to stop or block a surge. Facilities that can never have damage do not do what is foolish.
Your cable is required by codes and standards to already have effective protection. Unfortunately, those same ill informed consumers do not know anything about it. Then make conclusions only from observation.
A lightning strike exists because lightning seeks earth ground. Damage occurs when a surge is all but invited to go hunting for earth destructively via appliances.
Lightning striking AC utility wires far down the street means a surge is incoming to all household appliances. Are all damaged? Of course not. Another reality often forgotten by consumers. For current to be inside an appliance (cause damage) means both an incoming and outgoing path must exist. Incoming path is obvious. AC mains. Outgoing is to earth ground. Since telephone and cable already have effective protection, then that is a typical and destructive outgoing path.
A consumer see damage on a cable port. Does that mean a surge was incoming on cable? Of course not. Damage is often on the outgoing path. Lightning far down the street and incoming on AC mains, often finds and damages the outgoing path via an HDMI port, cable TV port, Ethernet, or any other path that connects to earth via telephone or cable TV wire.
Again, best protection means connecting to earth BEFORE a surge can enter a building. Once permitted inside, then nothing (not even disconnecting) provides reliable protection. Every facility that cannot have damage uses a 'whole house' solution. What is the most critical component of every 'whole house' solution? Single point earth ground. Then protection always exist without depending on unreliable humans.
Telephone, satellite dish, and cable must already have this installed - for free. But a most common source of surges exists when a homeowner does not learn about and does not earth a 'whole house' protector.
Effective protetion means a homeowner can always say where hundreds of thousands of joules harmlessly dissipate. Any recommendation that does not discuss those numbers is best considered bogus. Surges are about current - not voltage. A minimal 'whole house' protector is 50,000 amps. Because a direct ligtning strike is typically 20,000 amps. A protector is sized to not fail even after direct lightning strikes. Again, this superior solution also costs less money - maybe $1 per protected appliance.
Any recommendation that ignores numbers is best considered bogus. Numbers are often ignored by consumers educated by hearsay, advertising, and wild speculation. Protection is always about single point earth ground. No way around technology understood and demonstrated for over 100 years.