Is an item of electronic equipment safer from damage by lightning strike if it's inside the house, compared to being installed in a weather-proof box outdoors?
Yes! to your question. your house's electrical system is grounded to divert the energy to ground. A very good monster surge power bar or single outlet one will protect you if a strike hits your house...one that will also have a fast acting fuse.
Outside, you have no protection...if lightning hits your 3 meter metal rod your stuff will be fried, literally, in a fraction of a second.
Mark
Electronic devices inside a house are likely safer than those outside but not necessarily safe from lightning damage.
Borrowing from and modifying an old saying "Electricity follows all available paths." True, most energy from a lightning strike may be "diverted" by a building ground system but not all of it. Electrical devices connected to power, communication circuits, or ground can be "fried".
Several years ago a lightning strike hit a tree just behind my house and damaged:
Two telephones
Computer modem
Frequency counter
Graphic equalizer
Part 15 FM transmitter
Two garage door openers
Motion sensing light switch
The house wiring was grounded according to Code and the telephone land line ground block was grounded.
So it becomes a matter of probability rather than absolutes. Suppressors and grounding can lower the probability of damage but not to zero.
Neil
Every building I have ever occupied has been an "old city" building, with only two-prong AC outlets.
None of the buildings had 3-prong modern outlets with ground.
Because of that situation, even as I built a succesful recording business, I never quite understood the ground-safety issue, and floated by without any trouble.
Only by reading have I acquired an understanding of what the third prong is for, but even after learning that much I was yet to learn the importance of a good ground for AM transmitter performance.
Lightning wakes you up, and I lost a mixing board because lightning came in by the equalized telephone connection.
Of course I wonder if things would have been better if that building was wired for 3-prong service.
First, all bets are off if a lightning bolt actually hits your house or your antenna/transmitter outside away from the house. Fortunately, that doesn't happen very often. We mostly see damaging effects from lightning surges caused by "nearby" strikes.
When lightning strikes ground, it causes a voltage gradient in the ground: maximum at the strike point and tapering off with distance from the strike due to the resistance of the ground. As a crude example, suppose your transmitter is 100ft from the house and the transmitter has a ground rod under it, and suppose a bolt strikes the ground 200 ft away from your house on a line from the house to the transmitter and 100 ft beyond the transmitter. Because the strike is closer to your transmitter than the house, the transmitter ground rod will see a higher voltage than the house ground rod. This will cause current to flow in the power and audio wires from the transmitter to the house. Most likely, in this example, the power and audio wires will vaporize. In the instant before vaporization, the transmitter and the associated electronics in the house will be damaged. The "safety" ground rod under the transmitter didn't really protect anything.
The National Electrical Code covers this scenario by requiring secondary grounds to be connected with a heavy gauge wire to the primary building ground. So, the transmitter ground rod should actually have a #6 gauge, or so, wire running directly to the house service entrance ground rod. This wire will eliminate the voltage gradient between the transmitter ground and the house ground. This important connection isn't mentioned much in the part 15 information we see.
In actuality, the chances of this scenario happening is small so our response its to "hope it doesn't happen" and if it does, we tend to accept the vaporized wires and damaged equipment as a freak occurrence. Perversely, we then enjoy relating the story after it happens.
Both of the houses I was reared in plus my first apartment did not have grounded outlets and I don't recall lightning damage to anything except when a strike caused plasma lightning witnessed by my brother and me to come out of our wall crank telephone, bounce across the floor, and damaged the TV. (The strike mentioned in my previous post also caused plasma lightning in my yard as witnessed by myself and my neighbor.)
So why so little damage without the "protection" of the grounded outlet? I presume that there are a couple of factors, one being that electronics of the era were vacuum tube and not solid state. Tube equipment is more tolerant of spikes and surges than solid state devices. Another factor is that the telephone line was used only for the telephone and there was no interconnection of systems such as is done today.
The idea behind grounding through the third pin is to keep all electrical equipment cases and exposed conducting surfaces at the same potential so someone contacting them is not exposed to a potential difference. The same thing can be achieved by insulating all conductors (control shafts, chassis, antennas, etc.) from the power circuits and this was done back when. This equipotential exposure is the theory behind "double insulated" power tools where there exists only very high resistance paths from the power circuits to what the user might touch.
One of the biggest risks back when came from "AC/DC" receivers and the KnightKit Broadcaster where the chassis is connected directly to one side of the power cord and if the insulation failed or the device was improperly serviced the user could be exposed to the full line voltage. If the chassis connected wire in the power cord could be connected only to the neutral then this risk is minimized and this was the idea behind the polarized plug we have today where it can be inserted only one way into a recepticle. Don't bet your life that this was wired correctly! In my "new" home 27 years ago one third of the three prong outlets were wired reverse polarity.
Neil
The towers used by licensed broadcast stations typically are the tallest structures for several miles in all directions. As such they are natural targets for lightning, which is searching for the lowest resistance path to/from the clouds and the earth.
Such towers are "grounded" for d-c and lower-frequency a-c through several buried ground rods at their base. This puts the top of the tower essentially at the same potential as the earth around the base of the tower, which reduces, but does not eliminate the risk that the tower will attract lightning.
In any case these towers and the transmit systems using them typically can withstand multiple direct hits without serious damage. The pic below shows lightning hitting one of the antenna stacks on Sears Tower in Chicago, which is used by dozens of FM & TV broadcast stations and many other services.
Broadcast transmitters have protective circuits to sense the lightning energy on the antenna system and "kill" their r-f output during the strike event, so that it won't sustain any arcing that may be started by the energy in the lightning strike, and to protect the transmitter output amplifier(s). These off-the-air intervals usually are so short that they are unnoticed by listeners/viewers of those broadcast stations.
The bottom of a base-insulated tower used for AM broadcast can't be connected to a ground rod directly, as that would short out the base insulator. Their ground rod connection is made through a "choke" which puts a high impedance in that path for the r-f bandwidth of that station, but has a low impedance in the part of the spectrum where lightning energy is highest (below 100 kHz or so).
These AM towers also use a spark gap across the base insulator (pic below). Its gap is set just beyond the flashover distance for the base voltage during modulation peaks. The two rings are a transformer used to couple a-c power used for the tower lights across the insulator without shorting it out. The dark brown rectangular object at the left of the pic is the base insulator, itself.
Monopole towers (whether base insulated or not) that are used for licensed AM broadcasting must have much better r-f grounds than can be provided by ground rods. Most of them use a set of 120 horizontal wires, each 1/4-wave long, buried at 3-degree intervals around the tower base -- in addition to all the components shown in the pic above.
Not too relevant for Part 15 AM, but still might be interesting.