Prelude: Some common questions and misconceptions......

Q: When we "ground" things, doesn't that attract lightning?        A: No.

Q: Then what does "attract" lightning?     A: Any electrically conductive object, grounded or not.

Q: What's the difference in the way lighting leaves an ungrounded versus a grounded object?

A1 Lightning leaves an ungrounded object (to one that is) by converting enormous energy into heat. The destructive power that is possible in this kind of event can be measured in pounds of TNT.

A2 Once embarked on a bonded (continuous) and low-impedance path to ground, lightning ceases to convert as much energy into heat, and dissipates into earth in a non-destructive manner.

Q: Can lightning go through anything?    A: Not the actual strike, but it's powerful magnetic fields penetrate any non-metallic structure, attaching directly to interior wiring.  Lightning can also strike any outside object, and the less conductive the object is, the more damage usually results where lightning leaves that object. But a strike-attachment rarely transfers directly to the interior to the interior of buildings, normally racing down the outside of a structure to ground in the most direct path.  That unfortunately starts fires on most unprotected structures.  Direct strike currents follow the skin-effect principle on conductive materials and current remains on the outer edges of a building, car, sailboat rigging, etc. Almost anything between lightning and the ground will be considered "conductive" to lightning. It has after all, just burned through three miles of air to get here, and things we think of as insulators, "aren't" to lightning.

Like the Marines, lightning is made for travel to distant places, breaking stuff, and killing people.

Q: Is anything ever safe from lightning then?    A: Yes.  Equipment that is grounded and bonded will rise and fall harmlessly with the common-mode transients from nearby strikes. This is not protection from the (magnetically) induced-voltages or surges on AC or utility wiring, but we will cover that...

Q: Should I disconnect and ground antenna feedlines before a storm?   A: Lacking a total system*, yes.

* Refers to a comprehensive design for the control of all entrance and exit paths for lightning. Proper bonding and grounding with appropriate conductors, arrestors and suppression technology, taken as a whole, make a system.

Q: Do I have to disconnect equipment power supplies before a storm?   A: Same as above, yes.

 Equipment that is properly grounded and bonded inside a station is not going to be affected by lightning strike energy on or near the system ground. This includes GPR, or Ground Potential Rise,  where ground-current can enter the station from ground and leave (backwards) through AC wires,  cable or telco lines. This high ground potential from a strike is seeking a lower potential ground elsewhere in the system, and can destroy equipment by using the reverse path of coax arrestor devices or any wiring. Stations who fail to bond all equipment and neglect to create a well bonded path (with high current carrying ability) to the main utility service entrance ground may someday experience this destructive phenomenon.

However, the two other paths lightning takes is Electromagnetic Induction on interior utility wiring, and Power Surges that enter the building wiring from external utility lines (electric, phone and cable).

Q: But I thought my house wiring is grounded?    A: Not for lightning! Building electrical grounding is for one purpose only: equipment-fault protection from 120v and 240v.  But nearby lightning can cause surges of THOUSANDS OF VOLTS to enter via the power company lines, or it may be induced onto your house wiring from a nearby or direct strike. This energy can travel on your house wiring, and may damage or destroy anything plugged into AC, telephone or cable lines. The home electrical ground system by itself cannot prevent this. Neither does grounding and bonding the outer case of each piece of equipment protect from induction on the wiring or power surges coming in from a nearby strike on utility lines. EMI on wiring can only be controlled from careful application of surge suppression.

Q: O.K., and I use "Surge-Protectors" on critical equipment, so I'm o.k., right?    A: NO! Not if your "surge protector" is the standard "power strip"! The slow-clamping but high-voltage protection provided by MOV (Metal-Oxide Varistor) power strips all shunt surge voltage directly to AC (third wire) ground.

Vital Information:  Typical MOV-type surge suppressor power strips are Dangerous:

The ground system of your home electrical wiring was never designed to handle lightning surges. 

Magnetically or surge-induced transient voltage in AC wiring should only be referenced back to neutral (normal mode), never to AC ground.    Never to AC ground! Referencing EMI to ground requires sophisticated equipment not available at retail and should never be attempted by amateurs using equipment not designed for that connection (such as power strips).

There are technical reasons why these can also destroy sensitive equipment and data that relies on a zero-volt ground reference during even normal motor-starting surges, but the most serious reason is again.... The ground system of your home was never designed to handle lightning surges. All kinds of things are "grounded" for electrical safety that use inferior-sized conductors on un-bonded equipment in your home. 2,000-5000v surges can flashover, and burn or destroy many things, if not your entire home.

 Surge suppression for interior AC equipment should be normal mode only, which is Line-to-Neutral return. There are power-strips available that can do this (Transtector etc). They are never sold at retail.

Let's say a nearby strike imposes 4000v on the utility pole wires (above or below ground) heading toward your house wiring. At your AC service panel, ground is bonded to neutral (this is code). Most Common-Mode  (Line-to-Ground and Neutral-to-Ground) transient voltage stops right there by virtue of this bond. You could put a powerful MOV arrestor there, before the electric panel. Your electric utility may sell these, or you can have them installed (with utility permission).  As few homes go this expensive route,  we will assume that 4,000v wave is still riding on the hot wire (Line), with the differential being to neutral, as it always will be inside your home. Non-degrading silicone avalanching diodes *  in "normal-mode" suppressors will reference this over-voltage back to the neutral line in < 5 nanoseconds.  You will see later that at the radio station, referencing surges only to back to neutral makes all the difference in the world! This normal mode transient voltage suppression protects against not only lightning, but against all surges that typically happen in the house or station. Standard MOV power-strips are dangerous here, taking between 5 and 20 microseconds to begin to act, and then shunting surges very few times before they can short and burn.  Also, you will get a lesson in why bonding is important the first time your MOV-power-strip shunts 4,000v across an unbonded piece of "AC grounded" equipment. It can also be incredibly destructive when it short circuits high ground potential up into your house wiring from your own ground system!

MOV-type "surge" power strips are deficient and we were all misled by big names in the industry for decades.  MOV's are dirt cheap (pennies) to make and that's one reason they are so popular. They are not safe indoors! They may self-ignite when the MOV's eventually fail-shorted. Never use these in a home that employs lightning protection for communications equipment. It is possible to design a total system that uses only MOV type suppressors and this should be done by professionals only.  Never mix normal and common-mode surge suppression on the same branch circuit.

 I recommend only commercial-grade Transient Voltage Surge Suppression (TVSS) in normal-mode at both the service-mains panel and the individual circuits serving the equipment, or, the disconnecting of all equipment power supplies is required. Many people choose to simply disconnect before a storm. But how many of us go around disconnecting everything? Why not protect your whole house?

Normal mode from this side-><-Common mode "Strike"
Requires TVSS                                                                                                            Requires bonding & ground

Lightning directly striking the house, antennas, or the ground nearby causes common mode transients. Common mode is the step-potential or transient voltage between two ground potentials. It is defended against by lightning rods, grounded masts, coaxial feedline arrestors and complete and proper common bonding. The proper bonding is so critical that it cannot be over-emphasized.

The normal mode voltage differential is the only kind that can exist on inside AC wiring. It comes into the house via either power line surges, or it can be imparted directly onto the house wiring by electromagnetic induction. Surge protection is the only defense here, and it should be normal mode (line to neutral return only) for all interior AC-wiring systems. 

Satellite, CATV and telephone line protectors (which are vital to include in your plan) normally shunt surge voltages to ground. If you're thinking - "Which ground, AC ground?" NO! Remember the ground system of your home was never designed to handle lightning surges. The Telephone Company already has one protector installed. That more or less protects their lines, not your home. I added silicon-diode protection that protects telephone Tip to Ringer, Tip to Ground and Ringer to Ground, and installed it in the station.

Q: Can lightning also "induce" current inside my equipment?    A: No, if it is built with metal case enclosures, common bonded, grounded, and either surge-protected or disconnected from all external power, then only the outer "skin" of the equipment will carry energy. That surface-effect current will not harm electronics inside bonded metal equipment cases.

Q: Can I touch my bonded/grounded equipment during a storm then?    A: No. Unless you are electrically bonded to the ground system (insulated from any other ground potential) you could suffer burns if contact is made with both hands, a hand and foot, etc. "Step & Touch" hazards exist on any ground system. Safest practice is don't touch a bonded ground system during a thunderstorm. Use One-hand rule if you do.

Next: During a strike, the "ground" becomes deadly
The Forces and Affects of Lightning