Remember since a lightning event is a high frequency event it will also induce current/voltage into any loop circuit/systems, it is important to use some kind of isolation in any loops such as networks, audio (entertainment systems) and even phone and cable, a lightning attachment point even at a distance of a few hundred feet from the loop can produce enough voltage in a loop to cause damage, this LEMP (lightning electro magnetic pulse) has be in the 10's to hundreds of Mhz and can circulate current in very small loops of conductors.
Also since this is kind of a transformer effect this voltage is isolating and does not seek Earth as a return path rendering most Earth grounding and surge suppressor systems useless, this latter part is because of the way the loop is formed, take a computer, it might have two or more connections to it that when looked at as a whole will form a loop, if this loop is at the right impedance (think resonate frequency) a voltage will develop within this loop, and if the computer is part of that loop (as most will be) you will receive damage to the component of the computer that was part of the loop, take the network side, you have a network that connects to the router/hub that is connected to the power supply and EGC, the computer also has a connection to this router/hub and power supply EGC, the building wiring will complete the loop between the two devices (which can be several) now here is why most SPD's don't protect for this kind of event:
The lightning pulse (LEMP) induces the current on all the conductors in the loop as if they are one conductor, take a single Cat5 cable, all eight conductors are at one potential, same as the power cord supplying power to the computer, (in the US) the hot, EGC, and neutral are all one conductor in this type of event, kind of like having a circuit where the power cord is one conductor and the network cable is the other conductor, and guess what is inbetween, yes the network card.
since the SPD's only protect between each conductor in a single cable it will not protect between the power cord and the cat-5 where the damaging voltage will be.
Understanding this event takes a little out of the box thinking, and understanding how the wiring in a building can form loops along with other cables that will put many of our electronic equipment in these loops if we don't isolate to break the loop at a high enough voltage rating to prevent this type of damage, another way to prevent this type of damage takes no protection devices at all, just by wiring in what I call a spider fashion where you run your wiring in a method that does not create loops can go a long way to stop LEMP, one method I use is to run all conductors in a single path down the middle of a house then branch out to each area, keeping conductors close enough that the LEMP will treat them as just one conductor with no return path, but this would be impossible or not cost effective to do in existing installations, so isolation is the next best thing we have that can be easily done, fiber for network, wireless where possible, and other methods to achieve a high voltage break in any loops, it only has to be done in one point of each loop to be effective.
Do search on LEMP or lightning electro magnetic pulse, and it might open up a few eyes as to why SPD's are not protecting our equipment when a lightning strike hits a tree out side of our house.
