I've been researching on whole house surge protection devices and came across an Illustration of lightning current flow from a direct strike near
the (protected) structure.
From my understanding Type 2 residential SPDs divert surges from L-N, L-PE and N-PE. But the above illustration shows a Type 2 surge I never considered before, where inductance from a nearby lightning strike to the ground causes surge in the Ground (PE) wire. Can a normal type 2 SPD protect from this type of surge event?
P.S. I have a type 3 SPD (to protect terminal equipment) which I currently use and a whole house type 2 SPD which I haven't installed. It is my understanding that both essentially divert surges to Ground (PE) but I honestly have no idea what they'd do when it is the ground wire that causes the surge.
Best Answer
The diagram you link is ineffective - an indirect lightning strike bounces the local earth and, the whole earth system of the building follows that bounce (or surge). Usually (and provided there is only one earth point), there is no damage to building electrical infrastructure or appliances. This is because they all bounce together i.e. they remain at equipotential even if during that surge they might have been bounced to a kV or so.
The problems come when the building electrical infrastructure extends away from the building such as AC wiring and telephone wires because, if the far-ends are "earthy" those far-ends are not necessarily "bounced" by the same surge. The result can be tens or hundreds of amps flowing through that extended electrical infrastructure. That current now flows through the building electrical infrastructure and can lead to high levels of non-equipotentials and that can damage local equipment.
Surge protection devices can divert surges currents to earth and prevent common-mode AC surges damaging (say) computer power supplies. These SPDs are needed when the equipment cannot handle several thousands of volts to earth. An alternative is to design the equipment to handle that common-mode surge - more modern ethernet is a good example - generally the isolating magnetics are good for 6 kV surges.
Surge protection devices are also used to prevent differential effects damaging equipment. This is usually because a common-mode surge (as it normally begins) can produce differential artefacts that may rise to several hundred volts. These differential artefacts arise because two wires in a cable may not propagate the common-mode surge identically and therefore a differential surge is created.