Lightning does not have to strike your building to damage the electronics inside it. A strike a mile away couples energy into the power lines, the ground, and the structures that carry it to your board through doors a designer never considered — the mains input, the telephone or network line, the ground rod, and the capacitive coupling through the walls. Understanding where the surge comes from changes where protection goes: the diodes that save a board sit in the paths the lightning uses, and a board without them is a board with an uninsured weakness. This article walks the strike-to-board story in numbers, then maps the layered protection — grounding, then the TVS or MOV, then the last diode — that a real installation needs.
One Strike, Many Paths: How Lightning Finds Your Board
Lightning finds electronics through coupling, not through a direct hit. A nearby strike drives a transient into the mains wiring, which conducts it to every plugged-in board. The same strike injects current into the ground, which pushes a step up through the earth path and the grounding conductor into the equipment. Signal lines — phone, network, and data cables — carry induced transients from the electromagnetic field of the strike. Even walls couple a fast field directly into enclosed electronics by capacitive induction.
The consequence for a designer is that a board is exposed on every conductor that enters its enclosure, not just the power cord. The protection map must therefore cover the mains input, the signal lines, and the ground path. The transient voltage suppressor article covers the device that handles the mains-side surge; this article frames the full coupling story that decides where that device belongs.
The path view also explains why a strike far away can still be fatal: the coupling travels along conductors and bundles of wiring for kilometers, and the energy arrives at the board whether the strike was one mile or ten miles out.
Voltage, Current, and Time: The Surge Spectrum in Numbers
The surge a board must survive is a spectrum, and the numbers set the protection requirements. A direct strike current peaks in the tens of kiloamperes and lasts tens to hundreds of microseconds; a coupled surge at the mains input is smaller in current but can rise to the kilovolt range in microseconds, which is the event a TVS or MOV is sized to divert. An induced transient on a signal line is faster still — a few kilovolts in nanoseconds — which is the domain of the fast protection diodes at the pin.
| Event | Peak | Duration | Device class |
|---|---|---|---|
| Direct strike | tens of kA | tens–hundreds µs | Ground and structural protection |
| Coupled mains surge | kV | µs–ms | TVS / MOV at entry |
| Induced signal-line transient | kV | ns–µs | Fast ESD / protection diodes |
The table is the surge spectrum in one view: the bigger the energy, the slower the event and the heavier the protection; the faster event, the smaller the energy and the closer the diode sits to the pin. Reading the spectrum is what matches a protection device to the event it will actually see.
A worked coupling picture makes the numbers real. A strike a kilometer away injects a transient into the shared power grid, and that transient arrives at the building’s service panel already reduced but still in the kilovolt range. From the panel, the surge travels along the branch circuit to a wall outlet and into a plugged-in board; at the board’s input, the current is a few hundred amps to a few kiloamps, enough to destroy an unprotected rectifier and the microcontroller behind it within milliseconds. Compare that with the direct-strike path: a strike on the building’s own ground rod injects tens of kiloamps into the earth path, and a board whose ground connection is part of that path sees the step potential. The two cases bracket the spectrum — coupled surge at the mains, and ground-injected potential at the chassis — and both arrive at the board through wires the protection plan must cover.
The grounding detail is the layer most installations get wrong. A surge protector without a continuous low-resistance path to earth cannot divert anything; it simply passes the surge through the equipment looking for a way out. The field check is simple: verify the ground conductor is present, continuous, and sized for the surge current, and confirm the protector’s ground terminal actually connects to that conductor, not to a floating chassis screw. The surge and ESD test article includes the ground-path verification in its test sequence, which is why the grounding layer is treated as part of the protection chain rather than as a separate building service.
Protection Layers: From Grounding to the Last Diode
The protection a board actually needs is a layered defense, and each layer answers the coupling path in the first section. The first layer is grounding: a solid, low-resistance path to earth that steers the strike current away from the enclosure. The second is the entry diverter — the TVS or MOV at the mains input that shunts the coupled surge to ground before it travels down the rail. The third is the last diode — the fast protection at the signal pins that catches the induced nanosecond transient the slower devices at the entry cannot see.
The layers are not optional alternatives; each one protects a different timescale and a different coupling path, and a board with only one layer is exposed on the others. The surge and ESD testing article defines the tests each layer must pass, and the protection role map shows how the layers divide the same board’s protection plan between the entry TVS and the pin diodes.
Where TVS and MOV Participate in That Chain
The TVS and the MOV are the two most common entry-layer devices, and their roles in the chain differ by speed and energy. A TVS clamps in nanoseconds with a sharp, repeatable response — the right choice where speed matters and the energy per surge is moderate. An MOV is slower but absorbs far more energy per event — the right choice at a location exposed to repeated, higher-energy strikes, like the service entry.
The chain placement follows the geometry of the exposure: a small TVS at the board input handles the moderate coupled surge that reaches the equipment; a larger MOV upstream at the service panel handles the bigger energy before it propagates. The two are complementary rather than competing, and the decision between them follows the protection family comparison logic of speed versus energy.
A second field check closes the protection story. When a board leaves the bench for an installation, the engineer should be able to name each coupling path and the device that guards it: mains input to the TVS, signal lines to the fast pin diodes, ground path to a verified conductor. If any path has no named device, that path is the board’s lightning weakness — not because lightning will find it on the first strike, but because the coupling physics in the first section guarantees it will eventually. The TVS failure review is the field-side companion that names the mistakes this plan prevents, and the protection role map converts the plan into the device placement on the board. The result is a board whose protection matches the coupling spectrum: grounded, diverted at the entry, and protected to the last diode.
Common Lightning-Protection Myths for Electronics Users
The myths that get boards killed are worth stating plainly. Myth one: “I have surge protectors, so I am protected.” A cheap power strip with a low joule rating absorbs a small surge and dies quietly; the board behind it is then uninsured. Myth two: “Grounding is optional if the surge protector exists.” The protector diverts to ground, and without a real ground path the surge has nowhere to go and travels through the equipment instead. Myth three: “A direct hit is the only danger.” The coupling from a nearby strike does the damage most of the time, which is why the paths in the first section matter.
The correction that follows from this article: count the conductors, size the entry diverter for the real surge spectrum, verify the ground path, and add the fast last diodes to the signal pins. The lightning and electronics protection overview and the surge test article are the two published anchors for the numbers and tests this planning relies on, and the TVS families supply the entry-layer parts once the plan is drawn. In short, lightning protection is not a device — it is a map of every path the strike can take and a device on each one, verified at the installed ground before the enclosure is closed. The surge spectrum table and the coupling paths are the reusable tools, and they transfer unchanged to the next board, the next location, and the next strike, which is why the time spent drawing the protection map on this board is never wasted effort on any future install.