534.4.1.4

I guess the principle is that by putting the first SPD at the panelboard, you're not protecting the equipment downstream of that. If all you have is a simple switchfuse and cable and the chances of having to deal with lightning are remote (e.g. no LPS on the building) then I suspect it's not going to be a huge risk in practice. The regs however will be based on common principles and will have to assume the worst case where you incoming equipment really might not be able to withstand anything above 4kV (see table 443.2).

There's also the consideration for bonding - if main bonding were connected (directly or indirectly) to the intake position rather than the panel board, then SPDs at the panel board wouldn't keep the difference between electrical installation conductors and extraneous-conductive-parts as low as it might be during a surge - which can lead to larger than necessary potential differences all over the place as the extraneous-conductive-parts wend their way through the installation.

   - Andy.

Putting the regs to one side, the physics is that the surge voltage is more or less shorted out at the point of the SPD, but much in the same way that the cable near a short circuit fault has a voltage slope along it, so does the wiring to and from the SPD, only the voltages are higher, and the pulses are short so the inductance of the cable dominates over the resistance.  If there is anything delicate some distance from the spd, on the side the surge is coming from, the voltage it sees rises as you get more than a few metres to tens of metres from the SPD.

But if there is nothing of great value to protect then the SPD can be moved inboard towards the load without ill effect.  The other consideration is where the pulse gets shorted to - ground/ earth is not at 'zero' volts during the pulse, as the pulse voltage gets pulled down the earth gets pulled up.  This rise of earth potential can be a destructive influence, if there is other kit connected to ground  via another path, the equipment finds itself doing the splits between the bouncing earth and the non-moving one.  Common problems are things with external  antennas - radio, TV, satellite  or telephone or similar connections to outside.

In such a case the correct place for the SPD is near the connection to the non bouncing earth.

The advice in the regs is an attempt to simplify a complex subject.

Mike.