534.4.1.4

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.

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.