SPD arrangement

davezawadi (David Stone):

Simon, you have the wrong end of the stick. I said a HV strike, potentially many miles away.


You are right about very low power surges, the electronics looks after itself. So what is the purpose of the SPD? BS7671 says to protect against lightning surges, so I discuss them. You answer that nothing can protect against them, not necessarily true, but certainly not VDR based low power products. So what are they for? That is the question, as the Bard said. The slope resistance even at 4kV is still considerable, and unless operating against some significant impedance still cannot LIMIT the voltage. The only device which actually does is the Zener diode which has a low slope resistance and little inherent current limiting. Diodes with sufficient power rating to use for surge protection are quite expensive, many times the cost of a VDR (which is pence). A string of fairly low voltage diodes would be required, so even more expense, and as they are unidirectional two strings (or possibly a very high voltage bridge rectifier with a high current rating) would be necessary.

SPDs per 443 and 534 are NOT intended to protect against a DIRECT strike to the building.


The impedance in the supply network helps address overvoltages due to remote atmospheric activity.


The other thing that BS 7671 SPDs protect against is overvoltages due to switching surges

davezawadi (David Stone):

Simon, you have the wrong end of the stick. I said a HV strike, potentially many miles away.


You are right about very low power surges, the electronics looks after itself. So what is the purpose of the SPD? BS7671 says to protect against lightning surges, so I discuss them. You answer that nothing can protect against them, not necessarily true, but certainly not VDR based low power products. So what are they for? That is the question, as the Bard said. The slope resistance even at 4kV is still considerable, and unless operating against some significant impedance still cannot LIMIT the voltage. The only device which actually does is the Zener diode which has a low slope resistance and little inherent current limiting. Diodes with sufficient power rating to use for surge protection are quite expensive, many times the cost of a VDR (which is pence). A string of fairly low voltage diodes would be required, so even more expense, and as they are unidirectional two strings (or possibly a very high voltage bridge rectifier with a high current rating) would be necessary.

SPDs per 443 and 534 are NOT intended to protect against a DIRECT strike to the building.


The impedance in the supply network helps address overvoltages due to remote atmospheric activity.


The other thing that BS 7671 SPDs protect against is overvoltages due to switching surges