SPD Location In this setup.

Think of an SPD and its leads as a temporary jump lead that you use to briefly short together a L&N or whatever in a circuit where there is a transient. You would imagine this would bring the voltage at that point in the circuit to zero; however due to inductive effects etc and the fast nature of the spike, there will be in the region of 1.5kV across the SPD itself, and in the region of 1kV/m across its leads. So if the total length of the SPD's leads is 0.5m, then you're only reducing the spike in the circuit to 2kV. Generally, the electronics being protected downstream is supposed to be able to cope with 1.5kV.


The point where the "jump leads" attach to the normal path of the circuit being protected is where the length should be calculated from. So for example if the supply is at slot 1 on a bus bar, a circuit is at slot 2, and the SPD is at slot 10, then the length of the busbar between slots 3-10 contribute to the SPD's length as regards protecting that circuit. If you spurred the SPD from the Henley blocks, the total L+N path would be the tails x 2, plus a bit of busbar in the Henley blocks x 2, plus all the internal wiring in the SPD enclosure. For L+E protection, the length would include the E connection from the SPD to whatever point in the earthing system which is on the normal earth path for the circuit(s) being protected.

Think of an SPD and its leads as a temporary jump lead that you use to briefly short together a L&N or whatever in a circuit where there is a transient. You would imagine this would bring the voltage at that point in the circuit to zero; however due to inductive effects etc and the fast nature of the spike, there will be in the region of 1.5kV across the SPD itself, and in the region of 1kV/m across its leads. So if the total length of the SPD's leads is 0.5m, then you're only reducing the spike in the circuit to 2kV. Generally, the electronics being protected downstream is supposed to be able to cope with 1.5kV.


The point where the "jump leads" attach to the normal path of the circuit being protected is where the length should be calculated from. So for example if the supply is at slot 1 on a bus bar, a circuit is at slot 2, and the SPD is at slot 10, then the length of the busbar between slots 3-10 contribute to the SPD's length as regards protecting that circuit. If you spurred the SPD from the Henley blocks, the total L+N path would be the tails x 2, plus a bit of busbar in the Henley blocks x 2, plus all the internal wiring in the SPD enclosure. For L+E protection, the length would include the E connection from the SPD to whatever point in the earthing system which is on the normal earth path for the circuit(s) being protected.