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Required max discharge current of spd and cable length for connections

Alan B

A couple of questions on SPD's

I am specifying an SPD for a house supplied with overhead lines. Already know I need a type 1, will actually be a type 1/2 at the origin and considering a type 2 in the main DB.
Then comes the question of maximum discharge current required. I think where there are lightening conductor systems involved this is calculated, but for a domestic property without such a system, overhead lines and singe phase supply I expected to just find a number I have to comply with, but couldn't find anything in BS7671? Am I just missing the obvious.

Second part of the question. I noticed in table 3.7.3 of the on site guide that maximum cable length for spd connections is only defined for a type 1 SPD. Other parts of the osg and BS7671 extrapolate this to type 2 spd's as well. The cable length is to reduce ringing generated by large spikes. I was wondering if this limit was only intended for type 1 spd's dealing with very large spikes and has been incorrectly extended to any spd installation unnecessarily. Your thoughts.

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    The guidance I follow: In order to achieve optimal protection from the SPD, the connecting conductors must be as short as possible. This aims to reduce any additional voltage drops on the connecting cables. Therefore, the length of the conductors to the SPD and the earth to the bar should not exceed 1 metre. For the type 1 SPD, the risk of very high financial loss or irreversible loss must be assessed. The impulse current rating is either 25 kA/pole or 12.5 kA/pole. For the type 2 SPD, the risk of direct strike is low (20 kA), medium (40 kA), or high (65 kA).

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    The guidance I follow: In order to achieve optimal protection from the SPD, the connecting conductors must be as short as possible. This aims to reduce any additional voltage drops on the connecting cables. Therefore, the length of the conductors to the SPD and the earth to the bar should not exceed 1 metre. For the type 1 SPD, the risk of very high financial loss or irreversible loss must be assessed. The impulse current rating is either 25 kA/pole or 12.5 kA/pole. For the type 2 SPD, the risk of direct strike is low (20 kA), medium (40 kA), or high (65 kA).

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