Would a SPD within a CU at the origin of an installation cover other CU at the origin from surges too?

There will be some degree of spike reduction in almost any wiring arrangement with an SPD near when  compared to having no SPD, but its not a black and white thing, If you have the choice the suppression effect is best when the SPD iis nearer the source of the spikes you want suppressing.

As the current spikes have very fast rise times compared to 50Hz sinewave, the inductance of the wiring becomes far more important, hence the advice for short leads, and although the advice does not say so,  bringing the lines that carry the surge current through the SPD into close proximity so the magnetic field is partly cancelled, also helps, so cable or even a twisted bundle is better compared to a wide spaced loop of  singles.

Mike.

There will be some degree of spike reduction in almost any wiring arrangement with an SPD near when  compared to having no SPD, but its not a black and white thing, If you have the choice the suppression effect is best when the SPD iis nearer the source of the spikes you want suppressing.

As the current spikes have very fast rise times compared to 50Hz sinewave, the inductance of the wiring becomes far more important, hence the advice for short leads, and although the advice does not say so,  bringing the lines that carry the surge current through the SPD into close proximity so the magnetic field is partly cancelled, also helps, so cable or even a twisted bundle is better compared to a wide spaced loop of  singles.

Mike.