When connecting an SPD into the system, there are up to four modes of protection to cover. Line-to-neutral, line-to-ground, neutral-to-ground, and line-to-line. If the SPD is next to the transformer, line-to-line and line-to-neutral are sufficient, because the neutral and ground are bonded, making the neutral-to-ground meaningless and line-to-ground redundant. The further away it is from the line-to-neutral connection, the more important it is to add additional modes of protection. Also, be aware that SPDs can provide direct and indirect protection. For example, if two SPDs are connected directly between phase A and neutral and phase B and neutral, there will also be indirect protection between phase A and phase B.