Well, I'll kick off the debate on SPDs. Once again, the emphasis seems to be on domestic (we all, or virtually all of us live in a home of one sort or another) so are the proposals an improvement or not?
When talking about cable lengths for SPDs, think of it as a temporary jump lead being attached a two points somewhere on the active paths of the line and earth, or N+E (or whatever). The purpose of the jump lead is to briefly, and as far as possible, bring the L conductor and the E conductor as close as possible to the same potential. In practice, due to the length of the jump lead and inductive effects when faced with the very rapid rise time of the transient (e.g. 8us), the potential across the jump lead is likely to be in the region of 1.5kV. Reducing the length of the jump lead improves the effectiveness of the SPD.
So what do I mean by active path? Consider a socket outlet which is ultimately protected by an SPD in the CU. The earth path for that socket is [ socket -> T&E -> Cu earth bar -> earthing conductor -> etc ]. Suppose the earthing conductor uses hole 1 on the earth bar and the circuit is on hole 5. If the cable to the SPD is attached to the earth bar on any of holes 2..4, then it's effectively in the earth path for that circuit. Thus on the E side of the SPD, only the length of the earth-bar-to-SPD cable adds to the length. Conversely, suppose the SPD cable was attached to hole 10 in the earth bar. Then the total length from SPD to the circuit is now the SPD cable plus the length of the earth bar from holes 5 to 10.
So in a CU, typically put the EC in hole 1 of the earth bar, the E from the SPD in hole 2, then holes 3+ for circuit cpcs. If the SPD is protected by an MCB, put that next to the main switch, so that it's between the main switch and other MCBs, and so the bus bar isn't adding to the effective length.
When talking about cable lengths for SPDs, think of it as a temporary jump lead being attached a two points somewhere on the active paths of the line and earth, or N+E (or whatever). The purpose of the jump lead is to briefly, and as far as possible, bring the L conductor and the E conductor as close as possible to the same potential. In practice, due to the length of the jump lead and inductive effects when faced with the very rapid rise time of the transient (e.g. 8us), the potential across the jump lead is likely to be in the region of 1.5kV. Reducing the length of the jump lead improves the effectiveness of the SPD.
So what do I mean by active path? Consider a socket outlet which is ultimately protected by an SPD in the CU. The earth path for that socket is [ socket -> T&E -> Cu earth bar -> earthing conductor -> etc ]. Suppose the earthing conductor uses hole 1 on the earth bar and the circuit is on hole 5. If the cable to the SPD is attached to the earth bar on any of holes 2..4, then it's effectively in the earth path for that circuit. Thus on the E side of the SPD, only the length of the earth-bar-to-SPD cable adds to the length. Conversely, suppose the SPD cable was attached to hole 10 in the earth bar. Then the total length from SPD to the circuit is now the SPD cable plus the length of the earth bar from holes 5 to 10.
So in a CU, typically put the EC in hole 1 of the earth bar, the E from the SPD in hole 2, then holes 3+ for circuit cpcs. If the SPD is protected by an MCB, put that next to the main switch, so that it's between the main switch and other MCBs, and so the bus bar isn't adding to the effective length.
