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?
I think the focus on domestic, along with Part pee and the somewhat excessive (IMO) RCD requirements are a bit of a backlash because of the historically VERY lackadaisical attitude to domestic wiring. EAW and factories act have covered businesses for decades (although I could tell some stories about takeaways and weird stuff... shower run on 1.5 T&E? seen it.)
But I do think the SPD requirements are an improvement for domestic. The DNO's networks are falling apart due to lack of investment. One has declared any supply may become PME ... which means 'our cables rotted'. An open PEN WILL cause damage... however, a loose connection could cause short term transients of the type an SPD can deal with... plus many dozens of customers reporting SPD's failing in an area might clue the DNO into the issue.
There's a problem inasmuch that the public don't generally communicate with the DNO, but with their 'energy supplier' (silly term), but it might save a few people's bacon. And SPD's are no longer as expensive as they were in late 2018 when I sat my 18th edition course. Hopefully AFDDs will go the same way pricewise. but that's a separate thread.
Sorry if I sound bitchy about DNO's... it's NOT aimed at their employees, but at the political and corporate machinations that have led to the situation :(
I find SPDs an addition with little reason behind it. They have no "safety case" for PEOPLE at all. Electrical supply systems have always had transients present (although there do seem to be more now) and they did not cause a problem. "Electronics" is often used as a reason why they are necessary, but this is nonsense, any failures can always be found in poor design of the failed electronics! I think that the problem trying to be cured is that the change to LED lighting has put much more poor electronics into homes, and LED lamps have very simple design which is susceptible to transients because the manufacturer has saved tiny amounts of money, although the selling price is quite high. Again the fixed wiring regulations are being asked to fix problems elsewhere. This trend is very noticeable on car charging (RCDs), PME (additional earthing), domestic appliances (AFDDs), surges (SPDs) etc. I do not find that VSDs, SMPS in professional kit, quality computers, induction hobs etc, suffer failures due to surges or transients on the mains. The DC power connectors do not suffer from transients despite working on EHV lines and during thunderstorms. This problem is largely illusury and BS7671 is not the place to change anything.
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.
They seem to have converted the existing confusing criteria into a new set of completely bewildering criteria. If I am reading the draft 443.4.1 correctly, it summarises as:
Must protect if could result in risk of injury/life or financial / data loss.
Except single dwellings, if intolerable (as decided by all parties), do risk assessment to decide if protection required.
Protection not required for single dwellings with not much to lose (except it notes that most will have much to lose).
So they don't actually state what you should do for single dwellings with valuable electronics. It certainly doesn't state that you need SPDs, nor that you need to a risk assessment.
