BS7671 Amendment 2. Off-peak E7 consumer unit RCD and SPD.

As the storage heaters contain electronics, I'd say no to an AC RCD (would have to be A-type) and likewise it would need SPD protection unless the customer specifically opted out of it. With 34m of cable between the offpeak CU and the SPD (SPD protection is know to reduce significantly after just 10m) and effectively 17m of 'tails' between the SPD and the point where the off-peak supply tees off (compared with the usual limit of 0.5m) I can't see that the SPD in the other CU will offer much protection.

I'd maybe consider SPD protection at the origin too, unless you're convinced that the equipment there will be immune (as I read table 443.2 we can only assume that switch fuses, DBs etc will good only to 4kV).

  - Andy.

As the storage heaters contain electronics, I'd say no to an AC RCD (would have to be A-type) and likewise it would need SPD protection unless the customer specifically opted out of it. With 34m of cable between the offpeak CU and the SPD (SPD protection is know to reduce significantly after just 10m) and effectively 17m of 'tails' between the SPD and the point where the off-peak supply tees off (compared with the usual limit of 0.5m) I can't see that the SPD in the other CU will offer much protection.

I'd maybe consider SPD protection at the origin too, unless you're convinced that the equipment there will be immune (as I read table 443.2 we can only assume that switch fuses, DBs etc will good only to 4kV).

  - Andy.

AJJewsbury said:

With 34m of cable between the offpeak CU and the SPD (SPD protection is know to reduce significantly after just 10m) and effectively 17m of 'tails' between the SPD and the point where the off-peak supply tees off (compared with the usual limit of 0.5m) I can't see that the SPD in the other CU will offer much protection.

Would somebody kindly explain this 10 m rule in simple terms please?

Because of it, I raised the question at a recent NAPIT update whether it was better to have one (type 2) SPD at the origin, or one in each (SP) final DB. The answer was unequivocally one in each final DB.

I then asked about final circuits which are > 10 m in length and I don't think that I got a satisfactory answer. I think it was along the lines that at the DB is the best that you can do.

To my simply brain, if an SPD blocks the voltage disturbance, it blocks it and the location of the delicate little transistors downstream should not matter.