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

What's the off-peak CU supplying? 

A couple of new storage heaters will cost between £1500 and £2000, they have electronic controls.

I would not be asking the question if they were a couple of old heaters, how far do we go?

The tack that seems to be taken re the SPDs is that providing the client is made fully aware of the implications of omitting them, then they are treated almost like an optional extra. In other words, not safety critical. The AC RCD is falling from favour for good reason. Whether it is required in the circumstances you describe, I do not know, but I would err on the side of prudent design and go with a type A.

I have ordered a nine module consumer unit with a main switch for the off-peak.

Main switch two modules, SPD and MCB another two modules, plus the RCD again another two modules, which leaves three usable ways in a nine module consumer unit for the heater circuits.

The cost comes out about sixty quid more than a simple consumer unit with a main switch and MCBs as we would have installed a few years ago, although not a huge sum it’s tripling the cost of the consumer unit.

it is an interesting question about doubling the spds- presumably the off-peak electrics is never on without the other board being on as well, there will be some cases with very short meter tails where the projection being share may well be OK.

If the spd is on the far side of an rcd that may trip then while tripped it removes protection to the other loads, so they are at riskm rather than just an rcd costing less than the spd.but in any case a tripped RCD is normally quickly noticed and reset.

of course if on different phases this does not apply.

Mike

RCDwise I`d be happy with that. Unless your storage heaters are with some bristling electronics fandango that I`m not aware of.

SPDwise, if single phase I`d tend to be happy - ish . Along the lines of what Lyle and Map have said.

Just an aside. I`m assuming a TN system. If TT then I tend to err on 2 RCDs in cascade to help mitigate RCD failure which might well start to add costs considerably. I do tend to favour a plain DP Mainswitch & RCBO setup too , not as expensive as it used to be but again additional expense.

Having said all that, my own home ("TNS") has a front end RCD and MCBs for peek and same again for off-peak, no SPD and I`m not worried.

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.

Is is just me or are things getting so out of hand that electricity should be removed from domestic premises? Storage heater: electronics, OK so the electronics take 10mA, and of course they will have protection from mains "events", at this power level it is trivially cheap and the manufacturers warranty costs depends on it. RCD protection, there is no conceivable risk for additional protection of these circuits unless they are not installed correctly, you know the rules. I know that BS7671 is extremely unhelpful about these things and full of feature creep, but this is ridiculous. A type A RCD for an electronic circuit which might be able to leak 50 mA of DC if it suffers multiple failures? Pull the other one, the circuit does not need ANY RCD, the loads are Earthed and there is no access. 411.3.3

I am looking at Amd 2, we are going mad.

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.

Unless the heaters need an RCD for fault protection, installation method or manufacturer recommendation, i would be inclined to omit it. If you are going to fit one, check with heater manufacturer what type they recommend and amount of DC leakage per heater.

The cost of the heaters against the cost of an SPD+mcb means i would recommend to the client they be fitted. The SPD may be fine protected by one of the heater's 16A mcbs, That way if it trips it will be noticed, and it will save a few quid, plus less ways in the CU