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

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

I suspect the worry is that the electronics might also switch the heating current using triacs or thyristors or the like - so with some faults there could be a substantial residual current under semiconductor control, especially on TT systems where the earth fault current isn't going to be enough to immediately blow the electronics to smithereens, but even on TN systems under some circumstances (e.g. fault from part way along an element).

the circuit does not need ANY RCD, the loads are Earthed and there is no access. 411.3.3

The usual reason for 30mA RCD protection on a domestic is cables concealed in walls (which I presumed was the case here) - 411.3.3 doesn't tell you the whole story, you need 522.6.202 as well.

  - Andy.

I said that Andy, New storage heaters are not going to be fitted on non-compliant circuits are they?

Now the next bit, please explain how you think a circuit with semiconductor control elements can produce a single fault with a significant DC current being passed to Earth, yet this current is insufficient to cause the operation of the CPD or causing circuit failure of some kind? Now the hard bit, why does it matter? The answer is it only matters if there is a second fault involving direct human contact with a live conductor. Now the hardest bit, why do you think a type A is suitable because you know nothing about this dangerous fault current! Why not choose the most expensive device you can find because it is sensitive to any leakage from the load or cables to Earth, whatever its nature, pure DC included.

It appears that any evidence of actual accidents from the "DC problem" is unavailable, and I have discussed this with the policymakers. It is thought to be a possible theoretical risk, although I think most of the problem is that most of the Electrical people don't understand electronic faults and possibilities.

I am increasingly unhappy that "double faults" are rising on the "what if" agenda. 522.6.202 is there to deal with a particular possibility, that is adequately dealt with by a type AC RCD. To make this not work we need a number of complex details about a potential DC second fault, that in itself is as unlikely as contact with a buried cable. Even the 50mm regulation is a "wet finger in the air" number, I and probably you often drill holes much deeper than that, and many older installations do not have the RCDs on all circuits. As chasing a wall with 50mm of finished cover is virtually unheard of, it is likely that every buried cable is expected to have RCD protection against idiots. The justification came from a coroner from a single case, Such things tend to be full of holes and assumptions without proper technical understanding. The answer "it could be better than a type AC under certain conditions" is not a proper justification for a type A, is it?

I am increasingly unhappy that "double faults" are rising on the "what if" agenda.

But shouldn't there be room for safety improvements over time if that's what the public wants? The safety features of cars have moved on almost unrecognisably since say the 1970s - as society generally demands safety improvements where the technology and the cost permit. Why should the electrical industry stick with 1970s era 2-faults to danger doctrine when when the rest of the world moves on?

   - Andy.

It is fine so long as we realise that in times of economic stress, the value of life can fall , as well as rise when things are improving - under some conditions there is then a case for rolling back on mandating some improvements if they become disproportionately expensive.

It is exactly this reason that allows things to be made more cheaply abroad where the risk/life balance is further over than here.

Like in America’s and Ford’s most disgraceful episodes

www.spokesman.com/.../

Those new type storage heaters a just full of electronic components. I installed one recently. They will be very vulnerable to transient peaks I reckon.

Z.

Yep, modern storage heaters are controlled internally by large complicated electronic circuit boards with triacs or equivalent devices..

Z.

Does anyone recall how the old Robin D-LOC system worked? Was the d.c. blocking current applied L-N or L-PE?

   - Andy.

As usual that got a little bit more complicated than anticipated. 

I had to order a BG split load internal tails kit to make the off-peak consumer unit into a split load consumer unit to be able to install the surge protection device upfront of the 30 mA RCD, because BG use crimps that reduce the size of the internal tails to fit into the neutral busbar. 

I had some 16 and 25 mm Tri-rated cable and crimps with me,  but neither size will fit into the BG neutral busbar, so I had to go online and order a kit to be delivered to me. Whilst I was at it I ordered some spare kits, so I have some to hand now. 

There's also a "spare" circuit in the off-peak consumer unit, there appears to be another storage heater circuit,  but I have not found the other end of it, I think it's behind some long case Grandfather Clocks that I'm not going to try and move, so I have parked the circuit in the earth bar until another day.