Domestic electrical schematic for comments

It looks very clear!

Thanks, but I was wondering whether anyone had any comments on the electrical design. It's easy to miss things when a system evolves and, for example, I'd not realised that I had two 20A MCBs in series until I drew the diagram.

Yup! That's why I wanted to document it properly.

A 2.5 T&E ring. But you've spotted my deliberate ;-) drawing mistake - there's an FCU for the C/H

Discrimination is the problem - not guaranteed when they're different ratings but definitely poor to have them rated the same. Given the potential loads the second one could/should be a B6.

Yes, separating circuits gives a problem for Diversity calcs. Now that it's drawn out I can see opportunities to reduce a couple of breaker values and to combine a couple of circuits - but in a larger house it's useful to keep things separate.

Handy diagram to have.

One minor design issue - workshop CU has same RCD for lights and sockets. Depending on use that could leave you in the dark with still spinning blades!

You also have a RCBO upstream of of the feeds to workshop and Store. Is that a type S (i.e. with delayed operation) device? If not you have another possible similar situation there - 50mA of leakage from the socket circuit in the workshop could turn off the lights since cascaded RCDs don't discriminate on leakage current generally.

I am guessing it is just your diagram missing a main earth connection to the secondary CU and not the actual install?

You have a PME setup in the house, but also some external sockets that don't appear to be TT, so you are exporting the TN-C-S earth into a location where you are not also exporting the house equipotential zone. Technically this is not ideal (although quite commonly seen)

Your potential load is large before applying diversity[1], and doing so may not get to under your supply capacity, however in real life it is unlikely to be a problem unless cooking Christmas dinner, charging the car and taking a shower all at once! :-)

Have a look at:

Does your generator have a earth neutral bond in place connecting it to the supplemental earth as well? (you might find things like ignition detection in some boilers would fail without)

Your car charger needs to be on a type B or type A RCD (and A is only permitted if the charger itself has built in detection of any stray DC leakage current over 6mA).

There might be other things - but that is what jumped out at me based on a quick look.

It would be worse if they were in parallel.

I should do one for here as it's got quite complicated over the years.

Brian

A good point but the more likely situation is a power cut. The workshop has a diesel air heater with a battery back-up (to save the electronics if the power goes off) so I've connected a small emergency light to the battery to cope with breaker trips and power cuts.

That's a drawing artefact. The real situation is that the TT supply enters a double external socket that has integral RCBOs for the sockets. The feed (including earth, it's a short distance) to the workshop and store is connected inside the double socket enclosure but does not pass through the RCBOs. I should think of a better way of drawing this!

The secondary CU takes its earth from the (16mm2 T&E) sub main. Because the sums didn't quite work I added a secondary ground wire in parallel with the 16mm2 T&E.

Yes, but they're fixed to the house wall and it seemed OTT to provide an earth rod for each.

Yes, when things evolve and circuits get separated onto different breakers it's difficult to keep track. Drawing this diagram has shown a couple of areas where I can reduce the breaker ratings and, possibly, combine circuits. I'd like to get rid of the electric shower (which we never use) but management won't let me.

I'll be reducing the rating of the downstream MCB.

I tested it with floating Neutral and the (large oil) boiler seemed to work but I've included a N-E strap in the generator connection. I need to show this on the diagram.

It's on a B32. I just checked what was in the Rolec "dumb" charge pod - it's a C40A RCBO.

That was very useful, John. Thanks. If you have (or anyone else has) any other thoughts I'd be grateful to read them.

Yup, an emergency light should mitigate.

Yup probably better if the drawing shows the functionality and connections rather than reflecting the actual enclosures exactly. You could draw more boxes and add notes to say which share an enclosure.

Yup that is often the case with domestic sized submains where you also need to export the equipotential zone, since you will typically need at least 10mm^2 of copper to act as a main bonding conductor.

Indeed - just mentioned for completeness. Chances are any appliance you are likely to plug in will be double insulated (Class II) anyway.

Note sure how oil boilers do flame detection, but I expect flame rectification is probably not used so unlikely to be an issue.

It is the trip characteristic that matters in this case. I am assuming that the C40A applies to the trip curve, and the nominal trip curent of the MCB part of the device only. Many RCBOs will just have a normal type AC RCD element. These are not ideal for vehicle charging.

The secondary CU isn't outside, it's in the central part of the house. The meter, primary CU, etcetera are in the attached garage. I ran a

According to Gemini AI: Worcester Bosch oil boilers, such as the Camray or Danesmoor ranges, typically use a CAD cell (cadmium sulphide photocell) to detect flame presence and ensure safety. This photoresistor device monitors the combustion chamber for the bright light produced by the oil flame during operation.

The spec for the charge pod RCBO says: The Garo C Curve RCBO ... supports 6kA fault current and Type A residual protection to detect both AC and pulsating DC leakage.