From Newsgroup: uk.d-i-y

John Rumm wrote:

On 25/04/2026 13:47, Sparky wrote:

John Rumm wrote:

On 24/04/2026 10:37, Sparky wrote:

My house electrical system has evolved over the years and I've been
meaning to document it for quite a while so future owners will stand
a chance of sorting out any problems. I've recently finished drawing
a schematic and would appreciate any comments - I've already spotted
one MCB sizing that needs to be changed.
The drawing is here https://ibb.co/C3t0mWDn

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!

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.

Yup, an emergency light should mitigate.

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.

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!

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.

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

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.

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.

https://wiki.diyfaq.org.uk/index.php?title=Taking_electricity_outside#Exporting_an_Equipotential_Zone

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
16mm2 supplemental earth conductor between the primary and secondary CU,
in parallel with the T&E.

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)

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

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

Cascaded 20A MCB is not an actual problem in most cases - although in
the event of an over current trip, you don't know which device will
trip or whether both will.

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

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)

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.

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

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.

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).

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

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.

https://wiki.diyfaq.org.uk/index.php?title=RCD#Electrical_and_Trip_Characteristics

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.







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