Concern about electrical safety check

[Someone-Else]

where did the term "Ring Main" come from?

ericmark has already told you.

[glassescase]

where did the term "Ring Main" come from?

ericmark has already told you.

unfortunately when reading down the thread I think I got to the post I quoted before I got to the post with the explanation.

glassescase

[ericmark]

The ring final is the last circuit before power is used, there is a part exception with the fused spur, this is why for Part P the use of the fused connection unit (FCU) is not considered as a new circuit as it comes from the final circuit.

The ring final used cable rated 20 amp or more with a 30 amp fuse or 32 amp trip so it is important both legs are used, the ring main each leg can carry the full load, so any section can be isolated for maintenance.

The ring final system was invented at the end of second world war so the re-build would need less copper, and still have the loop impedance needed to ensure a fuse would rupture, or MCB trip within the required time.

To use radials you need three circuits to replace one ring final, and it is no where near as flexible. But there are drawbacks, the fuse is in line only, so very important the line and neutral are correct way around.

[OnlyMe]

Of course the advent of the RCBO has improved things, before if there was a slight earth fault, nothing much happened, no RCD's only re-wireable fuses, then MCB's came into widespread use, then the RCD (Its was preceded by the ELCB*) one RCD would control all MCB's, which obviously meant a slight earth fault and click, no lights no TV no freezer, so the split board came out, two RCD's. Then the RCBO cost came down and now all new installations have them, and the latest must have is surge protection, I wonder what is next.


*ELCB is Earth Leakage Circuit Breaker, they worked on voltage not current and were not that good.
MCB is Miniature Circuit Breaker (Like a re-usable fuse that is very quick) it operates if too much current is being drawn.
RCD is Residual Current Device, it detects current not coming back, so it must be going to earth.
RCBO is cross between an MCB and an RCD as it does both things.

Pretty good stuff but I can add to that

ELCB originally used to refer to both the voltage operated and current operated circuit breakers. On 80s installations it is common to come across a main switch labelled ELCB with an operating current of 30mA!

When the term RCD came along ELCB was then generally used to refer to the older VO devices.

You missed out the 16th edition installation where the term RCD was widely in use and only the sockets, shower etc were RCD protected with their MCBs sharing one RCD and the lights, CH etc were not RCD protected. As em says it was also common to have the freezer on a non RCD supply at the time.

The dual RCD protection came in with the 17th edition in 2008.


A device that operates just as an RCD (no overcurrent/overload protection) is now called a Residual Current operated Circuit Breaker (RCCB) although in general terms even at work will still call them RCDs.

Not always must the "current not coming back" have to be going to earth. How many times on here have we seen a post where someone has put the neutral into the wrong busbar

[ericmark]

When I bought this house I had a house buyers survey done, as part of this comments were made about the electrical installation. It said how there was a reasonable modern consumer unit, and the old disused fuse box was still in place between the old ceiling and new suspended ceiling, actually the fuse box was still supplying the main house, the consumer unit only supplied the garage converted into a flat under the main house.

The two units have been replaced with a new metal unit with all RCBO's fitted, and a SPD. Actually made an error, the boxes for the RCBO said type B which was better than I expected, type A would have been adequate, but then realised they were curve B type AC which really was not good enough, I tried ordering type A, but Colvid19 resulted in them not arriving and in the end I cancelled the order.

The regulations say "Every installation shall be divided into circuits, as necessary, to:" and has a list, the two which seem to have caused problems are:-
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation

I hate the "take account of" as it leaves it rather open, but what we don't want is should you have a shock from some thing plugged in, for the lights to go out as well.

So unless like with a caravan or boat where you have 12 or 24 volt lighting from a battery, you need at least two RCD's so lights and sockets are not from same RCD.

We tend to spit the sockets into a few circuits, for a few reasons, one is loading, but also to reduce the loop impedance, and to keep volt drop within limits, to do this best option is split side to side, this also means if one fails no need for extension leads up/down stairs.

However with lighting it was normally split up/down, so it is near impossible to arrange it all on two RCD's, unless sockets also split up/down.

Also we should ensure in normal operation the leakage does not exceed 9 mA on any 30 mA RCD, that can only be measured after the house is wired, and to be frank my clamp-on ammeter will not measure low enough.

So the designer has to try and guess what the leakage under no fault conditions will be. AC will always leak to some extent, and to my mind best option is all RCBO, but many feel two RCD's are good enough, or maybe with one or two RCBO's for some circuits.

I think all RCBO will cost around £200 more than 2 RCD's which is less than a freezer full of food, so I feel well worth the extra. However of late there has been a further consideration, the electric car charging and solar panels, in both cases faults can cause DC, and DC can stop a RCD working, so if better than type A RCD is required than can't use single modular width RCBO's using a 6 mA DC disconnection device is used by some charging units, so type A is good enough, but some cars what you to fit their system which requires a type B, And as well as being expensive they are at least two modules wide, so there is a case for having some circuits through a RCD when I say some, read that as solar panels and EV charging points.

So today the installation has to be designed, not simply thrown together using the excuse we have always done it that way.