must I use a single MCB to protect a ring?

[1john]

Can I ask the OP which he feels is a better installation, we will take a standard 3 bed house for instance and his hypothetical breakers although we shall have to limit them to 32A to comply.

Scenario 1
Since ring final circuits are order of the day, we shall install two, just to add to the inconvenience we shall install and upstairs circuit and a down stairs circuit, thus loosing all power on each floor in the result of a fault. This has used 8 ways on the consumer unit.

Some comparable facts -

-If a fault arises on a circuit then you have lost 1/2 the installation.
-Each circuit is capable of supporting 2 3kW fires
-These circuits require excessive testing and inspecting
-These circuits are difficult to modify or add on to in the future.
-Any amount of faults could arise on this type of circuit which will cause 1/2 the installation to fail, as well as any the radial could suffer from there are also broken legs of the circuit which can cause problems and no easy way to diagnose a fault.


Scenario 2
3 bedrooms = 3 radial circuits
lounge - 1 radial circuit
Dining room 1 radial circuit
Kitchen - 2 radial for sockets, seperate radial for appliances
Allprotected by RCBO's.
This has used 8 ways on the consumer unit.

Some comparable facts -

-If a fault arises on a circuit then you have lost an 1/8 of the installation
-Each circuit is capable of supporting a 3kW electric fire.
-These circuits are simple and straight forward to test and inspect
-These circuits are simple to extend, alter and add on to in the future
-If a break in the line or neutral conductor arrises then the circuit will fail to function onward from this fault and you know to get it repaired. Infact you will know that the fault lies somewhere at or between the last functioning socket and the first non-functioning socket on the circuit.


Which installation would you want in your home?

 

[ajrobb]

Can I ask the OP which he feels is a better installation, we will take a standard 3 bed house for instance and his hypothetical breakers although we shall have to limit them to 32A to comply.

Scenario 1
Since ring final circuits are order of the day, we shall install two, just to add to the inconvenience we shall install and upstairs circuit and a down stairs circuit, thus loosing all power on each floor in the result of a fault. This has used 8 ways on the consumer unit.

My idea was to have a single RCBO module with a 4-sensor CB (now 5 to limit supply to 32A). So 2 ring finals => 2 ways on the CU.

I was not initially aware of the 32A limit on the socket connections (or is it just on spurs?). If single 2.5mm² cable can carry 27A (not series pairs in a single conduit), there is little point in the existing ring final design limited to 32A. However, there are still the attractions of possible constant circuit testing and existing 2 circuit protection conductors that radials don't offer.

The ring final circuit was rightfully downgraded from 40+A (with 30A semi-enclosed fused protection) to 32A (with CB). This downgrading makes it less attractive to the 'chuck it in and forget it' brigade. At the same time, radials have only changed from 20+A (with 15A fuse) to 20A (with CB). With improved protection, a ring final could provide 40A but other factors seem to preclude this.

Some comparable facts -

-If a fault arises on a circuit then you have lost 1/2 the installation.

The ring final circuit is more tolerant of some faults. Without individual conductor protection, this tolerance is presently seen as a problem. However, with individual conductor protection and constant testing, not only is the circuit protected from excessive currents but also the fault is immediately diagnosed.

-Each circuit is capable of supporting 2 3kW fires
-These circuits require excessive testing and inspecting
-These circuits are difficult to modify or add on to in the future.
-Any amount of faults could arise on this type of circuit which will cause 1/2 the installation to fail, as well as any the radial could suffer from there are also broken legs of the circuit which can cause problems and no easy way to diagnose a fault.

My module would include constant circuit testing - something that radials don't offer. The ability to hang an unlimited number of radials off other radials makes them a nightmare. At least a spur off a ring final only goes to a single socket.

Scenario 2
3 bedrooms = 3 radial circuits
lounge - 1 radial circuit
Dining room 1 radial circuit
Kitchen - 2 radial for sockets, seperate radial for appliances
Allprotected by RCBO's.
This has used 8 ways on the consumer unit.

Some comparable facts -

-If a fault arises on a circuit then you have lost an 1/8 of the installation

and you are unaware that your freezer has stopped working

-Each circuit is capable of supporting a 3kW electric fire.
-These circuits are simple and straight forward to test and inspect

what is simple about a Christmas tree of spurs off spurs?

-These circuits are simple to extend, alter and add on to in the future
-If a break in the line or neutral conductor arrises then the circuit will fail to function onward from this fault and you know to get it repaired. Infact you will know that the fault lies somewhere at or between the last functioning socket and the first non-functioning socket on the circuit.

With a loose connection rather than a total failure, all the downstream current is forced though this high impedance and your socket burns. With a ring final, the current through a single loose connection is limited as there is an alternative low-impedance leg. Stir in the constant circuit test from my module and you immediately know there is a problem.

Which installation would you want in your home?

Mine!

• 2 separate protective conductors back to the CU
  constant testing
  fault tolerance

 

[Risteard]

Even though the law also permits them to be connected to 2.5mm² ring final circuits protected with a 30A CB, nobody here thinks that practice should continue.

I'm not sure what "the law" has to do with it, but I have no difficulty with ring final circuits being employed.

 

[echoes]

Sorry to go off topic slightly, but if designing 1 radial per non-kitchen room for a typical 3 bed house, is it usual to spur off a convenient circuit to provide a couple of hallway and landing sockets?

 

[Chri5]

Yes, Often done back to back depending on the in room socket position.

If your smart you set the position in room so that it's a good place in room and a sensible place for back to back in to hall / landing.

 

[echoes]

@Chri5

Just what I needed to hear! Thanks again.

 

[1john]

I dont know how to do the multiple quote thing, but will continue this debate as it is quite entertaining for me.

Your querie on the 32A OCPD and the capability of 2.5 t&e to carry 27A has this logic behind it - you can only have one spur from a socket on a ring final circuit, thus limiting the maximum current drawn to 26A (or should that really be 20A, although thats another matter)

I can see no way of a single module circuit breaker having the spave available to terminate 6 seperate conductors in, or infact have the physical capacity to house all of the electronics required to make your hypothetical device function as you intend.

You can have a 32A radial circuit, they are not only 20A/16A/15A.

I fail to see how a radial off of a radial circuit suddenly becomes a nightmare, or infact a christmas tree either?

I cannot comment about real life occurences with radial circuits vs hypothetical ring final circuits, i can only comment on 'real life' ring final circuits as they are installed today.

How much additional testing, on top of all the excessive testing already required for a ring final circuit would it take to ensure your hypothetical device is functioning properly?

Also you would know as much about your freezer not working as you would with any other protective device setup.

I do infact install ring final circuits for customers where demand required them, they take alot of time to plan and consideration to install. I very rarely do a PIR where a ring final circuit has not been unsucessfully altered, they are abused and not understood, opened, bridged and allsorts. They can add hours of wasted time onto testing procedures too

Rng final circuits are here, we have to accept that but they are a bloody nightmare. Stroll on the day when the regs forbid installing them. The only time they are required in a domestic setting is for the kitchen, and 2 20A radials would serve all the sockets in the average kitchen. On a side note there are installations in Spain running off of 40A supplies, some even off of 25A suppiles. I think the ring final circuit was first devised to be able to provide one socket circuit in a dwelling so save runnign additional cable for additional circuits some time around the 2nd world war when materials were in short supply, I should imagine someone with in depth knowledge of regs past and present could verfy or deny that.

John

 

[flameport]

My idea was to have a single RCBO module with a 4-sensor CB (now 5 to limit supply to 32A). So 2 ring finals => 2 ways on the CU.

While many of these far fetched ideas could be made to work, and the various items of equipment made (or in some cases already exist), they will never be used in any domestic property, mainly due to cost. No one will pay the significant extra to provide an apparent zero benefit. People generally do not care how their electrical system works or is designed - they just want to plug things in and have those things work.

There is also the issue of complexity, as increasing the complexity of any system dramatically increases the ways in which it can fail, be misused or generally ruined by persons who don't know how to use/maintain it properly.

20A radial circuits in general will not be overloaded by people with hoards of electric heaters, racks of electric kettles or anything else, since people don't use these appliances in that way.
Radials don't have to be 20A either.

Worthwhile to remember that for several decades when ring circuits were installed, it was common to have ONE 30A circuit for all of the sockets in the house. Did fuses blow every week? Of course they did not.
Does a house that has 1x 30A ring circuit suddenly use twice as much electricity when rewired with 2x 32A rings, or 3x 20A radials? Clearly not.

I don't install rings any more, since they are essentially a useless design and are likely to be damaged in the future when people make additions or alterations.

 

[ajrobb]

I dont know how to do the multiple quote thing, but will continue this debate as it is quite entertaining for me.

I accept that present ring final circuits are not a good idea. They are not as versatile now as they originally seemed 60 years ago, because the original idea was flawed. It is fair to say that radial circuits use tried and tested designs. They have evolved, but would still be understood by an electrician from 100 years ago. Their simplicity is a strong point in their favour. However, they are still open to abuse and faults.

My hypothetical 4-pole RCBO module is an intellectual exercise. I have found this thread very informative. Old ideas need challenging, even if they are right. No-one yet has said that constant testing is a bad idea. This is easier if you have both ends of a circuit available, as in a ring final circuit. So many other devices now include built-in diagnostics, why not circuit protection?

Your querie on the 32A OCPD and the capability of 2.5 t&e to carry 27A has this logic behind it - you can only have one spur from a socket on a ring final circuit, thus limiting the maximum current drawn to 26A (or should that really be 20A, although thats another matter)

If you run 2.5mm² T&E in individual conduits, its rating should go up by about the square root of 2 and I have seen mention of 27A. I have never seen a 32A 4mm² radial but I am guessing this is how they are wired, otherwise they would be rated at about 25A for two cables in a single (hypothetical) conduit in plaster.

I can see no way of a single module circuit breaker having the spave available to terminate 6 seperate conductors in, or infact have the physical capacity to house all of the electronics required to make your hypothetical device function as you intend.

Given that a 32A RCBO has 2 large terminals plus neutral and earth fly leads, I see no problem replacing each of the large terminals with 2 small terminals able to take single 2.5mm² conductors. I had not considered connecting the cable's protective conductors to the module. I wouldn't like to say how small circuit breakers and control circuitry can be made.

You can have a 32A radial circuit, they are not only 20A/16A/15A.

I fail to see how a radial off of a radial circuit suddenly becomes a nightmare, or infact a christmas tree either?

It has been stated here that a 20A radial circuit can supply any number of sockets to 50m². It has also been suggested that radials are easy to modify. I was taking these to their logical conclusion that you can add any number of radials off radials off radials... That is a tree structure.

I cannot comment about real life occurences with radial circuits vs hypothetical ring final circuits, i can only comment on 'real life' ring final circuits as they are installed today.

How much additional testing, on top of all the excessive testing already required for a ring final circuit would it take to ensure your hypothetical device is functioning properly?

The constant testing would include some form of impedance measurement of the line and neutral conductors. Hopefully, this would reveal loose connections early, before they become dangerous. I envisage a traffic light system, so consumers who spot the amber warning can get a sparks in good time. I don't suppose there are many electricians who would mind the extra business of tightening loose terminals.

Also you would know as much about your freezer not working as you would with any other protective device setup.

'Cos with a single ring final circuit the telly goes off with the freezer.

With a sensible multiple circuit installation, a nail through the freezer circuit cable might go unnoticed.

Rng final circuits are here, we have to accept that but they are a bloody nightmare. Stroll on the day when the regs forbid installing them.

I agree. The only thing they presently seem to have in their favour is two circuit protection conductors back to the CU.

 

[dingbat]

Ring final circuits are here, we have to accept that but they are a bloody nightmare.

They've been here since 1947.

Stroll on the day when the regs forbid installing them.

There was a fairly widespread hope that the 17th Edition would have ceased to recognise them. Oh well...

The only time they are required in a domestic setting...

They were only ever intended for new-build council houses and their entire rationale was based on domestic, domestic, domestic...

I think the ring final circuit was first devised to be able to provide one socket circuit in a dwelling so save runnign additional cable for additional circuits some time around the 2nd world war when materials were in short supply...

Yes, 1947. But in fact they use more cable than a radial circuit with multiple sockets. Trouble is, prior to WWII it was common to run one unfused 15A socket per 15A fused radial circuit and compared to this wasteful circuit architecture, the ring looked like the answer. It never was the answer, but nobody ever thought to check.

 

[ricicle]

I can't believe how long this thread is and how tedious it has become on such a simple subject.......

 

[securespark]

There was a fairly widespread hope that the 17th Edition would have ceased to recognise them. Oh well.

Yeah, that and the other dinosaur of electrical installation the 3036 fuse...

 

[ajrobb]

I can't believe how long this thread is and how tedious it has become on such a simple subject.......

In summary:
Is there a better way to protect a ring final circuit?
No, they're a flawed concept. You're much better off using multiple radial circuits.