Question about rings

[sirocosm]

When I first moved to the UK, ring circuits seemed a bit unusual, we only use radials in Canada in house wiring. Although ring finals seem to have a lot of benefits, I don't think they would be legal in Canada with a 2.5mm cable because you cannot use a 32A breaker to protect a cable rated less than that. Although there are 2x2.5mm conductors, if the ring is broken or if a contact becomes high resistance, it is possible that 32A could flow down a single 2.5mm conductor and be overloaded.

Assuming there is space in the consumer unit, why don't they don't use duplex breakers (2x20) so that each end of the ring gets its own breaker? and the cable is fully protected even if the ring breaks?

 

[EFLImpudence]

When I first moved to the UK, ring circuits seemed a bit unusual, we only use radials in Canada in house wiring. Although ring finals seem to have a lot of benefits, I don't think they would be legal in Canada with a 2.5mm cable because you cannot use a 32A breaker to protect a cable rated less than that.

No, but the whole point is that there is effectively 5mm² of cable protected by the 32A breaker.

The 'ring' was devised after the war to double the capacity on a 30A BS3036 circuit merely by extending it back to the CU - with associated restrictions and limitations.
Since the introduction of MCBs their use has become virtually pointless.

Although there are 2x2.5mm conductors, if the ring is broken or if a contact becomes high resistance, it is possible that 32A could flow down a single 2.5mm conductor and be overloaded.

That is true.

Assuming there is space in the consumer unit, why don't they don't use duplex breakers (2x20) so that each end of the ring gets its own breaker? and the cable is fully protected even if the ring breaks?

No point - plenty of other ways round the problem.

 

[flameport]

why don't they don't use duplex breakers (2x20)

When ring final circuits were introduced in the late 1940s, there was no consideration for circuit breakers of any kind. They were intended for use with 30A fuses.

It's a very old design which provided a solution to a problem at the time.
Unfortunately there are plenty of people who still believe them to be a suitable thing to install 75 years later.

 

[sirocosm]

In their defense, ring finals do have some advantages. Load flexibility, redundant grounds, lower resistance, etc. And one of the main drawbacks/safety issues could eliminated by using double pole 20A breakers instead of a single 32A. Given that breakers are cheap, the question remains, why don't they do this?

 

[EFLImpudence]

In their defense, ring finals do have some advantages. Load flexibility, redundant grounds, lower resistance, etc.

Why, then are such things not considered advantageous for other circuits?

And one of the main drawbacks/safety issues could eliminated by using double pole 20A breakers instead of a single 32A.

But then it would not be a 'UK ring circuit'.

Given that breakers are cheap, the question remains, why don't they do this?

There is no point. A single 25A MCB or 32A cable would also solve your supposed 'problem'.

 

[flameport]

why don't they do this?

'they' would be those persons designing electrical installations.
No domestic consumer units will accept duplex circuit breakers, and contriving such a circuit would require far more effort than using radials.
It's adding complexity and cost for no reason.

 

[winston1]

In their defense, ring finals do have some advantages. Load flexibility, redundant grounds, lower resistance, etc. And one of the main drawbacks/safety issues could eliminated by using double pole 20A breakers instead of a single 32A. Given that breakers are cheap, the question remains, why don't they do this?

If you used 2 20A breakers you would not need a ring. Make it 2 radials.

 

[sirocosm]

No domestic consumer units will accept duplex circuit breakers,

Is that true? I don't have enough experience here to know. In Canada you can also use a pair of single breakers and a handle tie. If you look closely you will see the one in the picture has a handle tie, even though the body looks like two breakers molded together.

If you used 2 20A breakers you would not need a ring. Make it 2 radials.

You could make it 2 radials, but then you would lose the load flexibility, the redundant earth conductors, and reduced resistance on long runs.

So the question remains, why is this not done? All it would cost is a consumer unit slot and an extra breaker, surely that must be cheaper that 4mm radials that can handle similar load. So I don't think it is about cost or the availability of parts. There must be some other reason.

 

[RandomGrinch]

There must be some other reason.

Conservatism - if it ain't broke...

 

[sirocosm]

Conservatism - if it ain't broke..

Haha, it could be as simple as that, but I was kind of hoping for a legitimate technical/practical reason.

 

[flameport]

In Canada you can also

What's done in North America with electrical systems is best never mentioned.

load flexibility,

Meaning what?
There is no requirement for a 30A / 32A circuit for socket outlets in UK homes.
Although there is far more electrical equipment in use today than there ever has been, most electrical equipment for home use uses less than 100 watts.
That which isn't is generally restricted to a single room and most of that is used for short periods.

The original intent of ring finals was to have one for the whole house.
That hasn't been a sensible design choice for decades, and today is completely unacceptable due to the use of RCDs.
Today you have people installing multiple rings in a single home, upstairs, downstairs, kitchen, utility and even in a garage with a couple of outlets. It's a nonsensical waste of cable. 20A or even 16A radials would serve the same purpose.

reduced resistance on long runs.

Resolved by larger circuit cables in the tiny number of installations where that could be a problem.

redundant earth conductors

Not required.
The road to redundancy never ends. What if 2 were to be damaged? Why not 3, 4, 50 or 100?

 

[ericmark]

I agree two 16 amp MCB's linked would seem to remove the over load one leg problem. But not sure if you can do that with RCBO but main advantage in using a ring final is it reduces volt drop, to replace the 106 meters of 2.5 mm² with radials you need to split it into 3, assuming at limit, so at £30 for a RCBO using a ring final saves £60.

Two 13 amp devices on a 16 amp supply will likely trip it, but on a 32 amp supply it would not.

 

[bernardgreen]

The road to redundancy never ends. What if 2 were to be damaged? Why not 3, 4, 50 or 100?

Some sources on High integrity earthing consider the risk of 2 faults occurring at the same time on the same Earth connection as being small enough to not be a problem,

"high integrity earthing" should not to be confused with clean earths for sensitive equipment

 

[sirocosm]

I agree two 16 amp MCB's linked would seem to remove the over load one leg problem.

With 2.5mm you could use a pair of 20A breakers (RCBOs), so the ring might actually have more capacity than with a single 32A. Although there is a balance issue there.

and today is completely unacceptable due to the use of RCDs.

I am not sure what RCDs has to do with this, but on the load flexibility, if the boiler quits you can plug a pair of electric heaters anywhere on a ring, not so if you need them both to be on the same 20A radial. Maybe this multiple electric heater use case is not so common in a house these days, but yet they still put in ring finals. So it is back to the original question, if you are going to use ring finals, why not connect them to a pair of tied 20A RCBOs rather than a single 32A.

 

[EFLImpudence]

With 2.5mm you could use a pair of 20A breakers (RCBOs), so the ring might actually have more capacity than with a single 32A. Although there is a balance issue there.

You can do the equivalent of that with any circuit - but then it would not be a "UK ring circuit" in the meaning of the term.

You seem to be saying that the "UK ring circuit" would be better if it were not actually a "UK ring circuit" - I would agree.