must I use a single MCB to protect a ring?

[ajrobb]

Unfortunately, that leaves you with a single double gang socket on each 20A circuit

Don't be ridiculous.

OK - apparently you can have as many double gang sockets as you like on a 20A radial over 50m² but the more you have, the more likely the MCB is to trip.

The ring main may not be perfect, but it is far more convenient to a bozo than multiple radials.

Don't be ridiculous.

As a kid I lived in a house with rubber insulated radials and blown fuses were a regular occurrence, especially when my sister was drying her hair with a hair dryer sitting in front of a fan heater and someone put the kettle on - her favoured socket in her bedroom was on the kitchen radial. After we had the ring finals, we never had a blown fuse again. Maybe the fuse protection was inadequate and we were living in a fools' paradise/death trap. However, the Pyrotenax cable wasn't going to melt.

When my sister moved to Sweden, it seemed like going back 10 years when she had to learn what she could plug in where not to blow fuses.

This has left me with the impression that a 30A semi-enclosed fuse 'protecting' a ring final is unlikely to blow, regardless of where you plug in your fan heaters etc. (A 30A MCB will trip more often than a 30A semi-enclosed fuse would blow.)

A couple of fan heaters, a hair dryer, iron and plasma TV at least.

What - all at once? What kind of house has all those going on in the same room?

A small house with a single living room, like the one I'm refurbishing to rent. A single fan heater won't heat the living room in cold weather. Fan heaters are generally supplied under a central heating service contract pending repairs.

With my hypothetical quad-pole RCBO

What's hypothetical about it?

The idea that two poles protect the live (line) conductors and the other two poles protect the neutral conductors, all linked to isolate the whole circuit so a fault occurs if ANY conductor carries over 20A. The RCD component protects the whole ring final rather than individual legs. The total ring capacity would be 40A (close to the old 30A semi-enclosed fuse blowing current). However, the ring final is protected from local heavy loads near the CU.

http://www.gepowercontrols.com/10086/pdf/residential.pdf

http://uk.farnell.com/merlin-gerin/c60hb420/mcb-20a-4pole-type-b/dp/1421093[/QUOTE]
Thanks, but not what I had in mind.

 

[dingbat]

This has left me with the impression that a 30A semi-enclosed fuse 'protecting' a ring final is unlikely to blow, regardless of where you plug in your fan heaters etc. (A 30A CB will trip more often than a 30A semi-enclosed fuse would blow.)

Ah. There's another of the things that have always been wrong with the British ring final circuit. 3036 semi-enclosed fuses have a high fusing factor and a 30A fuse will happily allow 40A+ to flow all day long. The reason old rings on 30A rewireable fuses do/did not trip very frequently is simply that they were not properly protecting the circuits from overload.

Many see this as a benefit of such an arrangement, little realising that in some installations, sections of cable are/have been regularly overloaded, degrading the cable's life. This may become a fire risk, but it's okay, because all the sockets still work:

http://www.xtranormal.com/watch/6907375/

 

[1john]

Another point to make here is that since the war we have had great changes in the way we live, we may use more electrical equipment but it is lower powered, we have central heating systems which also provide hot water too which scrap the need for 3 bar fires everywhere and an immersion heater is a thing used in an emergency in most houses. As dingbat said, a 30A fuse can run at 40+ amps for quite some time, potentially damaging a cable.

The worst thing about ring final circuits in my mind is how kitchen fitters used to (and probably still do) abuse them, I can picture it now, they seperate the two legs at the socket and are amazed that there is electric at both ends, now these two ends dissapear in oposite directions never to meet again, and no one knows any better until (if at all) some testing takes place, as every outlets works fine!

 

[ajrobb]

http://www.xtranormal.com/watch/6907375/

Very droll.

I still think my hypothetical quad-pole circuit protection would be useful, if only for replacing 30A semi-enclosed fuses on existing ring final circuits without reducing the perceived capacity of the protection like a 30A CB does. Whether it would make installers lazy is another matter.

The worst thing about ring final circuits in my mind is how kitchen fitters used to (and probably still do) abuse them, I can picture it now, they seperate the two legs at the socket and are amazed that there is electric at both ends, now these two ends dissapear in oposite directions never to meet again, and no one knows any better until (if at all) some testing takes place, as every outlets works fine!

By protecting the ring final as 2 linked radials, cables can't be overloaded if the ring breaks. I haven't mentioned for a while that the protection could include continuity checks. It could even warn of high resistance, something not easy with a radial. Heck, I can almost sense the ring final circuit being rehabilitated.

 

[ajrobb]

Another point to make here is that since the war we have had great changes in the way we live, we may use more electrical equipment but it is lower powered, we have central heating systems which also provide hot water too which scrap the need for 3 bar fires everywhere and an immersion heater is a thing used in an emergency in most houses.

My argument is that domestic circuits should be designed for emergencies. If the central heating breaks down, electric heaters are the usual replacement. Nobody will thank an electrician if they can't run those heaters. That is why I'd be happier with a ring final circuit capable of delivering 40A safely.

 

[dingbat]

http://www.xtranormal.com/watch/6907375/

Very droll.

Thank you.

I still think my hypothetical quad-pole circuit protection would be useful, if only for replacing 30A semi-enclosed fuses on existing ring final circuits without reducing the perceived capacity of the protection like a 30A CB does.

No. Because apart from everything else, a very large number of ring circuits are already in a poor state, having been altered, extended, broken, cross-linked to other circuits, overloaded, etc, etc, etc. And how could you easily replace a single module fuse, with a four module device?

Too many people think that a consumer unit change is as good as a rewire, on the assumption that, as the house is still standing, there are no problems with the wiring itself.

As John said:

The worst thing about ring final circuits in my mind is how kitchen fitters [and DIYers and builders and plumbers and some 'electricians']used to (and probably still do) abuse them...

There are thousands of potentially unsafe electrical installations throughout the country and in part this is because users and modifiers of circuits do not understand even the very basics, but somehow imagine that if the toaster works, all must be fine.

Rather than dreaming up ways of making a discredited circuit design even more complicated, we should be training domestic electricians to install simple, proven circuits and thoroughly test them, whilst dissuading the unknowingly incompetent from even attempting electrical work.

Electricians should also stop trying to appease tight-wad customers by going to all lengths to suggest alternatives to rewires. There comes a point when an installation simply must be replaced, but so long as cosmetic considerations are more important than safety, incompetent installers will continue to extend the life of dangerous installations.

I'll say it again - no apologies - the majority of fully-qualified electricians are insufficiently knowledgeable with regard to ring circuits to be considered competent to install, test, modify and maintain them.

KISS: Keep It Simple, Stupid.

 

[dingbat]

My argument is that domestic circuits should be designed for emergencies. If the central heating breaks down, electric heaters are the usual replacement. Nobody will thank an electrician if they can't run those heaters. That is why I'd be happier with a ring final circuit capable of delivering 40A safely.

Dream on.

Far better to properly insulate a home so that, in the event of a loss of heating, very little energy is needed to keep warm (in our remarkably mild climate!).

But properly designed and installed, simple radial circuits will do this very well indeed, without the need for a level of complexity which is beyond the comprehension of installers and users.

Trust me, you're wrong on this.

 

[ajrobb]

I'll say it again - no apologies - the majority of fully-qualified electricians are insufficiently knowledgeable with regard to ring circuits to be considered competent to install, test, modify and maintain them.

We agree that ring finals can be badly designed and maintained. However, radials can also be abused:

• it is possible to link the live (line) from one circuit to the neutral of another
  a terminal can come loose and cause local heating

I'll continue this pipe dream and list the features of my hypothetical ring main module:
1. all 4 conductors are protected from currents exceeding 20A
2. over current on any conductor will isolate the circuit
3. RCD between line ring and neutral ring (no crossed neutrals)
4. single module with 4 connections - one wire in each
5. automatic circuit test (detecting loose terminals etc)
6. increased capacity compared to 30A CB
7. simplified circuit design (fewer restrictions)

This addresses not only the particular weaknesses of the ring final but also some of the weaknesses of the radial.

 

[ban-all-sheds]

1. all 4 conductors are protected from currents exceeding 20A
2. over current on any conductor will isolate the circuit

So what do you think those 4-pole devices I linked to do?

 

[dingbat]

I'll continue this pipe dream...

...and that is what it will remain.

 

[holmslaw]

..

 

[ajrobb]

1. all 4 conductors are protected from currents exceeding 20A
2. over current on any conductor will isolate the circuit

So what do you think those 4-pole devices I linked to do?

One of them was 3 line + 1 neutral and they didn't seem linked together...

 

[ajrobb]

I'll continue this pipe dream...

...and that is what it will remain.

In the meantime, in the real world, I'll take on board the advice from you and others. I started off worried about ring finals and this has been confirmed. I'll be happier with radials in future. I shall consult/use electricians but not believe everything they say - especially if they tell me sockets must be on "ring mains". Love this site .

It seems the main choice of radial circuits are 2.5mm² at 20A or 4mm² at 32A. This does not seem to fit with adiabatic calculations unless they are installed differently. I can only assume that 4mm² cables are run singly rather than as series pairs in conduit.

 

[ban-all-sheds]

The 4 pole rcd would'nt work with 2 live and 2 neutrals, its designed for TP&N.

Yes - RCD/RCBO would be pants, as it would always be seeing an imbalance.

But an MCB?

How would it know?

What would it do?

What would it not do?

OK - it probably wouldn't monitor the current on one of the neutral legs, but other than that it has 4 connections going in and 4 out, and would trip, isolating all 4, if any of the 3 it was monitoring were too high, wouldn't it?

 

[Spark123]

I don't see why a 4 pole RCD would be an issue with 2 lines and 2 neutrals connected to it, as long as the total current flowing in the lines equals the total current in the neutrals, as they are all in the same ballancing transformer it will hold.
2 separate RCDs wouldn't work though.

I still don't like the idea of having 40A ocpd with a 1.5mm CPC, which any spur would be.