Individual switches at sockets / fuses in plugs and electrical safety

eMeS · 8 Oct 2019

Mod - split from this thread.

najaB said:
Individual switches on electrical sockets and fuses in plugs.

I think that evolved from a shortage of copper in the UK after WWII, and we moved from a radial wiring system to our now familiar ring main system.

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Bletchleyite · 8 Oct 2019

eMeS said:
I think that evolved from a shortage of copper in the UK after WWII, and we moved from a radial wiring system to our now familiar ring main system.

It did indeed. It's also got a major (dangerous) flaw in that if the ring is broken (e.g. due to a cack handed electrician or DIY job) you can have an excess load on half of it which is not protected by the 30/32A fuse/breaker, and the problem isn't visible until things start overheating. 20A radials are much more sensible and I think new houses are moving that way.

Edit: FWIW, I suspect the number of fires caused by this is quite low because almost all household appliances these days are very low-power, in particular the use of electric heaters is very low because people tend to have gas central heating. I suspect the draw on an average socket circuit is well below 5A even with most things on. Similarly LED lighting (though that tends to be radials anyway) reduces the draw substantially - the current draw on my lighting circuit (6A breaker) is something like 0.6A with everything switched on. Only really the kettle, vacuum cleaner, toaster and hairdryer really tax it, and who has more than two of those on at once?

eMeS · 8 Oct 2019

Bletchleyite said:
It did indeed. It's also got a major (dangerous) flaw in that if the ring is broken (e.g. due to a cack handed electrician or DIY job) you can have an excess load on half of it which is not protected by the 30/32A fuse/breaker, and the problem isn't visible until things start overheating. 20A radials are much more sensible and I think new houses are moving that way.

After we'd had our house rewired, and got used to the new concepts etc., I joined the RAF and was sent to Germany in my second year of National Service. Our laboratory there was wired radially with each spur fused at a central box.

kermit · 8 Oct 2019

Bletchleyite said:
It did indeed. It's also got a major (dangerous) flaw in that if the ring is broken (e.g. due to a cack handed electrician or DIY job) you can have an excess load on half of it which is not protected by the 30/32A fuse/breaker, and the problem isn't visible until things start overheating. 20A radials are much more sensible and I think new houses are moving that way.

Edit: FWIW, I suspect the number of fires caused by this is quite low because almost all household appliances these days are very low-power, in particular the use of electric heaters is very low because people tend to have gas central heating. I suspect the draw on an average socket circuit is well below 5A even with most things on. Similarly LED lighting (though that tends to be radials anyway) reduces the draw substantially - the current draw on my lighting circuit (6A breaker) is something like 0.6A with everything switched on. Only really the kettle, vacuum cleaner, toaster and hairdryer really tax it, and who has more than two of those on at once?

Washing machine and tumble drier? Cookers and immersion heaters get their own circuits, don't they?

Bletchleyite · 8 Oct 2019

kermit said:
Washing machine and tumble drier?

Missed those

In my case it's one unit so I couldn't have both on at once

Cookers and immersion heaters get their own circuits, don't they?

Yes (they have to per the regs).

kermit · 8 Oct 2019

....although Baby Belling cookers could draw a fair bit of juice, via a standard 13 amp plug (paired via an adapter with a 3 bar electric fire in a classic damp bedsit death trap!)

HSTEd · 19 Oct 2019

Bletchleyite said:
It did indeed. It's also got a major (dangerous) flaw in that if the ring is broken (e.g. due to a cack handed electrician or DIY job) you can have an excess load on half of it which is not protected by the 30/32A fuse/breaker, and the problem isn't visible until things start overheating. 20A radials are much more sensible and I think new houses are moving that way.

20A radials can cause you issues with large appliances however.
32A means you basically never have to worry about overloading a circuit, since even 2 full 13A appliances won't do it.

Bletchleyite said:
Edit: FWIW, I suspect the number of fires caused by this is quite low because almost all household appliances these days are very low-power, in particular the use of electric heaters is very low because people tend to have gas central heating. I suspect the draw on an average socket circuit is well below 5A even with most things on. Similarly LED lighting (though that tends to be radials anyway) reduces the draw substantially - the current draw on my lighting circuit (6A breaker) is something like 0.6A with everything switched on. Only really the kettle, vacuum cleaner, toaster and hairdryer really tax it, and who has more than two of those on at once?

Toaster and a kettle being on at once is hardly unusual.......

Belperpete · 19 Oct 2019

HSTEd said:
20A radials can cause you issues with large appliances however.
32A means you basically never have to worry about overloading a circuit, since even 2 full 13A appliances won't do it.
Toaster and a kettle being on at once is hardly unusual.......

The difference being that with a radial, the CB will trip when it is overloaded, whereas with a broken ring the cable can be seriously overloaded without the CB tripping. Toasters and kettles both tend to be plugged into much the same part of the ring (in the kitchen) and are therefore both likely to be on the same side of any break.

In my house, the ring has its own dedicated RCCB. It surely shouldn't be that difficult to design an RCCB that incorporates broken-ring detection. That such things aren't available is I suspect due to there being such a small potential market for them, the UK and Eire being the only European countries I am aware of to use rings.

HSTEd · 20 Oct 2019

Belperpete said:
The difference being that with a radial, the CB will trip when it is overloaded, whereas with a broken ring the cable can be seriously overloaded without the CB tripping. Toasters and kettles both tend to be plugged into much the same part of the ring (in the kitchen) and are therefore both likely to be on the same side of any break.

Well modern practice is often to provide a seperate ring main, solely for use in the Kitchen.

Belperpete said:
T
In my house, the ring has its own dedicated RCCB. It surely shouldn't be that difficult to design an RCCB that incorporates broken-ring detection. That such things aren't available is I suspect due to there being such a small potential market for them, the UK and Eire being the only European countries I am aware of to use rings.

I'm not really sure how you can detect a broken ring easily, it is likely to require something more sophisticated than the electrics present in a normal RCD or RCBO.
It'l be substantially more expensive.

Belperpete · 21 Oct 2019

HSTEd said:
I'm not really sure how you can detect a broken ring easily, it is likely to require something more sophisticated than the electrics present in a normal RCD or RCBO. It'l be substantially more expensive.

A substantial imbalance in the current being fed into the two sides of the ring. Same principle as detecting an imbalance between current in live and neutral legs of a circuit. Agreed it would be more expensive to provide the extra functionality, but the greatest expense would be down to the fact that because they would only be used in the UK, there would be so few sold to recoup the initial investment.

dgl · 21 Oct 2019

HSTEd said:
20A radials can cause you issues with large appliances however.
32A means you basically never have to worry about overloading a circuit, since even 2 full 13A appliances won't do it.

Toaster and a kettle being on at once is hardly unusual.......

The kettle + toaster problem comes to it's head when staying in a caravan. Where I work most caravans are on a 16A supply and so if you have a few devices on, switch on the kettle followed by the toaster it can trip. Naturally they don't realise it's the over current that has tripped it and think it must be the toaster as that was the last device switched on. Hence we get a lot of perfectly Good toasters being replaced and have gone from chucking them to giving them a once over and putting them back into service.

Bletchleyite · 21 Oct 2019

HSTEd said:
I'm not really sure how you can detect a broken ring easily, it is likely to require something more sophisticated than the electrics present in a normal RCD or RCBO.
It'l be substantially more expensive.

The easiest way is to lop the ring in half and wire each half to a separate breaker, making each a 15 or 20A radial. Though that only works if the ring isn't too "long".

HSTEd · 21 Oct 2019

Bletchleyite said:
The easiest way is to lop the ring in half and wire each half to a separate breaker, making each a 15 or 20A radial. Though that only works if the ring isn't too "long".

Some early rings were made out pairs of 15A radials, and were fed through pairs of 15A fuses, one to each leg.
You could make a double housing housing two MCBs with the trip bars connected, which would still allow a single point of isolation, and if either leg went over 20A it would trip both.

Cost of development is unlikely to be an issue as ring mains are installed by the hundreds of thousands thousand every year.

I believe the real alternative to a ring main in terms of capability is a 32A radial, but that needs a lot more copper

najaB · 21 Oct 2019

Bletchleyite said:
The easiest way is to lop the ring in half and wire each half to a separate breaker, making each a 15 or 20A radial. Though that only works if the ring isn't too "long".

You don't actually need to physically break the ring, it can be done quite easily using cheap digital signal processing using the same method as track circuits in electrified areas. Inject a RF signal into each half of the ring and trip the breaker if you can't see the signal coming back on the other side of the ring.

cactustwirly · 24 Oct 2019

Bletchleyite said:
It did indeed. It's also got a major (dangerous) flaw in that if the ring is broken (e.g. due to a cack handed electrician or DIY job) you can have an excess load on half of it which is not protected by the 30/32A fuse/breaker, and the problem isn't visible until things start overheating. 20A radials are much more sensible and I think new houses are moving that way.

Edit: FWIW, I suspect the number of fires caused by this is quite low because almost all household appliances these days are very low-power, in particular the use of electric heaters is very low because people tend to have gas central heating. I suspect the draw on an average socket circuit is well below 5A even with most things on. Similarly LED lighting (though that tends to be radials anyway) reduces the draw substantially - the current draw on my lighting circuit (6A breaker) is something like 0.6A with everything switched on. Only really the kettle, vacuum cleaner, toaster and hairdryer really tax it, and who has more than two of those on at once?

Do you not have an electric oven or hob? These draw quite a lot of electricity.
One reason why Mobile homes use a lot of gas appliances, is that some pitches don't supply enough electricity and the fuses trip.

Bletchleyite · 24 Oct 2019

cactustwirly said:
Do you not have an electric oven or hob? These draw quite a lot of electricity.

Ovens and hobs have a dedicated radial circuit and are not generally connected to the main socket ring.

cactustwirly · 24 Oct 2019

Bletchleyite said:
Edit: FWIW, I suspect the number of fires caused by this is quite low because almost all household appliances these days are very low-power, in particular the use of electric heaters is very low because people tend to have gas central heating.

Most fires these days start in the lounge or bedrooms (from objects like discarded cigarrete butts), or come from shallow frying etc.

AM9 · 24 Oct 2019

Belperpete said:
A substantial imbalance in the current being fed into the two sides of the ring. Same principle as detecting an imbalance between current in live and neutral legs of a circuit. Agreed it would be more expensive to provide the extra functionality, but the greatest expense would be down to the fact that because they would only be used in the UK, there would be so few sold to recoup the initial investment.

Plus the fact that relatively few rings out of the total installed are either broken in their lifetime or installed without testing. Yes I know that there are cowboys out there but the type of faults that they 'install' tend to be more like poor insulation within terminating accessories, e.g. lack of sleeving, and poor wire dressing. With 30/32A ring circuits having been around for over 70 years, almost every one of them has had at least some attention from a competent electrician evwen if they are still in use. The recommended life of a ring circuit was originally stated as 25 years, partly becuse early ring circuits were vulcanised rubbber insulated, (VRI). Those recommendations have not changed since PVC sheathed become the norm, but 2.5mm twin and earth cables are continuously rated for 32A when surface mounted, (or under floorboards etc.) and 26A when enclosed in conduit in a thermally insulating wall. The chances of continuous operation of 6kW on a ring circuit in a domestic environment are very rare, and unlikely to coincide with a poor wiring job. That is why domestic circuits diversity allowances whereas commercial/industrial ones don't. It would be impractical to prevent every cowboy malpractice, - as each new safeguard was introduced, they would devise new ways to subvert those 'inconveniences', - some of those new dodges could well be more dangerous than the original practices that the safeguards were designed to prevent.

Barn · 25 Oct 2019

Bletchleyite said:
Ovens and hobs have a dedicated radial circuit and are not generally connected to the main socket ring.

Usually true for floor-standing cookers (which often have two ovens and a hob), but many houses have built-in electric ovens which run off a 13amp plug.

https://markselectrical.co.uk/elect...single-built-in-electric-ovens&attributes=125

Bletchleyite · 25 Oct 2019

Barn said:
Usually true for floor-standing cookers (which often have two ovens and a hob), but many houses have built-in electric ovens which run off a 13amp plug.

https://markselectrical.co.uk/elect...single-built-in-electric-ovens&attributes=125

It's still usual for hobs to require the cooker radial (very few are under 13A, though they do exist and some can be configured to work that way), and so those ovens are normally plugged into the cooker radial in some form.

The most common scenario with separate oven and hob is for both to be wired into the cooker outlet together, with some others having the hob wired in and the oven plugged into a socket wired to it.

Individual switches at sockets / fuses in plugs and electrical safety

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