Euro Electrics

In fact even a 32A MCB will supply 40A for an hour or two before tripping. The wiring will also take it, so again no problem.

Yup, just got rid of the ELCB on my TT system - never had it trip once in over a decade (and it did self test ok!).

Reply to
John Rumm
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Well, given what you have described, it won't. TD+WM+K adds up to 6kW or so. Most irons are in the 500W to 1kW range and are thermostatically controlled. At best you have described 7kW of load for maybe as much as three or four minutes. 500W "in hand" to the nominal spec., and a lot more to the real limit.

[...]

With a broken connection I'd agree, but I have seen many cases of "loose" connections on radials which don't exhibit any obvious symptoms unless you happen to dismantle the problem junction whereupon you spot the blackened copper and bubbling PVC. Classic example recently was a

45A shower switch to an 8.5kW shower. As far as the people were concerned, the shower had worked just fine with no problems at all until a guest decided to use the 45A switch to switch off after having a shower (the householders usually left it on). The switch jammed in the off position (though indicating on) and they called me in. I found two or three loose screws in the switch and obvious signs of sustained overheating.

Hwyl!

M.

Reply to
Martin Angove

OK, so you don't agree. However I don't see why you feel a radial is better suited to diverse power supply for flexible loads however.

I would say it is about better engineering than about cost reduction...

Depends on what you mean by hidden. Given the same fault on both a radial and a ring (i.e. loose screw on a socket connection), the radial will melt a cable or overheat the socket, the ring will carry on working even if the socket will get warm to the touch. How is it better to have a melted cable?

This seems to be a very weak argument: "you need to wire it according to the design for it to work correctly".

Probably nearer 30A in fact...

30% overload for an hour will do it no harm. This is by design. This is another reason why diversity works well. However we are not talking about one cable, but two (unless there is a fault)

In many real world cases a radial will actually fair less well with a wiring fault. It is not safe to assume that all sockets downstream of the fault will simply stop working.

No, but it could be that you are exceeding its temperature budget and hence reducing the expected life.

Reply to
John Rumm

When they rate a cable for 27A, they are also saying that it can really take twice as much, but they've left a whole load of headroom to allow for short term (i.e. 10s of minutes) of transient loads (such as toasters/kettles/washing machines etc).

There's a reason that MCBs are only guaranteed to trip quickly at 5 times nominal current. It is because the cables are man enough to take 5 times current for a short period and there's no need to design a system that cuts out just because with a heavily loaded system you've put the kettle on for 2 minutes, or a motor is winding up.

Christian.

Reply to
Christian McArdle

Its funny to think one of the oft quoted objections to ELCBs was nuisance trips. Sure, they might once in a decade or two. The atrocious record of RCDs has left those objections looking odd.

NT

Reply to
bigcat
[diversity/ring circuits]

Without diversity modern living would be impossible. In the one-radial-per-room scenario we would have at least eight radials in this small house. Even if they were all 16A that's a non-diversified

8*16A you have to cater for which is 128A which is well over the capacity of even the largest normally available service fuse (100A). Without diversity you'd have to assess a cooker at its maximum rating in the same way as a shower. One 8kW cooker and one 8kW shower makes 66A while many houses are still on 60A service fuses.

Now I know that this isn't really your argument. You are arguing that a ring circuit is unsafe because in the event of a break it is 2.5mm2 cable fused at 30/32A and the cable is only rated for 27A at best. I don't know the figures, but Peter Parry has over the years proved to have done rigorous research (look for his postings when Part P was originally proposed) and I am much more prepared to believe him when he says that the UK system (and by implication the ring final) is amongst the safest in Europe than I am to believe you who states with no backround evidence that a many-radials method would be safer.

It has probably never saved very much money. The tortures of installing a ring in certain shapes of houses makes you wonder why people didn't use radials more often. In the terraces around here they often did. Where can you buy 20A fuse wire these days? Actually, on that subject, that's an area where a 3036-protected ring may actually be safer than a radial: fuse wire of 20A is not commonly available any more, so if a 20A fuse blows it may be replaced with 30A wire. Wire greater than 30A is likewise not commonly available so a blown 30A ring fuse is only likely to be replaced with 30A wire. (Let's ignore the cases where fusewire isn't used at all and blown fuses are replaced with bits of copper stripped from T&E)

To be at serious risk of damaging a correctly functioning ring you have to have a series of improbable coincidences. Either that or the central heating has broken down, it's the middle of the winter, and you've managed to find the one homeowner in the town who has more than three or four portable electric heaters.

Sure, a ring final *is* a compromise (of sorts), but it is a well-engineered, well-calculated, conservatively-rated and provably safe compromise.

Hwyl!

M.

Reply to
Martin Angove

You aren't looking at the long term use. These systems are going to be in use for decades and faults are going to happen.

With a ring you will get isolated legs which look just like a radial circuit and will suffer from the same problems as a proper radial circuit.

However a proper radial circuit has been designed that way and will typically have a ~20A fuse in the circuit and a known distribution of connection points.

The faulty ring will have a 32A fuse and a random distribution of connection points..

Now which is safer?

If it were possible for the user to know there was a fault in the ring then I would say a ring was safer.. but with rings the way they are the user has no hope of knowing there is a fault.

This makes them unsafe for long term use. Stick a 20A fuse in and they are safer than radials.

Reply to
dennis

You don't seem to understand the cause of most electrical fires. They don't get started (in domestic settings) by fixed cables being overloaded. They get caused by poor connections.

The ideal failure case is in fact the cable overheating, melting the insulation and shorting out to blow the fuse/circuit breaker. The fire risk in such an unlikely scenario is very low.

Where risk exists it is in poor contacts which generate heat sufficient to ignite local materials. You don't need much energy to do this (typically 100W will be more than adequate)and in many fires of electrical origin the fuse or MCB involved never trips or does so far to late to be of use.

Electrical fires are common in the USA. These are caused by a combination of extensive use of flammable construction materials, high currents, poor standards of installation and, most significantly, aluminium cable. Because of poor connections caused by aluminium corroding so quickly aluminium cable carries a fire risk approximately 100 times greater than equivalent rated copper cable.

This is why any domestic radial system is intrinsically far less safe than a ring. Firstly it involves more connections, and connections are the primary fixed wiring risk, secondly adding sockets to a radial installation is costly and time consuming, continental practice is to have far fewer sockets than are installed in UK houses. As a result the use of extension cords and multiadapters is far more common and these pose a far greater threat than fixed wiring of any type ever will.

Reply to
Peter Parry

Are they? If so what would you say is the most likely failure mode:

Complete break in a conductor, or, a terminal screw working loose leading to a higher resistance connection?

I would suggest the latter is the more likely. If that is the case then the ring circuit handles the fault better.

(not that relevant to the discussion, but a radial will often have a 32A breaker and 4mm^2 cable...)

There are some fault conditions where a radial will fail outright and a ring will "limp on", so you can argue the outright failure of the radial is more desirable, but in many cases you won't get clear black and white failures like this. So on the balance of probabilities, I would still say the ring.

Same applies to a radial unless the fault is completely open circuit conductor.

With no faults on the circuit, then the ring has clear advantages.

Reply to
John Rumm

Modern radials are multiple connection affairs not the old ones with round fuses in the plugs. 8-)

I think you will find you have to use a MCB in a modern radial circuit (probably for the reason you state).

Why is it so difficult to explain that designing a system which can (apparantly) function normally with a hidden fault present is bad engineering. It is worse if that fault drops the margins of that circuit below what is safe. This is the case for a ring main. It cannot be considered to be good practice these days when there are alternatives (like fitting a 20A MCB to the ring).

Reply to
dennis

Assuming the ring and the radial are both working correctly what are the advantages of the ring?

Reply to
dennis

I think you will find that it requires less connections on a radial than on the equivilent ring. There is one less length of cable so less connections.

This is also untrue, you only need to run a single T&E for the extra socket not two like on a ring.

I will take your word for that. However Part P will soon up the number of trailing extensions to the continetal level as they are exempt. I expect we will see somone electrocuted by a 4 way in the sink soon, especially if someone has bonded the taps/sink to earth.

Reply to
dennis

You can put lots and lots of outlets on it without worrying about where particular loads are going to go. This means you remove the need for extension leads which are a far greater hazard than the fixed wiring. They are also often much easier to add to. A spare bedroom with perhaps 4 outlets can be converted to a home office with

10-20 outlets quite easily and cheaply.
Reply to
Peter Parry

The usual addition, nearby existing sockets, involves very little extra wire, and none of it draped up conduit.

Indeed, something that was pointed out at the time. Part P, on its own calculations, will kill more than it saves.

Reply to
Peter Parry

What Peter said, and lower earth fault loop impedance (hence quicker disconnect times), lower voltage drop, less cable heating (hence power dissipation). Also the ability to cover larger areas (100m^2) when required. Easier to install (i.e. getting 2 x 2.5mm^2 cable into a socket is much easier that 2 x 4mm^2)

Reply to
John Rumm

How does that differ from a modern radial circuit?

Reply to
dennis

Hmm, An RCD would disconnect an earth fault in Also the ability to cover larger areas (100m^2) when required.

Thats just rules and they are there to be bent.

I would use 2.5mm2 for a radial myself and just use more of them.

Reply to
dennis

My parents house, built in 1966, has electrical storage heating on a switched (what would now be called Economy 7) night time, plus an afternoon boost, supply.

I've never paid it any attention but ISTR that each room with a storage heater is fed by one 20A fuse. However the kitchen and living room both have two heaters each, and I think these rooms are each fed by a 30A fuse.

The house is actually across two phases, so there are four fuse boxes,

Ring/Lights phase 1 Heating phase 1 Ring/Lights phase 2 Heating phase 2

The meter/fuse box area is rather large, my father built a cupboard around it soon after moving in !

I wonder how much of any of that would be reg compliant these days ?

Reply to
Mark Carver

Totally agree. They have electrics on a par with Eastern Europe

Should be about the same so don't agree with this.

That depends on the country. We all agree French, Spanish, most Low and all Eastern countries need re-wiring to a safe standard. That's the point of the whole standardisation process to kick it off.

But German extension adaptors are well controlled and so aren't that dangerous whereas these £2 UK ones appearing really don't look up to it. I expect there are just as many UK ones as in other countries in fact and so whilst the rest of Europe is rewired it would be a good idea to do us as well.

Reply to
Mike

I was referring to anything that breaks the supply.

We were talking about 70/80s housing or houses rewired in that period. What period did 15th edition cover ?

Reply to
Mike

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