415v Supply to Domestic Homes in the UK

When I was doing service work I don't remember a failure caused by a wirenut. What do you use to join wires. At devices do you use screw tightened pressure contacts instead of wrap around? Anything else?

'Heavy duty' US cords have a rubber or plastic jacket over the insulated wires - probably the same as sheathed. Would be interesting to see actuarial insurance info on electrical causes of fire in the US and UK.

They're all screw tightened pressure contacts, with no possibility of the wire escaping as it's tightened. There's an insulated free-standing type which is used instead of wirenuts ("chocolate block" connectors, because they come in strips like bars of chocolate which you break down to the required number), but we don't use anything like as many, as we don't use pigtails -- most accessories will not need any extra connections behind them. If an accessory does normally need extra wire jointing behind it, in some cases an extra terminal is included on the rear of the accessory for this purpose.

Electrical installation fires in the UK cause around 5 deaths per year, and electrical appliance fires around 18 deaths/year, with the figures reducing over time. In perspective, total deaths from fire in UK homes is around 500/year, the main causes being smoking (particularly falling asleep whilst smoking), followed cooking related (oil/fat catching fire). Again, the trend is figures reducing each year.

On Fri, 22 Jul 2005 15:16:27 -0500 Bud wrote: | Andrew Gabriel wrote: | |> |> One thing most UK people notice when visiting the US is the way |> your lights get brighter and dimmer when other loads in the house |> switch on and off. That's very rare here in UK and when it does |> happen, it will normally trigger a search for a fault, which is |> probably why it initially alarms visitors to the US. |> | I suspect the major cause of dimming is mixing lighting and other loads | on the same circuit.

And they don't do that in UK?

On 22 Jul 2005 16:42:56 GMT Andrew Gabriel wrote: | In article , | snipped-for-privacy@ipal.net writes: |> On 22 Jul 2005 00:39:29 GMT Andrew Gabriel wrote: |> |>| No. That particular type of fault seems to be much more of a |>| problem in the US than elsewhere. Our approach over the last |>| 50 or so years was to engineer out the situations which give |>| rise to arcing in the first place. |> |> How has this been done? | | I don't claim to know all the areas, but some of the better known | areas where problems of this sort had been observed... | Wirenuts ceased to be used (actually, that's over 60 years ago, but | their use was never very widespread for mains wiring even before then). | Wrap-around screw terminals have been gradually phased out.

What replaced them? I'm sure it can't be backstabs.

| Plugs and sockets have much larger contact areas, higher contact pressure, | and firmer grip -- no wobbling around (this is also well over 60 years | ago, as it was started with the design of our previous round pin plugs).

Mine don't wobble around.

| We didn't use Alumin{i}um wiring in houses.

Good thing, too.

|> A series arc is one example. Twice the voltage and half the current |> does change the character of the arc. But over twice the distance |> it can still dissipate the same energy with half the current. That |> would be an arc that would drop out at a lower voltage. | | Arcs are sustained by current. The higher the current they pass, | the lower voltage they need to sustain them. They aren't ohmic.

But there is a voltage drop within the arc if it dissipates power. Otherwise it would be an interesting way to get free heat from electricity (it isn't).

|> Are the cords leading to appliances mechanically protected to avoid |> stepping on them causing damage? That would be something I'd like to |> see here. | | Yes. All appliance cords are sheathed, since 1960's I think. | All except low current ones were sheathed long before that. | They probably get stepped on quite often, but I don't ever | recall any damage due to stepping on one. | Unsheathed cords are limited to 50V max, so they can be used | for LV side of LV lighting.

What kind of sheathing is required?

The fixed lighting is on dedicated circuits, but people routinely plug table/portable lamps into regular sockets, and we don't get lights changing brightness even with those, as is common in the US. So I don't think this can just be blamed on mixing lighting with other loads on the same circuit, because when we do that, it still doesn't happen.

Straight blind hole for the conductor, with a screw clamp entering it from the side. These were always more popular than wrap-around screw terminals in any case, but the wrap-around's have now pretty much vanished from the scene.

There is, but an arc naturally seeks to minimise this by increasing its current density. As the current density increases, the arc voltage plumits. To keep an arc stable, you need something to limit the current flow in it.

I don't know what the precise rules are, but there's a second layer of insulation around all flexible cables, tougher the thicker the cord. Cords are not damaged by standing on them, nor if you occasionally drive over them (although I doubt the manufacturers would claim they are designed for that). There are also special sheaths for different purposes, e.g. silicone for things like irons and soldering irons, rubber for tools where the cord is likely to come in for rough use, PVC for most other things.

The figure-of-8 zip cord and other single-insulated cord isn't allowed for mains use here (or anything over 50v).

On 24 Jul 2005 12:01:16 GMT Andrew Gabriel wrote: | In article , | snipped-for-privacy@ipal.net writes: |>On Fri, 22 Jul 2005 15:16:27 -0500 Bud wrote: |>| Andrew Gabriel wrote: |>| |>|> |>|> One thing most UK people notice when visiting the US is the way |>|> your lights get brighter and dimmer when other loads in the house |>|> switch on and off. That's very rare here in UK and when it does |>|> happen, it will normally trigger a search for a fault, which is |>|> probably why it initially alarms visitors to the US. |>|> |>| I suspect the major cause of dimming is mixing lighting and other loads |>| on the same circuit. |>

|>And they don't do that in UK? | | The fixed lighting is on dedicated circuits, but people routinely | plug table/portable lamps into regular sockets, and we don't get | lights changing brightness even with those, as is common in the US. | So I don't think this can just be blamed on mixing lighting with | other loads on the same circuit, because when we do that, it still | doesn't happen.

I see lights dimming here, even on different circuits, so I would have to agree that sharing the same circuit is not the sole cause of this problem. This was an issue discussed maybe here a few weeks/months ago, and it then focused more on the fact that in the USA, most of the loads tend to be on 120V, and hence draw twice the current as their UK counterparts. Given that a 100A service is going to have roughly the same gauge wire in both countries, that means one will see twice the drop in voltage in the USA as in UK. Then, that twice voltage drop has an even greater impact on the lower working voltage. If a motor draws enough current on 120V to cause a 6V drop on some given wiring, that same motor would cause a 3V drop on the same wiring at 240V. That's a 5% drop vs. a 1.25% drop.

Makes me want to switch to 480V or 600V service :-)

Still, even with that, people here report lights dimming even though the load in question causing the problem is wired to opposite poles and gets

Where I live, one single phase pad mount transformer of unknown capacity is serving a group of about 8 homes. I don't see or notice (doesn't mean it isn't there) any flickering or dimming except when I turn on the garbage disposal unit. That causes a lot of lights to dim rather dramatically. The GD is on 120V. The air conditioner (on 240V) has never caused any dimming. And I cannot recall any dimming from unexplained sources other than what appeared to be line fault problems way upstream. Trees like to drop branches on, or fall directly on, primary lines around here. Given the topology, it is nearly impossible to keep trees trimmed back enough to avoid this in all locations.

|>What replaced them? I'm sure it can't be backstabs. | | Straight blind hole for the conductor, with a screw clamp entering | it from the side. These were always more popular than wrap-around | screw terminals in any case, but the wrap-around's have now pretty | much vanished from the scene.

I'm sitting here holding a receptacle that hasn't been installed. It happens to be one intended for 240V, 15A, in the USA. It is a duplex, and each outlet has two horizontal blade holes aligned end to end, instead of the two parallel vertical ones usually seen. The ground is in the usual position (above or below or sideways depending on which way you turn it). It fits the same plate opening as the common 120V receptacles. There are screw terminals on the side. But there is a plate under the screw that forms a clamp. The back as an indentation makred "strip gauge". Stripping insulation off the wire at the length shown would just fit completely under the clamp. Then the screw would be tightened down pressing the clamp onto the wire and the wire only the base metal around the screw hole. The majority of wire contact is the clamp and the base. The screw may have a slight contact with the wire along the edge of the threads and that is all. You can't get the wire around the screw for a few reasons. The screw will not come out all the way. You can't bend it around the screw under the clamp piece. The clamp has some lips on its upper side at the back to prevent putting the wire around the screw above the clamp piece. It seems to be a very good design to me. It is made by a company called Hubbell.

|>|> A series arc is one example. Twice the voltage and half the current |>|> does change the character of the arc. But over twice the distance |>|> it can still dissipate the same energy with half the current. That |>|> would be an arc that would drop out at a lower voltage. |>| |>| Arcs are sustained by current. The higher the current they pass, |>| the lower voltage they need to sustain them. They aren't ohmic. |>

|>But there is a voltage drop within the arc if it dissipates power. | | There is, but an arc naturally seeks to minimise this by increasing | its current density. As the current density increases, the arc | voltage plumits. To keep an arc stable, you need something to limit | the current flow in it.

I'd say the arc is sustained by ionization, which results from power dissipated, which as we all know is a function of both current and voltage.

| The figure-of-8 zip cord and other single-insulated cord isn't | allowed for mains use here (or anything over 50v).

The zip cord is still used here, but it's found in a lot fewer places now.

I finally found where I hid time-current curves for a circuit breaker. For comparison a generic curve for Square D breakers (which are probably typical) are instantaneous at over 10x rating with a maximum clearing time of 1 cycle. This is right between your B and C rating. For a 2x overload the clearing time is 9-40 seconds, a lot shorter than I thought. May have to be faster than UK because we can have #18 wire rated 7A on a 20A circuit (of course with no fuses in the plug).

Bud--

Another US quirk - US circuit rating is 80% for continuous loads over 3 hours). My understanding is that this is because fuses and circuit breakers may open/trip if 100% load is applied over 3 hours. (There is an exception for 100% rated devices.) Seems like an odd design.

Bud--

Much Snipped

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Hello all,

Could the difference in lights dimming possibly be to the difference in size of the pole or distribution transformers? I understand the USA tends to use transformers for suburban areas of 16 - 50kVA range? And that in the UK transformers of 300kVA+ are much more common due to higher density living in a lot of areas? The available short circuit current at the terminals of the main switch will have an effect of how much the lights will dim.

Here its not unusual to see lights dim a bit in the suburban areas when a large airconditioner cuts in, especially if there is some distance to the pole transformer. In say a medium size block of units (appartment, condo, whatever they're called elsewhere) where there will be a large transformer within the building its unusual to see lights dim when switching on large appliances.

We have similar supply characteristics to that of the UK - 240/415 volt three phase supplies, but the actual method of delivering it to homes varies on area. It can be underground, overhead, single phase, three phase, big transformers or small transformers - whichever is most economical when the area is being subdivied or developed.

Cheers James