My original response was to Bob Eager's point blank "no it doesn't" which is not specific at all as to why it "doesn't". I inadvertantly accused Bob Mannix snipping here and lack of clarity ... apologies, I mis-read the sender in the thread.
Alex
Ah full circle. Apology accepted :o). My comment about the trap was because I guessed Bob Eager put what he did (out of devilment) to see how long people argued about the reversing direction bit before realising it was the
50 that was wrong (which, if I may say, you weren't clear about realising/accepting, hence my other comments). Of course it may not have been like that at all, only he can say.I dont want to be pedantic but AC current is just swapping the positive and negative electrodes around 50 or 100 times a second,
so positive and negative are switched around by AC current,
Alternating current is current which constantly changes in amplitude, and which reverses direction at regular intervals. You learned previously that direct current flows only in one direction, and that the amplitude of current is determined by the number of electrons flowing past a point in a circuit in one second. If, for example, a coulomb of electrons moves past a point in a wire in one second and all of the electrons are moving in the same direction, the amplitude of direct current in the wire is one ampere. Similarly, if half a coulomb of electrons moves in one direction past a point in the wire in half a second, then reverses direction and moves past the same point in the opposite direction during the next half-second, a total of one coulomb of electrons passes the point in one second. The amplitude of the alternating current is one ampere. The preceding comparison of dc and ac as illustrated. Notice that one white arrow plus one striped arrow comprise one coulomb.
See
great article. the current goes from live to neutral and back again 50 times a second as explained here
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But still, every once in a while a Customer will get a really nasty shock from a cheap plastic appliance that's wet. Or perhaps an electric hair dryer will fall into the bathtub, but the short-circuit current in the water won't be enough to blow the fuses in the house, and the bathtub water will become lethal. In theory there is a way to prevent this. These Customer shocks are happening because the customers' bodies offer a path for current between the appliance and the lightning-safety earth grounds. During the shocking event, some of the flowing charge is going in (and out) of the "hot" wire, but it is NOT going back into (and out of) the "neutral" wire as it's supposed to. Instead, it's going through the human, and also going into the grounded pipes of the plumbing.
If we could measure the current that's taking the "wrong" path, maybe we could detect the problem and turn off the power before anyone dies. We can't measure the current in the plumbing, but we can measure it in the "hot" wire, measure the current in the "neutral" wire, then subtract them. This tells us what level of current was escaping via the "illegal" path through the human to ground. The subtraction should normally give a zero result, since there never should be a current path to ground that isn't using the neutral wire. If we amplify the subtraction's result and use it to trip a circuit breaker, we'll have a new type of appliance which turns itself off immediately when a human gets into the electrical path. These devices are now required in wet areas of homes (bathrooms.) They're called Ground Fault Interrupters. And so we've finally entered modern times, the "Age of the Electric Outlet with the Little Red Button Which Pops Out!"
You're absolutely right! Although I'm surprised that Alex didn't just put his hand up and admit he was wrong...!
I snipped because I wasn't answering the other part...
On 5 Oct 2005 09:16:56 -0700, "blackboab" scrawled:
Er, this is uk.d-i-y, note the UK bit.
Higher voltages are not unknown on isolated overhead LV supplies in the sticks (cue "Wanderer").
An HV supply won't usually have a distributed neutral conductor (just
3-phase, 3-wire). There'll be a neutral point at the transformer or generator, earthed via a known impedance, but only the 3 phase conductors head off outwards.i know.
but most of the information is still relevant.
say what you want about the yanks being imperialist , war-mongering bastards but their technical knowledge is pretty impressive, at least they make good websites,
Oh you would say that ;-)
Why be surprised?
To be honest shortly after sending my first reply the sinusoid entered my head and I realised exactly 50 changes = 25 Hz but did not consider it was a trick question ... and my attention was elsewhere for a while ... After Bob M's post and decided if trick questions were the name of the game then actually I was not wrong in the strictest sense. Rob was right about wriggling ... but hey why not if you've been set up.
That's an approximation.
If the frequency is high enough, and/or the distance is long enough, a lot of the power gets carried as an electromagentic wave around the conductor.
I'm used to seeing this at MHz and GHz ranges in short conductors, but I'm pretty sure this is why HT pylons have 4 wires with airgaps for each conductor.
Andy
If the conditions were right, I'd touch the Phase. Since 33kV is probably going to be 3 phase, I'd say any one of them...
Not in the UK it couldn't (330kV that is)
They always seem to have cables in multiples of 3 whenever I look. One group for each phase.
275,000V is a bundle of two, and 400,000V is a bundle of four. This is to reduce the electric field strength which in turn reduces the corona discharge.
Or maybe you are referring to the 3 or 6 lines carried, which will be for one or two 3-phase circuits?
Nothing wrong with multiple RCDs. If you are on a TT install then that is the way to go anyway. The ultimate solution you could argue would be a RCBO for each circuit that needs one.
The argument against RCDs, is the problems of unwanted trips as a result of "normal" leakage (some types of appliance are prone to it because they have mineral insulated heating elements, or capacitive filters on their mains inputs). For circuits that present very little real risk of electrocution the negatives may outweigh the positives.
Why not add a non maintained emergency light then? About £20 from TLC. Wire it to a feed on the lighting circuit in there. That will keep it charged. When the power goes off it lights up.
Apart from the bit about having sockets allowed in bathrooms...
Alas their electrical installations are of state of the ark in comparison to ours. So of very limited use as a source of reference material.
really ? how do they manage to do things like the space shuttle ? then again two of those crashed and they lost 2 space probes they sent to Mars. Perhaps they arent as great as they like to think.
what is the worst thing about their electrical systems ?
Err, how about twisted joints?
OK, OK everyone! I accept that such systems are 3-phase without a neutral return. I thought 330kv was one of the standards (it clearly isn't). I was making a general point about the context of the original question determining the supply details - of course I realised almost immediately there would be nationalgridophiles who would descend on me :o)..
Actually it struck me there is an interesting psychological point - the overhead supplies in our street are 240V 3phase with neutral (every house is connected to the neutral and they are spread across the phases). While I am happy to touch the neutral in the house, I would be less happy to touch the neutral on the pole, but they are connected together so that's illogical but there you go.
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