Electricity: 3 phase query

Hi gang,
AIUI, out in the road there are three live wires and one neutral. The phase
s of the lives are 120 degrees out of phase with one another. The potential
difference between any two live wires is 440V. The potential difference be
tween any live and neutral is 230V. Domestic supplies only tap one live and
the neutral to give you a single phase supply, so if you want a 3 phase su
pply for a workshop or whatever, you have to get the leccy blokes in to dig
up the road and tap you into one of the other live wires. Do I have that r
ight?
cheers.
Reply to
orion.osiris
Oh, and before some helpful soul offers up a practical solution, such as "g et yerself a phase converter, mate, it's a lot cheaper" I should just point out that I'm NOT looking for solutions, here, I'm simply enquiring as to w hether my understanding of the basic situation is correct or not. Thanks!
Reply to
orion.osiris
On Sun, 7 Jul 2013 06:03:20 -0700 (PDT), snipped-for-privacy@virgin.net wrote:
From a legal, technical or practical angle?
AB
Reply to
Archibald
"get yerself a phase converter, mate, it's a lot cheaper" I should just po int out that I'm NOT looking for solutions, here, I'm simply enquiring as t o whether my understanding of the basic situation is correct or not. Thanks !
Er, technical and theoretical. I'm already aware that the most cost-effecti ve practical solution is to hook up a phase converter to the distribution b oard via its own spur and that's what I'll probably end up doing.
Reply to
orion.osiris
On Sun, 7 Jul 2013 05:59:03 -0700 (PDT), snipped-for-privacy@virgin.net wrote:
No good tapping into just one of the other live wires you need two more.
Five wires in total
L1 L2 L3 N E
Plus a new three phase meter (or another two single phase ones)
and a big hole in your bank account
Reply to
The Other Mike
I don't really see any 'legal' angle provided the leecy board or their agents does the work. In addition to the cabling, you will need a change of meter and distribution board. I doubt that they will want to balance the loads for domestic use so your main house load can stay on one phase and the three phase go off to your workshop etc When selecting your wiring to the 3 phase load, you may want to run a neutral as well because some machines use the neutral for control circuits. Latest machines tend to use a 440-110 or a 440-24 transformer for controls. If you are buying secondhand stuff, my suggestion would be to be prepared with a neutral too.
Incidentally if you go down either the converter or inverter route, you might find this of interest.
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Some converters will use the single phase neutral as one of the output phases thus not providing a neutral in the correct relationship to the other phases. Inverters will usually only provide 3 phase output at the same voltage as the input. i.e. run from 240v single phase will only produce 240v three phase output so motors in your machines need to be dual voltage - marked as 240/440v or similar.
hth
Bob
Reply to
Bob Minchin
It's curious. I've been heavily into electronics for decades yet nothing I' ve learned it seems, is relevant to domestic/industrial electric installati ons. Though I'm intimately acquainted with the concept of phase, phase-shift and know the difference between current, voltage and power inside-out I'm stil l struggling to get to grips with this 'macro' stuff. These two fields are like chalk and cheese. I suppose an electrician would be equally perplexed if required to calculate the output impedance of a buffer/current-amplifier stage. :-/
Reply to
orion.osiris
:
The basic numbers in the UK are 415v 3Ph. 240v single phase = 240v from any of the three phases, to neutral. 415v as measured from any phase aay of the other two.
If you have 1ph + N, you are lacking two more phases to make a 3ph supply.
Reply to
Harry Bloomfield
I mean they're clearly two different disciples entirely. In my typically mi llivolt/milliamp world, we don't have to contend with forces sufficient to rip a machine's internals to pieces as you do with power engineering, so th e need for nice, smooth 3 phase supplies simply doesn't arise in the first place!
Reply to
orion.osiris
Right, Harry. So with only two live phases you'd have your 415V potential difference alright, but it would be effectively unusable?
Reply to
orion.osiris
...
Since 2003, it has been 400v and 230v, but with wider tolerances, so that neither we nor any other EU country had to make any change to existing equipment.
Colin Bignell
Reply to
Nightjar
I don't really understand why we don't go for 3 phase at say 800V. Let's fa ce it: even 230V is well into the lethal zone so going higher isn't going t o be any more dangerous. Plus 800 is still low enough not to arc or flashov er - given remotely adequate insulation. And just think of the saving in te rms of conductor! Given the price of copper (notwithstanding it's slipped b ack lately) I'd have thought pushing up the supply voltages would be obviou s thing to do from the pov of conserving natural resources.
Reply to
orion.osiris
One problem is the huge installed base of equipment, including cable that is rated at the current voltages. Current insulation standards sometimes stuggle with the current voltages, shown by exploding cables as frequently reported in this newsgroup.
Another reason is that along every street in the country is a set of three cables carrying the current voltages of 230V to earth, and 400V (+-) between them. To have 800V available, the powers that be would need to run new cables from new substations to only those users wanting 800V.
Another way to save money on the grid would be to increase the frequency, allowing the use of smaller transformers for the same power. The same objections apply, although aerospace applications often use 400Hz AC for this reason.
Reply to
John Williamson
I've found that most sparks I talk to don't have an in depth understanding of the subject. They are taught largely by rote both in college and as apprentices. They know all the how but very little why.
There are exceptions naturally.
My background like yours is in electronics but I've learned to adapt over the years.
Bob
Reply to
Bob Minchin
If you pay them, I should imagine, but if its industrial, then you might have the gods of the council down for some business rates. Brian
Reply to
Brian Gaff
Trouble is even though substation efficiency would increase, your main transmission line losses will increase dramatically due to skin effect. It is still significant even at 50Hz.
Andy
Reply to
Andy Bartlett
In article , snipped-for-privacy@virgin.net writes:
When you plot all the costs against each other (e.g. conductor costs, insulator costs, switchgear costs, etc) with a very broad brush, you tend to find the optimum is around 1000V per mile over which you are transporting the electricity. So 240V is good for up to 1/4 mile runs (i.e. distance from substation), and 415V (same thing strung as 3-phase) can go a bit further with a roughly balanced load.
So between villages, you might want various mixtures of, say, 1000V to 5000V, but then the next factor comes into play - the cost of conversion, and you don't want to do that too many times. Also, standardisation - let's stick to some specific voltages so we don't need loads of different transformer models, cable insulations, etc. We use 11,000 V as the voltage between most substations, and the next one up is 33,000V, these being good for transporting power around 11 and 33 miles respectively. The actual power carried also has to be factored in, and you might well get a shorter 33kV run where large amounts of power are required, or generated to be transported to the grid.
132kV, 275kV, and 400kV are used for transporting power over longer distances across the country, particularly as generation increasingly moved further away from consumption. (There may still be the odd 66kV line, but most of these were phased out as the National Grid was constructed.)
Reply to
Andrew Gabriel
Conflicting requirements. DC is best for long distance transmission, and HF AC for smaller transformers...... It's Edison vs. Westinghouse all over again.
Could this be the reason why we use 50 - 60 Hz worldwide, as the best compromise for a national grid?
Reply to
John Williamson
s face it: even 230V is well into the lethal zone so going higher isn't goi ng to be any more dangerous. Plus 800 is still low enough not to arc or fla shover - given remotely adequate insulation. And just think of the saving i n terms of conductor! Given the price of copper (notwithstanding it's slipp ed back lately) I'd have thought pushing up the supply voltages would be ob vious thing to do from the pov of conserving natural resources.
You see a lot of 3 wire transmission lines across the countryside. The line s are spaced about a meter apart and are supported by relatively close-to-t he-ground wooden poles which aren't dissimilar to telegraph poles in appear ance. You could reach them with a fishing rod they're that low in some plac es. I saw some hapless tree surgeon lop off a branch once that landed acros s two of these wires. Two little fires ignited at the contact points. Didn' t look anything special at first. I expected an upstream breaker somewhere to activate and shut off the current, but that didn't happen. The fires gre w rapidly and within one minute at the most there was the most *enormous* f lash and ***BANG*** and everything then fell earily silent. It was one of t he most spectacular things I've ever witnessed. Turned out the power to a c ouple of hundred thousand homes had been lost as a result. Would this have been one of your 33,000V lines? The homes affected were as far away as 35 m iles from the incident, btw.
Reply to
orion.osiris

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