110v vs 240v on building sites.
- Thread starter mattysupra
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Like an electron it is just a phase he is going throughOr shall we not go there again?
An appliance with only two active terminals can only be single phase, the supply to it can a single phase or two phases.
If the appliance has three active terminals it can be two phase in star or three phase in delta.
If the appliance has four active terminals it is most likely to be three phase in star. But it could be something else.
Our 240V system is (in most places*) part of a larger 415V 3 phase system. Along a street, there will be a 3phase and neutral cable - and each house will get a single phase and neutral tapped from it. There are significant benefits - not least the saving in copper by being able to share a single neutral between all the devices.
If someone wants a 3 phase supply, then all that's needed is to tap off 4 wires instead of 2.
* Yes there are places that have single phase supplies, but I think the majority of homes, at least in urban settings will just be tapped off a 3 phase supply down the street.
Actually, they use a split phase system which is not the same thing. In effect, they have a 220V supply with the centre tap of the transformer secondary supplied as the neutral. Large loads are connected across the two hot wires (for 220V), while most loads are connected between one hot and neutral.
At the end of the day it's all a trade off between different factors. Higher voltage means less current, and vice versa. Split phase means more cable and transformers, 3 phase (or one of 3 phases for a single phase supply) means higher voltages.
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Tech question here then as what you say has got my brain working for the first time in a while ! (interested basically and totally off topic with my original question)
So on a average street there is 3 phases and a neutral . My question is - is that neutral cable the same size as the phase cables ?
If i had a 3 phase supply then should the neutral be 3 times the size as the 3 phases coming into the property? (i think it 3 wires in total isnt it?)
If no then how come 2.5mm twin+earth as a example, how come the neutral is the same size as the live?
On a 3 phase supply as the phases are 120deg apart, if the load on all the phases is the same the net current in the neutral is zero.
There are a number of locations where the neutral is actually 50% of the phase conductor size. (this is permitted for us)
For some loads (motors being one) there may not even be a neutral.
Similarly HV cable and lines are only the 3 phases without a neutral as they are usually well balanced.
Going back to 110V there used to be parts of the UK where this was the LV voltage (Blackpool being one of them)
There are a number of locations where the neutral is actually 50% of the phase conductor size. (this is permitted for us)
For some loads (motors being one) there may not even be a neutral.
Similarly HV cable and lines are only the 3 phases without a neutral as they are usually well balanced.
Going back to 110V there used to be parts of the UK where this was the LV voltage (Blackpool being one of them)
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hence in 110 volt countries these are very common.
What - dustbins stuck up telegraph poles?
I stood scratching my head at a set up like that in Eire once, it was 3 single phase transformers (which is what they are) but wired in delta/star to give 3 phase LV!
I think in the very early days there were all sorts of different voltages (all DC, I believe) in different parts of the country. That was before the national grid when everyone was supplied from a local power station.
If the 3 phases were "all the same" then you would be right. However, 3 phase is more complicated than that - as already said, the voltages are not "in phase" with each other and the end result is to reduce the neutral current.
Consider this diagram, which comes from Wikipedia
If you have a normal UK single phase supply, you'll get a connection from one of the phases - could be any of them. In a 3 phase supply you get all three plus the neutral - single phase loads are connected from any phase to neutral.
Now consider you connect three heater elements, each between a different phase and neutral. One will have a current that looks like the black line, one a current that looks like the red line, and the third a current that looks like the blue line. If you look at the left of the graph, the black line is at 0, the blue line at about 0.86, and the red line at about -.086 - so all the currents sum to zero. At 90˚, the red and blue lines are at -0.5 which balance out the +1 on the black line. As you progress across the graph, you'll find that at all points, the sum of the currents comes to zero.
Now, you'll be thinking ... where does that 0.86 come from ? Grab a calculator (or your book of tables if old school) and look up sin(120˚
. As shown on the graph, the three phases are identical other than being offset by 120˚ and 240˚ - this would be achieved by having three sets of windings in the generator which are spaced out like that round the frame.If you only had two heaters, then things are a bit more complicated, but if you sum the (say) black and red lines on teh graph, you'll find that you still get a sine wave, but the height is less than either the black or blue red lines, and the phase (angle) doesn't match either.
So in a 3 phase system, the worst case (with resistive loads) is when 2 phases carry no load and the neutral current matches that in the loaded phase. Once you add load to more than one phase, the neutral current will be less than the highest of the phase currents. The best case is when the 3 phases are identically loaded - then the neutral current is zero (and if this were internal to some equipment, we might even omit the neutral wire for this heater bank). In reality, except for specific cases (such as the supply to a 3 phase motor or balanced heater bank), this is rarely achieved exactly.
Note that I added the caveat "with resistive loads" above ? Well there are some types of load where this doesn't apply. Fluorescent (and other forms of discharge) lighting, power supplies without power factor correction, etc. These can have a current waveform that isn't a nice clean sine wave - typically being narrower but higher at the peak. Try drawing a cosine wave, and another cosine wave at lower height but 3 times the frequency - at t=0 you'll find that the two add to a "pointy peak", while at other times they subtract. This is the sort of effect that some loads have on the supply current.
Because this "3rd harmonic" ends up in-phase across all the phases (3 * 120˚ = 360˚
, the 3rd harmonic current actually add up - and the neutral current can be significant. There have been incidents in the past - for example, consider a large office block built when the only electrical equipment would be the cleaner vacuum. Then add in a load of early computer equipment - power hungry and with poor quality power supplies. Everyone may assume that the neutral current is low - but in fact, because of the crest factor on the poor quality power supplies (and the fluorescent lights), the neutral current is anything but low and the cable is overloaded.That's why there are strict regs on the current waveforms allowed to be drawn by equipment.
EDIT: The above may explain why single phase is 240V, and 3 phase is 415V (in the UK). If you draw a phasor diagram, mark a dot which we'll say is neutral. Draw a line vertically upwards from the dot for 240 units of length - the top of this line is one of your phase supplies. Now draw two more lines, also 240 units long, outwards from the dot - but spaced out at 120˚ and 240˚. You now have your 3 phases. Draw lines between the ends of these phases, and you end up with an equilateral triangle - some trigonometry will tell you that the lengths of the sides of this triangle are 415 units long.
Now, going back to our heater example. Instead of using 3off 240V heaters, we could use 415V heaters and connect them across the phases - so heater one between A and B, heater 2 between B and C, heater 3 between C and A. No neutral involved. Instead of heaters, consider them the windings of a motor, and you can see why 3 wire, 3 phase supplies are common for motors - no need for a neutral wire at all if we also run the control circuits at 415V and connect across 2 phases.
Well, in most uses of T&E, the power for the device goes out along the live wire, and returns along the neutral wire - so the currents are exactly the same. There are instances (lighting circuits) where there may be more than one live feed, but sharing a common neutral - and for those, the neutral would (in theory) need to be larger (in practice, the conductor sizes are more likely to be determined by the type/rating of the circuit protection and there'd be no practical scope for using reduced live conductor sizes).
Unfortunately not all equipment follows them
Correct me if I am wrong, will delta star transformation will also come into it?
For example L1 on the LV side is derived from L1 and L2 on the HV side so the L1 (LV) ends up something like 60° out of phase from the L1 on the HV side?
Yes, distribution transformers are usually Delta/Star. But the phase shift is 30 deg not 60. Depending on whether the transformer is Dyn1 or Dyn11 will decide on whether that is +30deg or -30deg from the primary.
Yes indeed.Unfortunately not all equipment follows them
I guess there are a lot of points where that takes place, and I was thinking about this only the other day - but don't ask me how my rambling excuse for a mind got onto it
As long as all circuits have the same combination or aren't part of any loop, then it shouldn't matter. So I know that (for example) the 132kV system has loops and redundant paths - but since there won't (I assume) be any star-delta transformations within it then that won't matter. I assume star-delta only happens between the 11kV and 415V parts, and there won't be loops in the 415V network so even if two transformers are setup differently then that won't matter (it would only matter if someone decided to take supplies from two different substations and try to connect them At the generator end, they are all spurs - so again it won't matter what happens before the final grid connection.So unless I've missed something, it doesn't sound all that bad.
Now, how they work out that there's been no errors or crossed phases when completing a new loop at high voltage ... that would be interesting to know, and I don't think getting it wrong would do a long for the switchgear when they find out.
As an aside, not far from me I happen to know that they are running without the neutral/earth wire connected in the 132kV system. It broke in the recent bad weather - taking out the power at work which was "interesting" - and I believe that they just fastened it to the towers out of the way, presumably waiting for better weather before they try and fix it. I imagine Westie knows where I mean.
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