Monday, May 06, 2013

Electronics lessons: Mains Power -single phase and three phase

Whilst writing this blog I've stayed away from things that I would consider dangerous.

That is to say I've completed some projects that I've felt were either too big to document, (like when I put up my new storage shed in the garden and put electrics and lights etc in.) -at the time my thoughts were like this. it's OK to tell someone to stick bits of wood together to make a speaker, it's OK to say to people use this step down transformer and deal with the safe voltage that comes out the other side. I even thought it fine to tell people to stick electrodes into water to generate hydrogen/oxygen gases!

But big scary voltages have been something that I've stayed away from, the thing about mains utilities is that, you can smell gas, and know to shut off the supply and get out. you can see water and hopefully have the intelligence to get out before drowning. Electricity is completely invisible, it's not a substance, it's a force.

Anyway, that might give you an insight as to how I feel about mains electricity. it's dangerous to the point of deadly, and too much confidence can lead to mistakes that ultimately can kill you.

As a point of reference I live in the UK, this means that for me, voltages come in 120v for site tools, (where the 120v is considered safer due to lower voltages) this voltage is obtained by using those little yellow transformer boxes.

240v comes into every homes, (at least every connected home, I suppose off grid homes might make their own standards).

440V is considered industrial and is what most three phase power outlets are going to be using.

Voltages are measures with respect to neutral, live - neutral.

Single phase

Single phase power is an alternating waveform going from the positive extreme of the voltage swing to the negative extreme of the voltage swing in one wave form.

Single phase is what I've mostly dealt with, it's easy to visualise, easy to look at and easy to understand. remember that the positive is measured with respect to neutral, not with respect to ground.

Sometimes a single phase supply is actually a 2 phase supply, where instead of a virtual earth, or earthed neutral, there are two hot phases where the phases are 180 degrees apart.
the sum of the phases is zero, (e.g at 0 degrees phase 1 is 0v, phase two is at 180 degrees and is also 0v, at 90degrees, phase 1 is +120v and phase two is 270 degrees and -120, (+120 + -120 = 0v), the total potential difference between the two phases is 240 volts, (120 - -120 = 240)

Three phase

Here's where it starts to get a little more complicated.
Now there are three phases, each phase is 120 degrees behind the next, they are still sine waves, and they are still measured with respect to neutral, but there is no neutral in the transmission line, -when power enters your premises in a single phase set-up there is a live and neutral line. when you get three phase there is only 3 live wires, no neutral. (you can make a neutral by connecting all the phases together, because the sum of the phases is zero) -but I'll cover that in a later lesson.

You can see at 90 degrees now phase 1 (blue) is at at the top of it's rising cycle 240v, and phase 2 (green) is just coming up from the bottom (so during it's rising cycle) and is -120v, whilst phase 3 (red) is getting towards the bottom of it's falling part of the cycle, and is -120V
240 + -120 + -120 = 0

Colour Codes

Colour codes for wires are important, in that they will let you know what wire does what. everyone knows that green/yellow stripe is earth, and it's safe to touch. but what if I wired a plug so that the earth wire was used as a live conductor.
it's just a bit of copper, fundamentally there is nothing to stop me from doing that, provided I wire the other end the same way, where's the problem?

The problem is what happens when I'm out, and whatever appliance I've wired badly breaks, what if my wife/girlfriend/boyfriend/husband/friend/son/daughter/mother/father/grandparent etc decides that they just want to check the fuse on the plug, then they see the wires connected wrongly, so think that might be the problem. or something see's an earth wire, and decides that they can splice that wire to earth something else, -except it's not earth, it's live, and they are probably now dead.

Colour codes are set by the IEE.

for single phase systems.
Brown = Live, (the old colour was red)
Blue = Neutral, (the old colour was black)
Green and yellow stripe = Earth, (the old colour might be plain green)

for three phase systems,
Brown = Phase 1 (old colour was red)
Black = Phase 2 (old colour was yellow)
Grey = Phase 3 (old colour was Blue)
Blue = Neutral (old colour was black)

This means that if you are adding new wiring to an old system, don't just connect the new blue wire (N) to the old blue wire (P3), or the new black wire, (P2) to the old black wire (N)

you may also find three phase systems where the conductor colours are Brown, brown brown for P1, P2 and P3, with a blue for earth.

Earth (as always) is green yellow stripe.

earth may also be the bare wire in fixed installations, (where solid core wire is used rather than stranded wire).

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