What does the Neutral wire do ?

Postby DIYvirgin1975 » Wed Jan 23, 2008 2:35 pm

I understand the earth cable is to prevent having an electric shock but what exactly does an neutral cable do in a circuit ? :?
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Postby kuzz » Wed Jan 23, 2008 5:03 pm

In very simple terms it completes the circuit. no neutral would be like having a battery with only one end!
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Postby ericmark » Wed Jan 23, 2008 5:17 pm

Any circuit must have feed and return. Batteries have two connections. So in a house Live is Feed and Neutral is return. Any wires not connected to earth in some way can pick up voltage from all sorts of places. Any overhead cable can pick up static so even without a fault it could become 1000s of volts different to the ground. So this can't happen one of the pairs of cables is at some point connected to earth. This is called the neutral. By time it goes around your house it may no longer be exactly the same as earth but it is close enough that it can't brake through normal insulations and the live is only 230volt different from neutral so that also can't brake down normal insulation. Because neutral is very close to earth we don't bother switching it off for many items. In fact unless special double switch are used we are not allowed to. Normal house light switches only switch the live. It is very like an earth but normally carries current earths only carry current when there is a fault. Three phase is a little different and becomes more involved if you want I can try to explain but I should hope this answers your question?

Postby kbrownie » Wed Jan 23, 2008 5:42 pm

Hi DIYvirgin1975,
The neutral in a circuit returns the current back to the supply.
Is that a good enough answer or do you need more
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Postby DIYvirgin1975 » Wed Jan 23, 2008 9:14 pm

Superb thanks for your help. To take this disscussion further whats the difference between single phase and three phase ?
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Postby kbrownie » Wed Jan 23, 2008 9:49 pm

Hi DIYvirgin1975,
Single phase: you would normally find most domestic households being supplied with this, Its your everyday Live (phase), neutral, earth supply 230volts that can quite easily deal with domestic lighting and power circuits.
Three Phase: is more common to industry, workshops, garages, big machines, motors, stuff that needs more voltage to run well.
This consists of not just one live (phase) but three, hence three phase (I don't know who they come up with these names)
This can produce a supply voltage of 400v.
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Postby 333rocky333 » Wed Jan 23, 2008 10:18 pm

They have one live or 3 lives
L1+N or L1,L2,L3+N
Previously 3ph was R Y B +N
Both methods have only one neutral

Draw a circle, that is 360degrees put a dot in the middle.
Divide into three segments of 120 degree each radiating from the centre ,like a Y
That is your 3 phases and neutral in the centre

220 volt from any phase to neutral

As the three are 120 deg out of phase with each other you now get 400 volt between each phase

With 3 phase you will get more power for less current.
With single phase the live current say 100amp would be same as neutral return also say 100amp
Although each phase of a supply to a 3 phase building would have say 100 amp each you would have 300 amp of power
But coz there out of phase the neutral would not be 300amp but could actually be lower than 100amp
With 3phase it is essential to balance the load best as poss across the three
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Postby DIYvirgin1975 » Thu Jan 24, 2008 7:22 pm

It's started to make sence. Why is it on site the voltage tends to be 110v but at home it's 230volts. Why can't it be 230v at work as well as home ?
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Postby ericmark » Thu Jan 24, 2008 11:41 pm

It is only building sites and farms that have to use 110 volts or "reduced low voltage" and it is not really 110 volts but either 55-0-55 or 63-0-63 depending if from single or three phase transformer. As with most things it's a trade off and the lower the volts the higher the amps and as amps increase the fire risk also goes up but the chances of injury from shock goes down. At work there is a higher chance of cables being damaged than at home and so lower voltage. Also I am sure it is also done to stop people "Borrowing" items from work?

Postby ericmark » Fri Jan 25, 2008 12:18 am

Sorry missed question on three phase. Single phase has no direction, and motors need some way to get them to rotate, and they need something to start them. They don’t have much start torque. Three phase has direction and motors have very high start torque and have to have some way to reduce it in many cases. Most power is produced as three phase even when used as single phase and the three supplies when used as single phase are balanced so near equal draw on all phases. The whole of AC theory is wound around the sine wave and rotation. Returning to your original question neutral if you look at a Mercedes badge neutral is in centre and three phases at end of each spoke. For normal supply you have 230 between centre and end of spoke and 400 between each spoke end. With 110 volt system three phase then 63 between centre and spoke end and 110 between spokes. Some countries like Algeria have 110 centre to end of spoke and 190 end of spoke to end of spoke but USA does not feed 3 phase to houses but centre tapped single phase 110-0-110 one phase upstairs one phase down stairs and both to cooker. Because they have 0 – 110 and not 55 – 0 – 55 using their 110 volt tools in the UK can be dangerous as they have single fuses and switches so on our system can still be live when switched off and fuse blown. Most DC systems have now gone last I worked on was in Llwyngwril mid Wales about 1960 but with wind chargers etc. we may see some limited return and we still buy and sell power to France as DC.

Postby sparx » Fri Jan 25, 2008 12:32 am

Hi. site supplies can be 230V as long as on RCD, but most sites encourage use of 110V because when used from a transformer (Yellow coloured) you actually only get 55V to earth, which will not allow a dangerous amount of current to pass through the average human body to do damage,
The transformer has 2 windings one connected to 230V supply, the other has half as many turns of wire so producing half the voltage, unlike the 230V (primary) side the 110V (secondary) winding is not connected the same way, it has a connection in the middle (centre tap) which is connected to earth and the two ends are in fact 55v + 55V to give 110V between the 2, so it would need you to have hold of both wires to get a shock.
very basic explaination, would be easier to understand from drawing,
regards SPARX
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Postby kbrownie » Fri Jan 25, 2008 7:45 am

110v (actually 55v because centre tapped to earth) on construction sites because 230 volts a greater shock risk. (Can Kill!)
Construction site cables and hand held power, because of the nature of the work are more likely to incure mechanical damage (they get knocked about a bit and run other) so the risk of comimg in contact with a bare live cable is more likely and the condiction on site regarding damp and rain etc... Make the human body less shock resistive. So the need for a lower voltage (safety)
230 volts can be used on fixed lighting (ie floodlights but this must be installed using the correct cables and added protection)
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Postby curious1 » Fri Jan 25, 2008 9:59 am

hi all,
just reading the posts regarding 3 phase and single phase and was wondering if there are any specific reasons why can can or cannot use ring mains with either? For instance.. can you use a ring main with 3 phase supply or should it be a radial circuit and if so could you explain why?

Also, how would you balance the supply if you had 3 phase?


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Postby DIYvirgin1975 » Wed Feb 06, 2008 7:59 pm

Why is their no neutral on the light switch ?
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Postby thedoctor » Wed Feb 06, 2008 8:41 pm

A light switch is simply an interruption in the live feed to the bulb. Current is not returning anywhere, just on its way to the light. When the switch is on there is no interruption. See the Electrical projects on lights and switches to answer your questions with pictures
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