Question on live and neutral wire

[Lim Y K]

Why must a switch be at the live wire instead of the neutral wire?

 

[Dale]

Safety.

 

[Bystander]

... and, to cut off leakage paths.

 

[Lim Y K]

In what way does that make the appliance safe?

 

[jbriggs444]

In what way does that make the appliance safe?

Take the classic example of an electric toaster and a person in a bathtub. If you turn off the wall outlet with a switch on the live wire, the person in the bathtub is (reasonably) safe. If you turn off the wall outlet with a switch on the neutral wire, the person in the bathtub is at risk.

 

[NascentOxygen]

Why must a switch be at the live wire instead of the neutral wire?

In addition, if the wall outlet had its switch break the neutral line and not the active, then every unoccupied socket even though switched to "off" would expose a dangerous voltage to the idle probing of inquisitive toddlers (and pets).

Notwithstanding the hazard, some countries, I understand, do allow wall outlets without an associated switch; you have to plug and unplug from a constantly "live" socket. So try to keep fingers clear!

 

[pbuk]

Notwithstanding the hazard, some countries, I understand, do allow wall outlets without an associated switch; you have to plug and unplug from a constantly "live" socket. So try to keep fingers clear!

In the UK all domestic sockets must be fitted with shutters which prevent contact with the live (or neutral) supply unless there is a pin in the Earth socket. The idea that a socket is made safer from an inquisitive child's fingers by providing a switch which the same child can easily operate is ludicrous.

 

[Dale]

In what way does that make the appliance safe?

The most basic hazard with electricity is providing an alternate path to ground. If you grab a grounded wire then your body is just part of the ground. If you grab a live wire then your body may form a path to ground. By switching on the live side you make most of the wiring grounded when it is off, and only a small part of the wiring is live. Thus if you grab a random section of wire you are less likely to have grabbed a live wire if the switch is on the live side.

 

[William White]

In what way does that make the appliance safe?

the simple answer is if the circuit is switched on the neutral, then when the circuit is 'OFF' the load is still "LIVE" (at mains voltage).

Neutral switching is (generally) illegal in the UK.

In very special circumstances, you can use double-pole devices, but they must break the live conductor first and make the neutral conductor first (this could be a "suitable precaution")EAWRs
Integrity of referenced conductors

9. If a circuit conductor is connected to Earth or to any other reference point, nothing which might reasonably be expected to give rise to danger by breaking the electrical continuity or introducing high impedance shall be placed in that conductor unless suitable precautions are taken to prevent that danger.ESQCRs
No generator or distributor shall introduce or retain any protective device in any supply neutral conductor or any earthing connection of a low voltage network which he owns or operates.

 

[mr166]

The hot wire can supply full voltage and power to any true ground. If you touch a 3 prong appliance the odds are that the metal parts are grounded. The neutral wire gets connected to ground in the circuit breaker box. Thus putting a switch in the neutral wire leaves the hot wire hot all of the time and if you touch it and ground at the same time you could be killed. That is why the hot wire must be switched and not the neutral.

BTW just because a wire is white that does not mean it is a neutral. Here in the US it is very common to have 220V wiring that has black and white hot wires. The black measures 110v to ground and the white measures 110v to ground. They are 180 degrees out of phase so you measure 220V across the black and white wires.

 

[sophiecentaur]

There was a time when some electrical equipment had internal fuses in both the live and the neutral lines. I can't think why that idea ever caught on. I have actually used such equipment which had been modded to eliminate the neutral fuse.

 

[Nugatory]

BTW just because a wire is white that does not mean it is a neutral. Here in the US it is very common to have 220V wiring that has black and white hot wires. The black measures 110v to ground and the white measures 110v to ground. They are 180 degrees out of phase so you measure 220V across the black and white wires.

There's a similar problem with switch loops even in 110V circuits... Local code where I live (US) says that hot whites must be marked at both ends (usually by putting a piece of black tape on the end of the insulation).

 

[insightful]

Notwithstanding the hazard, some countries, I understand, do allow wall outlets without an associated switch; you have to plug and unplug from a constantly "live" socket. So try to keep fingers clear!

One of those countries is the US. Switched outlets are unusual.

 

[rcgldr]

One of those countries is the US. Switched outlets are unusual.

To compensate for this, some of the outlets like ones in a bathroom or laundry room (any room that could have water on the floor) have ground fault interrupters. Wiki article:

http://en.wikipedia.org/wiki/Residual-current_device

Also in the US, the neutral wires can have some small voltage since they are only grounded at the junction box, while the optionally used third ground wire is apparently grounded in more locations in a household.

 

[nsaspook]

There was a time when some electrical equipment had internal fuses in both the live and the neutral lines. I can't think why that idea ever caught on. I have actually used such equipment which had been modded to eliminate the neutral fuse.

I know that US military ships use floating AC for power, there is no neutral wire so both wires (single phase) are 'HOT' and are fused.

 

[Lim Y K]

Oh ok got it. Thanks

 

[sophiecentaur]

I know that US military ships use floating AC for power, there is no neutral wire so both wires (single phase) are 'HOT' and are fused.

And what is the rationale behind that? If the two conductors are truly floating then 1. how does the fuse provide a protection against shock and 2. How does an extra fuse in series do a better job of overload protection? There must be some scenario that prompted double pole fusing but I can't think of one at this minute.

 

[FactChecker]

And what is the rationale behind that? If the two conductors are truly floating then 1. how does the fuse provide a protection against shock and 2. How does an extra fuse in series do a better job of overload protection? There must be some scenario that prompted double pole fusing but I can't think of one at this minute.

My two cents:
I can think of two possible reasons: water danger and battle damage.
1) Residential building standards require ground fault breakers where there might be water (kitchen, bath, outdoors). So it makes sense for a ship to assume there might be water dangers everywhere. It's probably cheaper to add normal fuses on every line than to put ground fault breakers on every line.
2) Battle damage might short out wires all over the place. So maybe they want to assume that any wire can become hot at any time.
3) The combination of both. Battle damage causing water danger anywhere on board.

 

[Bystander]

rationale behind that?

Damage control? Never know which side of a circuit's going to be open to salt water?

 

[tfr000]

Simple example... I once repaired a spotlight lamp in a garage. When you operated the switch, it would go from bright to dim instead of from on to off. I found that the switch had been wired into the neutral side. The lamp current was obviously finding another path to ground when the neutral was interrupted. I moved the switch to the hot side, problem solved.

 

[CWatters]

I know that US military ships use floating AC for power, there is no neutral wire so both wires (single phase) are 'HOT' and are fused.

Google found this..

http://www.brighthubengineering.com/marine-engines-machinery/38231-electrical-grounding-on-ships/

The requirement ashore is the safety of human beings. So, inorder to prevent human-electrical accidents, the neutral is earthed. The priority is neither the safety of the machinery nor the continuous necessary operation of the machinery. But the scenario onboard ship is totally different. The priority is the continuous operation of the machineries which are classed "essential". The distribution system followed onboard is "insulated neutral" system. The main priority onboard is the safety of ship which includes navigation & fire safety..etc. If due to Earth fault, the machinery classed as"essential" gets isolated, say for eg: steering gear, then the safety of ship is at question, which may lead to collison, grounding, fire & pollution etc..So the priority onboard ship is to maintain the continuity of the supply to the machinery in the event of "single Earth fault occurring". Continues..

 

[William White]

There was a time when some electrical equipment had internal fuses in both the live and the neutral lines. I can't think why that idea ever caught on. I have actually used such equipment which had been modded to eliminate the neutral fuse.

In the UK there are STILL many old cast iron cut-outs with a very heavy fuse-wire in the neutral.

Hangover from the days of DC.

The DNOs were given until end-2013 to comply with the ESQCRs; but many still remain and get noticed every so often when the cut-out blows, or when an electrician is doing some work on the consumer unit and notices it.

 

[FactChecker]

Simple example... I once repaired a spotlight lamp in a garage. When you operated the switch, it would go from bright to dim instead of from on to off. I found that the switch had been wired into the neutral side. The lamp current was obviously finding another path to ground when the neutral was interrupted. I moved the switch to the hot side, problem solved.

That sounds dangerous. Like the kind of thing where you might touch the spotlight with one hand and something correctly grounded with the other and get a serious shock.

 

[Nugatory]

That sounds dangerous. Like the kind of thing where you might touch the spotlight with one hand and something correctly grounded with the other and get a serious shock.

No kidding that's dangerous. It's one of the many reasons why switched neutrals are evil.

My first switched neutral story involves standing on damp concrete while working on an overhead fixture. The experience was exciting, and not in a good way.

 

[nsaspook]

Google found this..

http://www.brighthubengineering.com/marine-engines-machinery/38231-electrical-grounding-on-ships/

True, the safety of personal was secondary to the operational safety of the ship. To operate commercial equipment with a grounded neutral we always used a isolation transformer to prevent a 1 wire short light on the distribution panel for the space.

 

[mr166]

An ungrounded electrical system is probably safer than a grounded one when you are in total control of the entire system. In theory, in a system powered by an isolation transformer you could stand in water and touch either leg and not get a shock. In a grid you have no such controls and one leg or the other will become grounded someplace so it is better to define which leg is grounded. Also, using the Earth as one conductor, does reduce wiring costs for the power companies.

 

[William White]

There are installations that have to be isolated from "grid" Earth - explosive atmospheres for example (petrol station pits).

Using the Earth as a return path (SWER) is used in remote areas in countries like Australia.

 

[mr166]

WW you are 100% wrong. In the US the average home is fed by a 11KV or so feeder line in which one leg is grounded. This 11KV is fed to one leg of a transformer whose other leg is grounded. The secondary consists of a 220-240 volt winding with a grounded center tap. Thus the average home is fed with 2 110-120 volt legs that are both referenced to ground.

 

[Aaron w.]

Why must a switch be at the live wire instead of the neutral wire?

Alternating current travels in a circular path along the "hot" and the neutral conductor. The switch breaks the circuit in the hot.

The purpose of the hot conductor is to feed the device (outlet, appliance, electrical panel etc...) with voltage. The purpose of the neutral is to carry the electricity BACK to the source. (The transformer. Many of them are on poles or in big green boxes on the ground.) if the neutral was broken, the device being controlled by the switch would still be energized because the electricity had already reached it. Hope that makes sense!

 

[William White]

WW you are 100% wrong.

where?

 

[mr166]

Sorry WW you weren't exactly wrong. I just poorly pointed out that
SWER is used in the US also.

 

[puf_the_majic_dragon]

So, forgive my ignorance, but I think I'm going to take this opportunity to educate myself just a little bit about AC grid electricity.
While I've done a lot of DIY electrical, and of course I read the instructions so I always wired up the hot and neutral correctly, I've never fully understood the difference between hot and neutral in AC from a physics perspective. Granted, most of my experience and knowledge is all in the home, after the mains - the description of the US grid using SWER helps a little bit...

So, basically, a generator has a magnet more or less "pushing" electrons around a coil of wire. In order for those electrons to respond to the "push", they need a place to go, hence a closed circuit with a source and return. Now I understood AC generators to more-or-less change the direction that they "push" the electrons in, which would mean that on the down phase the return wire became the source wire, and thus both wires could be "hot". But if the neutral is earthed, whether at the breaker panel or a grid transformer, then it could never possibly be hot - which leaves me wondering where the heck does the hot wire in an AC grid get its electrons? My hypothesis might be that on the down phase the generator is "pulling" electrons instead of pushing them, kind of creating an electron vacuum along the coil. Yes? No? I know I'm using layman's parlance, but I'm not afraid of a more technical explanation.

And another question - in an SWER scenario, what's to prevent the grounded neutral from acting as a short, especially with a capacitive load?

 

[sophiecentaur]

So, basically, a generator has a magnet more or less "pushing" electrons around a coil of wire. In order for those electrons to respond to the "push", they need a place to go, hence a closed circuit with a source and return. Now I understood AC generators to more-or-less change the direction that they "push" the electrons in, which would mean that on the down phase the return wire became the source wire, and thus both wires could be "hot". But if the neutral is earthed, whether at the breaker panel or a grid transformer, then it could never possibly be hot - which leaves me wondering where the heck does the hot wire in an AC grid get its electrons? My hypothesis might be that on the down phase the generator is "pulling" electrons instead of pushing them, kind of creating an electron vacuum along the coil. Yes? No? I know I'm using layman's parlance, but I'm not afraid of a more technical explanation.

It's really best if you realize that, as far as the generator and the load are concerned, there is no difference or order of importance between the two conductors. The Potential Difference between them just alternates at mains frequency. The fact that one of the wires happens to be connected to (or near) Earth has no effect whatsoever on the way the circuit works. Current flows alternately clockwise and anticlockwise. If you connect your 'scope probe to the live wire, its voltage will oscillate about zero volts and the neutral wire will have, nominally, no voltage swing relative to Earth. If you uncouple the Neutral back at the transformer, the two voltages will still waggle about and the difference between the two voltage waveforms will still the 'mains voltage' waverform but the absolute value of the mean between the voltages could be anything, depending on where one of the lines happens to be connected. The term Potential Difference says it all.

PS, many people feel that using the term 'electrons' somehow makes posts more learned or easier to understand. If you look at the wording used by 'people wot know', you will notice that they very seldom use electrons in explanations. They nearly always use the terms 'Current' and 'Charge'. because the nature of the charge carriers is not relevant in 99.9% of circuits. Electrons are far too hard to cope with when dealing with circuits. They do so many strange things.

 

[puf_the_majic_dragon]

It's really best if you realize that, as far as the generator and the load are concerned, there is no difference or order of importance between the two conductors.

Which is what confuses me, since that means the "neutral" is no different from the "hot".

PS, many people feel that using the term 'electrons' somehow makes posts more learned or easier to understand. If you look at the wording used by 'people wot know', you will notice that they very seldom use electrons in explanations. They nearly always use the terms 'Current' and 'Charge'. because the nature of the charge carriers is not relevant in 99.9% of circuits. Electrons are far too hard to cope with when dealing with circuits. They do so many strange things.

"Current" and "Charge", in this context, are rather abstract terms. I've known way too many electricians who use the words "current" and "charge" and don't have a clue what an electron is, much less what either word really means. Of course electrons do crazy things - this is a physics forum, after all, so why should we not talk about the crazy things they do?
So in physics, "current" describes the flow of electrons, and "charge" describes the number of electrons. "Potential difference" just describes a state where there are extra electrons on one end of a conductor and a shortage of electrons on the other end, and therefore the electrons will have a tendency to flow from higher to lower in order equalize the charge over the length of the conductor. If the local power plant operated strictly off of an electrical potential, electrons could move in either direction along the conductor in order to equalize charge - but that is not the case, as the direction those electrons move is strictly enforced (by the motion of the magnetic field of the generator). And those electrons can't flow if there is not a closed circuit between the point of negative net charge and the point of positive net charge.

Now, obviously there is a significant difference between the hot and neutral conductors, or this whole thread would make absolutely no sense, as a switch would have to switch *both* in order to completely disconnect the load. But the neutral is a dead wire when the hot is disconnected and, apparently, it just gets buried in the dirt at some point. So if we go back to the power plant, that generator has a giant coil around a spinning magnet and one end of that coil runs off to power your house - where does the other end of that coil go? Since it doesn't also run back to your house to close the circuit, it's got to close the circuit another way - is it just buried in the dirt as well? How does that actually work in order to form a closed circuit?

 

[sophiecentaur]

and don't have a clue what an electron is,

Exactly. And neither, I suggest, do you have a solid grasp of what they truly are (along with the vast majority of us). I would venture to say that you probably see them as tiny charge carriers. Does your understanding the way circuits work involve anything more than the charge on an electron? So why not avoid the term - which can only add complication - and use the accepted terms, using the Joule and the Amp instead of the number and rate of electrons? Like I said, it's common practice and there's a very good reason for it. It is not a cop-out to use the 'right' terms.

 

[sophiecentaur]

Now, obviously there is a significant difference between the hot and neutral conductors

Actually, as far as the circuit operation is concerned, there is no significant difference at all. The difference is in only terms of the way the distribution system works and relates to safety and protection of devices and cables - purely practical matters. The reason that the 'neutral' issue is raised so frequently on PF is that people don't start at the beginning of the story (basic circuits) and apply that rigorously when moving to AC and then to Mains supply. If only people used the term Potential Difference all the time (a long winded term which is shortened to Voltage) then the confusion would be less likely to arise. This would be because the word 'Difference' is so important. You could mount an AC generator and load on an insulated platform which is held at +5kV above Earth and the system would operate in exactly the same way, the 'electrons would all go in exactly the same direction and in the same numbers. Or you could connect one of the transmission cables to the +5kV source (relative to Earth) and the same thing would apply.

 

[Dale]

"Current" and "Charge", in this context, are rather abstract terms. I've known way too many electricians who use the words "current" and "charge" and don't have a clue what an electron is, much less what either word really means. Of course electrons do crazy things - this is a physics forum, after all, so why should we not talk about the crazy things they do?
So in physics, "current" describes the flow of electrons, and "charge" describes the number of electrons.

In many circuits, including circuits with batteries or other electrolytic conductors, the charge carriers include positive ions. So the terms "charge" and "current" are more corect and general than "number of electrons" and "flow of electrons". The electromagnetic effect of a current or charge typically does not depend on the sign of the charge carriers.

 

[Dale]

So if we go back to the power plant, that generator has a giant coil around a spinning magnet and one end of that coil runs off to power your house - where does the other end of that coil go?

Usually a power plant generator will have three coils, each producing a voltage 120 degrees out of phase with the other and all three referenced to a common neutral wire. The common neutral wire is usually grounded for safety and in normal conditions carries no current. If you look at power distribution lines you will usually see three large lines and a fourth much smaller line.

 

[sophiecentaur]

Whilst the above is all true, might it not be a lot to swallow for someone who thinks that, in a single phase system, there I s a fundamental difference between the two wires?

 

[Dale]

Probably. From a physics/electronics standpoint I agree that there is no fundamental difference whatsoever between the two wires. The only difference is with respect to safety, as described above.

The generator/fields/charge/current doesn't care if the load is an appliance or a human body. Power engineers do.

 

[William White]

Whilst the above is all true, might it not be a lot to swallow for someone who thinks that, in a single phase system, there I s a fundamental difference between the two wires?

in a real-world practical sense, there is a working difference: if you break the neutral, you've got a world of pain; if you touch the live you've got a world of pain!

in a three phase system breaking the neutral is not only very dangerous to people, it damages electrical equipment and can end up being a real nightmare
the conductors are clearly labelled differently for a reason, placed and protected differently for a reason, and defined in law as different for a reason.

 

[sophiecentaur]

If you read the contents of some of these posts, you can see that the 'difference' between the functions of the two conductors is actually perceived as fundamental. That was what I try to correct.
The people who 'know' do not seem to see what the problem is for the people who 'do not know'. To be helpful, PF needs to be aware of more than just the Engineering Facts. The Misconceptions are important.

 

[Nugatory]

Which is what confuses me, since that means the "neutral" is no different from the "hot".

You can interpret "neutral" as meaning "always at the same potential as the Earth nearby"; we make it so by connecting the neutral wire directly to the Earth (for example, by connecting it to a copper stake driven several meters into the ground).

The generator maintains a potential difference between the two wires. For a North American 240 V 60 Hz system, the potential is a sine wave with 60Hz frequency and peak-to-peak amplitude 310 V (Yes, 310! 240 is the RMS average potential across one full cycle. Google for "root mean square" if that doesn't make sense). In other words, the potential difference between the two wires is given by $V(t)=155\sin120\pi{t}$ and as far as the generator is concerned, the two wires are indeed not different in any way.

So if one wire is at the same potential as the earth, and the potential difference between the two wires is given by $V(t)$, then it must follow that the potential difference between the Earth and the other wire is also given by $V(t)$. That will be the hot wire. If you touch the hot wire while touching anything grounded, you'll be shocked; if you touch the neutral you won't. (Don't try this at home! The perfectly neutral neutral wire is an idealization even if some clown hasn't screwed everything up by putting a switch or fuse in the neutral, which is what this thread started out to be about).And to answer your original question: The current flow is through the hot and neutral, with the charge carriers moving first in one direction then the other. There's not a lot of current flow through the connection between the ground and the neutral, just whatever is needed to keep them at the same potential relative to one another while the potential of the hot wire oscillates relative to both.

 

[sophiecentaur]

And, on the subject of the charge carriers moving one way and then the other - how far will they actually move when they have only 1/100s, moving at a mean speed of a few mm/s??

 

[puf_the_majic_dragon]

You can interpret "neutral" as meaning "always at the same potential as the Earth nearby"; we make it so by connecting the neutral wire directly to the Earth (for example, by connecting it to a copper stake driven several meters into the ground).

The generator maintains a potential difference between the two wires. For a North American 240 V 60 Hz system, the potential is a sine wave with 60Hz frequency and peak-to-peak amplitude 310 V (Yes, 310! 240 is the RMS average potential across one full cycle. Google for "root mean square" if that doesn't make sense). In other words, the potential difference between the two wires is given by $V(t)=155\sin120\pi{t}$ and as far as the generator is concerned, the two wires are indeed not different in any way.

So if one wire is at the same potential as the earth, and the potential difference between the two wires is given by $V(t)$, then it must follow that the potential difference between the Earth and the other wire is also given by $V(t)$. That will be the hot wire. If you touch the hot wire while touching anything grounded, you'll be shocked; if you touch the neutral you won't. (Don't try this at home! The perfectly neutral neutral wire is an idealization even if some clown hasn't screwed everything up by putting a switch or fuse in the neutral, which is what this thread started out to be about).

And to answer your original question: The current flow is through the hot and neutral, with the charge carriers moving first in one direction then the other. There's not a lot of current flow through the connection between the ground and the neutral, just whatever is needed to keep them at the same potential relative to one another while the potential of the hot wire oscillates relative to both.

THIS answered my question :) and in such an elegantly simple way (way more simple than I was expecting).
And yeah, I did kind of hijack this thread, but I rationalize it because this understanding puts all of the other answers to the OP into a context. Your answer to my question answers the OP way better than any of the other responses did.

 

[NascentOxygen]

The generator maintains a potential difference between the two wires. For a North American 240 V 60 Hz system, the potential is a sine wave with 60Hz frequency and peak-to-peak amplitude 310 V (Yes, 310! 240 is the RMS average potential across one full cycle. Google for "root mean square" if that doesn't make sense). In other words, the potential difference between the two wires is given by $V(t)=155\sin120\pi{t}$ and as far as the generator is concerned, the two wires are indeed not different in any way.

Something is amiss here. Your figures fit a 110VAC supply with its peak value of 155V and peak-peak of 310V.

Any voltage difference of 240VAC 60Hz sine wave is $V(t)=339\sin120\pi t$, exhibiting a peak-peak amplitude of 678V.

 

[Nugatory]

Something is amiss here. Your figures fit a 110VAC supply with its peak value of 155V and peak-peak of 310V.

Any voltage difference of 240VAC 60Hz sine wave is $V(t)=339\sin120\pi t$, exhibiting a peak-peak amplitude of 678V.

Yeah, yeah, yeah... I slipped into 110V thinking there. Thanks, good catch.