So, I was told in my class that Safety devices like fuses should be connected on the live wire. But isn't the conventional current opposite to the direction of the flow of electrons? And what actually happening is that electrons are flowing into the load from the neutral wire? So how does that add up?
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$\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$– tpg2114Commented Jul 23, 2020 at 18:17
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3 Answers
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You may want to check out this answer on our sister site: https://electronics.stackexchange.com/questions/189867/ac-why-differentiate-between-ground-and-neutral
In summary here:
- "Ground" and "neutral" are both grounded in the sense of being attached to the actual earth somewhere in the vicinity of your house.
- Despite both being grounded, they are different. Under nominal conditions, the neutral wire is carrying significant current and the ground is carrying none.
- Wires aren't perfect conductors. So while ground and neutral are at equal potential somewhere - usually outside of your house - they are at different potentials at arbitrary points inside your house.
The ground is there as a safety only. If something, say your cloths dryer, shorts into its metal housing and the housing is connected to the safety ground, that's supposed to provide a low-resistance path to ground for just the shorted-out current (vs. all of the current your house is pulling). In particular, it's hoped to be lower resistance to ground than your body when you touch the metal housing.
The live wire, on the other hand, is being driven +/- hundreds of volts relative to the ground and neutral wires. Under normal conditions the direction of current through live and neutral reverses many times per second through a completed circuit connecting a load to live and neutral.
And now we get to your question. A short between neutral and ground is not great, but those wires will typically be at relatively similar voltages, so it's not as bad as a short between live and ground. A short between live and ground allows for much greater voltage difference and therefore greater current. Greater current, in turn, creates a variety of other hazards: For example, bad for you if you are part of the short, more heat in wires, etc. Putting the fuse (or circuit breaker) on the live side will cause it to blow in the more dangerous of the two cases.
This is not the only reason, but it's one good one to get you thinking in the right direction.
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$\begingroup$ Thank you, still a question: Does the current in our homes change directions once every 1/60th of a second? And if it does, So why does the live wire always remain the same? $\endgroup$– Doodoo28Commented Jul 23, 2020 at 16:23
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$\begingroup$ How often it changes depends on where you are, but AC power induces a current that changes direction. You seem confused by the fact that the "live" wire can be +120 V or -120 V peak to peak. (US - other places different values like 240 V). That causes current to flow in different directions, but the magnitude of the current is the same. The magnitude of the current, not the direction, is what causes most dangers to you and your house in a faulty circuit. The neutral is neutral because it's always approximately 0 V (see details above). In all cases voltage relative to local ground. $\endgroup$– BrickCommented Jul 23, 2020 at 16:27
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$\begingroup$ Thank you. One last side question that isn't related to the topic: In my country, the AC varies at 50Hz and the standard voltage is 230V. I have a habit of roaming around the mains and observing the electricity meter. It displays the the power, Current entering my house and the voltage. Is voltage variation a common thing? Sometimes I observe readings as High as 260V and as low as 220V. What are the possible reasons? $\endgroup$– Doodoo28Commented Jul 23, 2020 at 16:33
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$\begingroup$ Yes, it's common that voltage is only within a range of the "nominal" value given for your country. Typical reason is that the wires are not perfect conductors, as in my answer and the link it provides. Your last side question probably has a full Q&A devoted to it either her or on electronics.SE. $\endgroup$– BrickCommented Jul 23, 2020 at 16:36
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When an overcurrent protection device like a fuse is placed in the ungrounded (live) circuit conductor it disconnects line voltage from all downstream conductors and electrical components when it opens, making all downstream circuits dead and not a shock hazard.
Hope this helps
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$\begingroup$ What if we hook the fuse up to the neutral wire? $\endgroup$– Doodoo28Commented Jul 23, 2020 at 16:02
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$\begingroup$ @user810213, Then, when the fuse blows, current will stop flowing, but everything in the branch circuit, including the supposed-to-be-neutral wire, still will be connected to "hot." At that point, if you put one hand on a water pipe, and your other hand on the supposed-to-be-neutral wire, current will flow, in a loop, from the transformer's actually neutral wire, through the Earth, through the water pipe, through your heart, through the supposed-to-be-neutral branch circuit wire, through the load, through the "hot" wire, and back to the transformer... And, of course, it will reverse at 60Hz. $\endgroup$ Commented Jul 23, 2020 at 16:07
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$\begingroup$ @user810213 I guess Solomon Slow answered the question meant for me. I would add that while the fuse will cut off the power say to an appliance regardless of its location, it will not remove hazardous voltages within the appliance if it is connected to the grounded (neutral) conductor. One might get a false sense of security that the appliance is safe because the fuse opened, whereas the potential for electric shock remains. The same requirement is often also applied to single pole switches- i.e. they should be connected to the ungrounded (live) circuit conductor. $\endgroup$– Bob DCommented Jul 23, 2020 at 16:37
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you call it the life wire , so it is an AC current, the second pole than is usually earth. If you put the fuse on the earth part, and you touch the life wire you still are exposed to the full voltage and you are the best connection to earth.
