Help with Understanding Mains Electricity Watch

Horseinahat
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Why is the neutral wire in a mains circuit not dangerous? In a circuit in the lab you can put the ammeter after the components and get a reading for the current.

Also, if we have an ac supply, why do the live and neutral wire not change over, in time with the changing direction of the alternating current ?
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LuigiMario
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it is a bit complex as you are talking about domestic mains electricity, light industrial mains electricity.

most power is generated, traditionally, from a three phase system, which has a neutral wire, the "return" for each of the 120 degree apart sinusoidal phases. This tends to be distributed at very or medium high voltage, 3 x 'live' with different phases & a single neutral.

When a single phase is broken out from the three-phase supply, you get say a "hot" live (which eventually connects back to the generators feed point via transformers & switchgear) and the neutral wire (this wire is 'close to ground', and in some installations it is actually earth bonded at the generator station)
In many (recent) UK installations - so called PME Protective Multiple Earthing systems https://www.voltimum.co.uk/articles/...tiple-earthing the neutral wire of a domestic system is earthed at the consumer unit, with a link, where the cable enters the house.


(that Voltium link is behind a paywall, so here's another website http://www.emfs.info/sources/distribution/uk/
ignore the 'magnetic fields stuff , just notice the "traditional" earth neutral at 3-phase generator & the newer, but not everywhere, PME systems http://www.emfs.info/sources/distribution/uk/)

but not everywhere, in some older houses it is not PME, and the neutral wire is 'close to ground' by being earthed at the neighbourhood power transformer, or somewhere, hopefully

you have to treat the neutral wire as dangerous, because a faulty piece of equipment could couple the full 'live' voltage to neutral, and if the system has a faulty earth wire (and where there is one fault, you often see a fault ladder of many faults) then you could get a shock from neutral.

the live and neutral are PHYSICALLY treated differently, one is more closely tied to the actual balanced (star or delta) output of a generation system, whilst the neutral is often earthed somewhere or manywhere.

(my house was wired up in one light fitting with live 'hot' connected to the metallic earth of the metal ceiling light fitting, and was like that for years, I found out whilst standing on top of a ladder - changing the bulb, so assume ALL wires are dangerous, in practise)
Last edited by LuigiMario; 4 months ago
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Horseinahat
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Thank you for your help. Unfortunately I don't really understand most of the answer. In practical terms I get that the neutral wire should be safe but may not be. However, I don't understand the theory of where the current actually goes. More help please!
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Joinedup
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(Original post by Horseinahat)
Thank you for your help. Unfortunately I don't really understand most of the answer. In practical terms I get that the neutral wire should be safe but may not be. However, I don't understand the theory of where the current actually goes. More help please!
Short version - the neutral is supposed to be at the same potential as earth* in normal conditions
but some faults could break the neutral conductor at which time if would be connected to line through the appliance and be at the same potential as line up to the point the break occurs.



*(or near to earth potential it because the neutral conductor can have some resistance and is carrying the same current as the line (used to be called live) conductor.

Some more info about domestic earthing in this video - that guy's done a few videos and draws a lot of diagrams which I think is useful.
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LuigiMario
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(Original post by Horseinahat)
Thank you for your help. Unfortunately I don't really understand most of the answer. In practical terms I get that the neutral wire should be safe {NO!} but may not be. However, I don't understand the theory of where the current actually goes. More help please!
OK, no problem! you did very well by asking the questions, that will get you far

so answering your first question Q.Why is the neutral wire in a mains circuit not dangerous?

facts: The neutral wire is not safe. (it is connected to ground or earth somewhere, but in practise that could be missing due to faults. Also neutral wires can be physically labelled as "neutral" by a nametape which may be put there wrongly, or by a colour code (blue?), but the wrong colour wire could have been used) (all the colours changed in 2006!)


answering your second question Q. you can put the ammeter after the components and get a reading for the current?

for your experiment, I don't have your circuit diagram, but you will have connected a "load" across your "live" and "neutral", and measured things like voltage & current. The conventional description of current is that it flows from the live wire, through the load (a resistor or a heater for example) then the same current flows back to the source through the neutral wire. Important components in your house, a safety protection system is nowadays used to check that the exact current that flows down the live wire, then flows back through your local neutral wire. This measuring device is called an RCD , residual current device, used to be called an ELCB, earth leakage circuit breaker.

if 3 amps flows down the live wire to your load, and 2.69 amps flows back through the neutral wire to the lab wiring, then 0.31 amps have gone somewhere else! maybe someone has touched a wire (do not do this test) and the RCD should cut the power for safety reasons. The balance check device senses that sent live current is equal to return neutral current is made in a little magnetic circuit, and can detect differences of 30 milliamps, in current flows of ten to twenty amps. This is a very important safety device.


your other question ac supply, why do the live and neutral wire not change over,
That's why I mentioned the four wire national grid, with three different live wires at 120 degree offset from each other, at a fixed phase angle at generation, and the fact that the neutral wire in your home comes from the neutral wire in the power distribution system, and your live wire is just one of the three possible phases. There is a sinewave of voltage (and current) when you measure between live and neutral, but it is not power alternating between these wires, it is a voltage that is increasing and decreasing fifty times per second, that is supplied by the live/phase wire, and this alternating current is sent back to the generator over the neutral wire. the live and neutral wires are physically different.







your home/domestic mains colours are here https://www.electriciancourses4u.co....athing-bs7671/
Last edited by LuigiMario; 4 months ago
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RogerOxon
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(Original post by Horseinahat)
Why is the neutral wire in a mains circuit not dangerous? In a circuit in the lab you can put the ammeter after the components and get a reading for the current.

Also, if we have an ac supply, why do the live and neutral wire not change over, in time with the changing direction of the alternating current ?
When there are no faults, neutral is close to ground potential, so can drive little current through a person.

It's the current that can be driven that is dangerous, which requires a larger potential difference (voltage). A low resistance wire can carry a lot of current with a very low voltage drop, which, if at a low PD to earth, would not be dangerous to touch.

Current travels through a device, so is the same before and after it (assuming no earth path in the device). The PD each side will be different - the live side is high, so dangerous, and the neutral side low (when correctly wired).

Live goes negative rather than neutral's potential difference changing. UK distribution uses three 240V phases, which sum to zero, i.e. neutral. Each house receives one live phase. In the US, each house receives two 120V lives, in antiphase, with 120V loads using only one phase, allocated to minimise the neutral current. 240V loads (US) span two lives, so don't have a neutral.
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