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What happens exactly when AC neutral and active lines are switched ?

  1. Nov 7, 2010 #1

    I have a basic understanding of electrical wiring from when I was in school, and have a question that I have always wondered about that I hope someone could answer.

    My electrician mate tells me for household wiring its important for active and neutral wires not to be mixed up. That its common for 'cowboy's to get them mixed up.

    I believe him but don't understand why, because my limited understanding of mains power is that its AC, and that the voltage alternates across these wires, which suggests it doesn't matter if their mixed up.

    I describe my confusion to my mate , but he doesn't give me a clear answer.

    Why does it really matter ?

  2. jcsd
  3. Nov 7, 2010 #2
    Hello Steve, welcome to PF

    From the point of view of supplying most electrical devices with power it doesn't matter. They will still work.

    However, they would be potentially unsafe because both the power switch and the fuse/breaker are connected to the what is supposed to be the active (called line ) wire.

    So when you switch off or the breaker goes the active wire is disconnected from the apparatus.

    Swapping the wires so the neutral is connected to the switch /breaker leaves the active wire connected when you switch off or there is a fault, either of which could be seriously, even fatally, dangerous.
  4. Nov 7, 2010 #3
    Thanks Stu,

    So , in normal use of some circuits the neutral is connected permanently to the circuit ?

    So a neutral line won't kill you if you touch it ?

    I had thought it would, just that the electron flow be in the opposite way.

  5. Nov 7, 2010 #4


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    In the U.S. the neutral is connected to a ground rod driven into the earth. Normally when everything is installed correctly touching the neutral will not shock you. However, if the neutral is interupted (never permitted by the code) with a switch, fuse, etc., things change.
  6. Nov 7, 2010 #5
    The electricians' mantra: "White is the color that kills you". I think this is USofA specific and there are other color codes elsewhere, but our electrical code decrees that the White wire be the "neutral" which is eventually connected to earth-ground and thus has no "potential". Black and Red are the most common household wire colors which are "hot". If someone reverses the colors making White the hot line, the next person working on the wiring will grab the White wire thinking it's safe and discover differently.
  7. Nov 7, 2010 #6
    Are you quite sure?

    I am not wholly conversant with US standards, but that would be contrary to the electrical requirements in most countries, where a double pole disconnect switch is required at the supply point.

    Common sense and good safety practice suggests you should never grab a wire of any colour unless you have determined it is not live.
  8. Nov 7, 2010 #7


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    Yes I am sure. In the U.S. the neutral is NEVER interupted. Never.
  9. Nov 8, 2010 #8
    In the US there's usually a "two-phase" feed to a home -- two hot wires with a ground/neutral line from the pole. The two hot's are switched and fused at the service entry but as (not)Average says, the neutral is never switched.

    As to "colour" testing, your safety practicum {is,should be} the net result of my electrician's mantra...
  10. Nov 8, 2010 #9


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    Just to add one more safety item about hot/neutral in the US. Even with 2-prong plug devices, some of them have polarized plugs (the hot prong is narrower than the neutral plug). This is done for devices that need to reliably know which wire is hot and which is neutral.

    The most common example is light fixtures and lamps. A common screw-in light bulb fixture needs to have hot connected to the end button (down inside the screw fixture), and the less dangerous neutral connected to the outer screw part of the female receptacle. If hot were connected to the outer screw part of the female receptacle, it would be a significant shock hazard any time you went to replace a light bulb.
  11. Nov 8, 2010 #10
    This is a good guide to current safety considerations in european electronic equipment.

    http://www.elektor.com/magazine/construction-electrical-safety.83362.lynkx [Broken]
    Last edited by a moderator: May 5, 2017
  12. Nov 8, 2010 #11


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    Good link, Studiot. Could you post it in the stickie thread at the top of the EE forum on "Useful EE Links and Search Engines"? Thanks.
    Last edited by a moderator: May 5, 2017
  13. Nov 8, 2010 #12
    Generally speaking the white wire is ground but when a fixture like a ceiling light is wired sometimes the power is brought to it. Then a 2 wire cable is brought to a wall switch. The switch is connected to the white and black. This becomes an example of a case where the white wire is not ground and is a shock hazard.
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