Why can't the neutral and live wires be exchanged when wiring a plug?

  • 1
    Because touching the live wire will kill or injure you. Touching the neutral won't. And people want know where the things that can kill are located.
    – Eugene Sh.
    Nov 3 '16 at 15:51
  • Because simple switches only switch one of the wires. Cutting power to the neutral wire will e.g. switch of a light bulb but still have the socket at dangerous voltage. Users could be fooled into thinking that the socket is safe to touch.
    – 0x6d64
    Nov 3 '16 at 16:02
  • It is arbitrary in any country just like driving on the right side of the road. The 2 pronged units don't care since they have "double insulation" whereas some 3 pronged units care about which side is hot with respect to ground for internal reasons such as a switch to remove unsafe voltages when off near human fingers when servicing it. Nov 3 '16 at 16:03
  • 1
    @TonyStewart.EEsince'75: only where grounding is done in a way that the plug cares about the orientation. There are countries where you just turn it by 180° and you exchanged live and neutral.
    – PlasmaHH
    Nov 3 '16 at 16:13
  • @PlasmaHH Yep! Definitely! In Italy plugs are symmetric: the prongs are three in a line and the center one is mains earth. They can be inserted either way in the socket, so the appliance "cannot know" which of the two "external" prongs is live and which is neutral. Nov 3 '16 at 17:16

There are two fundamental requirements for safety unless the design of a device guarantees that no electrified portions will have a path to ground other than via the neutral:

  1. The hot wire needs to be fused unless the wire can pass 20 amps continuously.

  2. If the hot wire is disconnected, the neutral wire must be disconnected as well.

If a device didn't have a fuse on the hot wire, nothing would control the over-current in the event something shorted to ground. If the device fused both sides and--under a different failure condition--the neutral-side fuse blew before the hot fuse, the device could appear to be inert while its innards were all connected to the hot wire.

There are two says a device could be constructed to be safe without a polarized plug:

  1. Design it so that there is no plausible accidental current path anywhere except between the hot and neutral (whichever is which).

  2. Use a double-pole fuse which will open both sides under an over-current condition. While it's possible to construct fuses in such a fashion, such designs are much more complicated and expensive than fuses that merely need to open the wire carrying the excess current.

Using polarized plugs is often easier than doing either of the above.

  • (+1) especially for the mention to double-pole fuses: I didn't know such a thing existed! Always learning something here on EE.SE! :-) Nov 3 '16 at 17:18
  • @LorenzoDonati: Circuit breakers are counted as fusing elements, and double-pole breakers are pretty common. Common residential breakers in the US are designed so that a tie bar can connect a pair for double-pole operation, but they're mechanically more complex than the simpler breakers found in things like power strips.
    – supercat
    Nov 3 '16 at 17:38
  • I know double-pole breakers, but I thought you meant something different by "double-pole fuse" (I imagined some strange kind of "real" fuse that could interrupt two different circuit). It appears I was mislead by the terminology, then (I never heard a breaker described as a "fusing element"). Besides the obvious different language, in Italy when you talk about a "fusibile" (technically translated as "fuse") or "elemento fusibile" ("fusing element") you really mean a device where something melts down in order to break a circuit. Nov 3 '16 at 19:38
  • In other words, in the Italian technical usage, the fuse is a kind of (one-pole) breaker, not the other way around! Nov 3 '16 at 19:40
  • @LorenzoDonati: I've seen fuses where the fusible element holds a spring-loaded actuator in place, and have read of some where the actuator operates a contact (on the ones I saw, it caused a tab to pop out for easy visual identification). Two fuses rigged so that either would open a contact controlling the other would serve as a double-pole fuse; other slightly-simpler arrangements may be possible. I don't know that such things have been used since the invention of the modern circuit breaker, though.
    – supercat
    Nov 3 '16 at 20:09

Because the electrical code (at least in US) requires neutral and ground to be connected, usually at the service panel as shown below. While simple systems might not care which input to the load is live, many systems will have some non-trivial connection between their neutral input and ground.

Note, also that fuses will be more effective if they are located on the live wire than on the neutral.

NEC Wiring requirements

  • 1
    The majority of appliances however are built in a way to not care about which line is neutral and which is live because it is easier to have one model work for the whole planet.
    – PlasmaHH
    Nov 3 '16 at 16:15
  • @PlasmaHH - I agree 100%. But even if the majority do work that way, the remainder that don't can still burn down your house.
    – Seth
    Nov 3 '16 at 16:19
  • @Seth: a question: what's the point having an isolation transformer when you ground the secondary?
    – Janka
    Nov 3 '16 at 16:43
  • @Janka - Reduces common mode noise. NEC still requires neutral-ground connection, however.
    – Seth
    Nov 3 '16 at 18:40
  • Isn't the power company's transformer floating relative to the customer's ground connection?
    – supercat
    Nov 3 '16 at 20:11

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