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What is the purpose of switching both load and neutral lines of AC power as opposed to just switching the load wire?

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    \$\begingroup\$ Safety. Who know how and with which polarity it is connected to the mains.... \$\endgroup\$ – Eugene Sh. Aug 12 '16 at 16:18
  • \$\begingroup\$ So does that mean I should have a fuse on both wires too? I mean, if the load and neutral are reversed and there's a short, a fuse on neutral won't save someone from being electrocuted. \$\endgroup\$ – squarewav Aug 12 '16 at 16:47
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    \$\begingroup\$ Fuses usually don't protect from electrocuting, they protect the equipment from high currents. The current passing through a human connected to the mains is pretty low and won't blow a fuse.. \$\endgroup\$ – Eugene Sh. Aug 12 '16 at 16:50
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    \$\begingroup\$ Also, there are situations where you can have 2 hot lines. Like residential 230VAC in the USA, for example, or any circuit drawing two phases out of a 3-phase power source. \$\endgroup\$ – bitsmack Aug 12 '16 at 19:10
  • \$\begingroup\$ My answer to [this question] (electronics.stackexchange.com/questions/211010/…) while not a direct answer may help a little in explaining some potential fault conditions. \$\endgroup\$ – Transistor Aug 13 '16 at 16:28
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If there is a fault where the supply active is shorted to the supply neutral, the neutral will go live.

If there is a fault where supply active and supply neutral are swapped, the neutral will go live.

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  • \$\begingroup\$ If the supply live is shorted to the supply neutral, then there is a short circuit and emergency switch will turn the supply live off in a fraction of second. \$\endgroup\$ – Incnis Mrsi Aug 28 '16 at 18:01
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The purpose is to isolate the circuit such that no voltage is present on the live or neutral to make it safe for maintenance etc.. As @Incnis Mrsi says you can't take the potential on neutral for granted, the system may have earth sat between live and neutral (where earth may be to a centre tap on the substation transformer) or there may just be significant volts between neutral and earth where you are on the circuit (this is common where the supply doesn't provide an earth).

It is worth noting that for these reasons it is quite possible to trip an incoming RCD if you touch neutral.

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One shouldn’t take a low potential on the N wire for granted. If this wire failed (got open) somewhere between the source and consumer, with some load present between N and L, then the N conductor will effectively become live in a part of the grid connected to the load. Think of it as of extreme case of voltage divider:

source L————————————————————————[load]
source N———————— (fail) ————————————[  ]
         all voltage drops here  live!

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    \$\begingroup\$ The editor toolbar has a built-in schematic editor to save us all from ASCII diagrams. ;^) \$\endgroup\$ – Transistor Sep 27 '16 at 19:00
  • \$\begingroup\$ @Transistor: Not cute. First, “—” and “ ” are not ASCII. Second, my text labels are searchable by Internet index services. \$\endgroup\$ – Incnis Mrsi Sep 28 '16 at 8:43

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