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In the two-way switch below, the bulb can be connected to either two hot wires or two neutral wires to turn it off

What will happen if a bulb is connected to two live wires? Will the current flow through it, and will the bulb be damaged soon?

two way switch

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    \$\begingroup\$ bulb is connected to two live wires ... but it's not connected to two live wires ... both sides are connected to only one wire \$\endgroup\$
    – jsotola
    Apr 14 at 5:11
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    \$\begingroup\$ This setup could be very risky. I'd suggest avoiding it for safety reasons. If someone tries to change a damaged bulb while thinking the power is off - they might still get shocked or trip the GFCI (but not guaranteed). Normally, you'd operate the light by toggling the switches oppositely (one up, one down), but standard two-way (staircase) wiring should only switch the live wire, much like an XOR/XNOR logic gate. This makes it safer, ensuring the live wire is disconnected when the light is off. You can find such safer diagrams online. \$\endgroup\$
    – 15 Volts
    Apr 14 at 5:27
  • \$\begingroup\$ If you want to experiment with electricity, I recommend using something around 12 V instead of mains. You can easily get 12 V from a mains adapter, and 12 V bulbs are readily available. A switch rated for 240 V AC will also work for 12 V DC bulbs. \$\endgroup\$ Apr 14 at 14:08

3 Answers 3

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It's an unconventional 'staircase light' circuit. enter image description here

Both the 'line' and the 'neutral' are switched.

This circuit is not recommended for safety reasons since, in one 'bulb off' position, both the terminals of the bulb are 'live'.

The recommended circuit is as shown below.

enter image description here

In this case, only the 'line' is switched.

It is safer since the bulb terminals are not 'live' when the bulb is switched off.

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What will happen if a bulb is connected to two live wires? Will the current flow through it, and will the bulb be damaged soon?

Nothing, no, and no.

The voltage that the light bulb experiences is the difference between the potential that its first terminal is connected to and the potential that its second terminal is connected to. Both terminals are connected to the same potential (namely, the potential of the live wire), so the difference between the potentials is 0 V, which means that the voltage that the light bulb experiences is 0 V, so nothing will happen.

However, for safety reasons, it's good practice to never connect the "neutral" terminal of the light fixture to a live wire, because of the risk that a human will accidentally touch the socket and get shocked. This way of connecting a light fixture is illegal in some places.

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In your setup, no current will flow through the bulb since both hot wires are obviously derived from the same phase. If they were two of three different phases, then you would get the line voltage, \$\sqrt{3}\$ times the phase voltage; if they were opposite phases from a split-phase setup, then you would have double the line voltage.

This setup was also known as the Carter system for controlling a light with two switches. It is now no longer allowed (at least in the United States) since the light can be off (as in not illuminated) but still energized and dangerous.

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    \$\begingroup\$ Oop yep good catch; changed in post to reflect this. Thanks! \$\endgroup\$
    – vir
    Apr 14 at 13:07
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    \$\begingroup\$ \\\$\sqrt{3}\\\$ will give \$\sqrt{3}\$. (Escaping the codes with extra \ allows the mathjax to be displayed.) \$\endgroup\$
    – Graham Nye
    Apr 14 at 13:55

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