If an AC reverses direction 60 Hz then why are there different-sized receptacle slots for hot and neutral since v and i are alternating direction each half cycle?

Answer 1

It has been stated that: "AC outlets have polarity specific outlets because some devices can be ruined if the device is plugged in backwards." Although this might be true in some sense, the most important reason is enhanced user safety. This is a very good question; one that deserves a good answer. It is true that most household "alternating current" devices may work with a reversible (two-prong) plug inserted either way. It simply doesn't matter which way the current is going at any given moment. Many devices still have two identical prongs. The "polarized" plug is for extra user safety (and a three-prong plug is even safer, see below). Two-prong devices may be eventually phased out of production in many countries. AC outlets have polarity-specific sockets because some devices can easily become dangerous if the polarized device is plugged in backwards. This happens when the switch or fuse inside the appliance is designed to disconnect only the "hot" and not the "neutral" wire when the device is switched off because it is cheaper than switches that disconnect both wires. The "neutral" wire is typically connected to ground at the main panel, so it is safer to use the switch to open the "hot" wire. Also, in lamps (and light fixtures), the neutral should be connected to the lamp shell for added safety, and this could also become dangerous if reversed; the socket shell could still be "hot" even when the switch is off (and the light is out). Without a polarized plug, you can't tell which wire the switch will disconnect and may receive a shocking surprise! In theory, safe devices are designed to keep a user from touching either the hot or the neutral. In practice, if designers know which wires will be "hot" inside the device, they can take extra precautions to make sure they cannot break apart and electrify anything a user can touch. A broken "hot" wire can electrocute a user if he/she touches the metal enclosure and the user "completes" the circuit to ground. Therefore, many appliances now have a polarized plug so the switch always disconnects the "hot" circuit and extra attention is placed on safe disconnection and insulation of that conductor throughout the device. A so-called "double-insulated" (IEC Class II) device may have a non-polarized plug because the same safe design has been made for BOTH conductors, regardless of which one may be "hot", and without relying upon a third prong for safety ground of the enclosure. No single internal fault would be likely to cause an electrocution hazard. Furthermore, even with a polarized plug, a single internal fault can be deadly in an ordinary appliance. The third prong (ground) can save your life if there is an internal fault of the hot touching exposed metal, and a GFCI can save your life if there is an internal fault of the neutral to exposed metal (where you could otherwise become the missing "neutral" connection to ground as you are electrocuted).