All ac power cords sold in North America must meet a NEMA standard and UL standards that spell out factors such as the minimum wire gage and the connector style. Most cords sold for industrial use, and even many of the cords targeting home appliances, have a third contact in addition to the power and ground connections. When the third prong plugs into the wall socket, and assuming the building has been wired correctly, the prong makes contact with earth ground.

As a quick review, neutral is the return path for an ac circuit. In normal conditions, it carries the same amount of current as the ac hot wire. In three-phase systems, the neutral wire may not carry the same amount of current as the hot wire because of factors such as phase current imbalances. In contrast, earth ground connections carry leakage or residual currents. Any current flowing in the earth ground lead should be coming from secondary sources. Current flowing on the earth ground wire is usually much less than that on the neutral connection and is generally on the order of milliamps.

One of the two main prongs on the plug is also wider than the other. The wider prong is sized so it fits in the grounded side of the ac outlet. Again, this assumes the outlet was wired correctly. Two-prong versions of these plugs are common in simple appliances like table lamps.

A differentiating factor for power cords is the connector on the end that goes to the instrument or appliance drawing power. The formal name for that connector is the appliance coupler. There are several niche versions of appliance coupler connections for arcane uses like enterprise-class servers. Some Xbox 360 game boxes also have their own special coupler connector.

But most PCs and test instruments these days have what’s called a C13 and C14 connector though the moniker is seldom used. Perhaps the most important thing about this connector is that it is rated to handle ten amps.

One other appliance coupler in wide use is the C5 which is more frequently called a clover leaf or mickey mouse connector because of its shape. Small switch-mode power supplies as often find use in laptops may use this type of connection. But a point to note is that this style coupler is only rated to handle two and half amps, not the ten amps specified for rectangular couplers.

The other widely used version of this is unpolarized and is called the C7. It has only two connections so it can only be used with equipment that has a double-insulated power supply. Again, this one handles only 2.5 A.

Finally, there is a variation of this version that has one squared side for the sake of polarization. But this little variation isn’t a standard version — there’s no C designation for it.