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Polarized 1-15P plugs will fit 5-15R grounded receptacles, which have the same wider slot for the neutral blade. Many plugs manufactured since 1948 are polarized; the neutral blade is 5⁄16 in or 7.9 mm wide, 1⁄16 in or 1.6 mm wider than the line blade, so the plug can be inserted only one way. The neutral blade on 5-15P plugs is not always wider than the line blade, since the ground pin enforces polarity. A major barrier to wider adoption of merchant transmission is the difficulty in identifying who benefits from the facility so that the beneficiaries pay the toll. A three-phase wye-connected (four-conductor) transmission circuit. One of the other recommendations in the report was the introduction of the final ring circuit system (often informally called a "ring main"). This can be demonstrated by putting one end of a pair of wires in a hand drill and turning while maintaining moderate tension on the line.

More than one pantograph on a train can cause a similar problem when the leading pantograph head sets up a wave in the wire and the rear head can’t stay in contact. These prevent contact by objects like keys or paper clips inserted into the socket. In older plug designs both blades were the same width, so the plug could be inserted into the receptacle either way around. This is accomplished by an interlocking mechanism that requires hot and neutral blades inserted simultaneously to release the small doors blocking the slots. As commonly used, 10-30 and 10-50 plugs required the frame of the appliance to be indirectly grounded via a strap connecting to the neutral blade. The 5-20R receptacle has a T-shaped neutral hole, to accept both 5-15P and 5-20P plugs. The 5-15R and 5-20R are by far the most common electrical receptacle in North America in buildings built since the mid-twentieth century.

Internationally, the NEMA 5-15P plug and NEMA 5-15R receptacle are the basis for the International Electrotechnical Commission's IEC 60906-2 standard IEC system of plugs and sockets-outlets for household and similar purposes - Part 2: Plugs and socket-outlets 15 A 125 V a.c. The Japanese plug and socket with narrow insulating faces appear and work physically identical to NEMA 1-15, and such non-grounded receptacles are still common in Japan (though grounded 5-15R and 5-20R receptacles are slowly becoming more common). Polarized 1-15P plugs will not fit into unpolarized receptacles, which possess only narrow slots. Some devices that do not distinguish between neutral and line, such as internally isolated AC adapters, are still produced with unpolarized narrow blades. Although standards exist for 2-15, 2-20 and 2-30, this series is obsolete, and only Hubbell still manufactures 2-20 devices (for repair purposes). NEMA L20 series devices are three-pole, four-wire, non-grounding devices for three-phase 347/600-volt devices. NEMA 20 series devices are specified for 347/600Y three-pole, four-wire, what is electric cable non-grounding devices. This series of devices is specified for 600-volt, two-wire, non-grounding devices. This series of devices is specified for 277-volt, two-wire, non-grounding devices. There are however NEMA L22 series locking devices for 20- and 30-ampere devices specified and available for these applications.

These larger sizes are uncommon, as twist-locking plugs are generally used for high-current applications. There are some obsolete 1-15R or 1-20R (120 V, 15 or 20 A) receptacles which are mechanically able to accept 1-15P (120 V), 1-20P (120 V), 2-15P (240 V), or 2-20P (240 V) plugs. A non-polarized 1-15P plug has 2 identical blades. Generally the plug is the movable connector attached to an electrically operated device's mains cable, and the socket is fixed on equipment or a building structure and connected to an energised electrical circuit. In the UK, this clock connector seems to be the only option but only if the socket is sunk into the wall. In stage lighting for film and theater, this connector is sometimes informally known as PBG (Parallel Blade with Ground), U-ground, Edison or Hubbell, the name of a common manufacturer. NEMA 10s are classified as 125/250 V non-grounding (hot-hot-neutral), and were designed to be used in a manner that indirectly grounds the appliance frame to the neutral, which was common before the requirement of a separate safety ground was incorporated in the National Electrical Code. The Electrical Safety Foundation International has stated: "Never remove the ground pin (the third prong) to make a three-prong plug fit a two-prong outlet".