no ground connection but will fit a grounding NEMA 5-15 receptacle. Early versions were not polarised, but most plugs are polarised today via a wider neutral blade. (Unpolarised AC adaptors are a common exception.) Harvey Hubbell patented a parallel blade plug in 1913, where the blades were equal width (). In 1916 Hubbell received a patent for a polarised version where one blade was both longer and wider than the other (), in the polarised version of NEMA 1-15, introduced in the 1950s, both blades are the same length, only the width varies. Ungrounded NEMA-1 outlets are not permitted in new construction in the United States and Canada, but can still be found in older buildings. NEMA 5-15 grounded (Type B) The NEMA 5-15 plug has two flat parallel blades like NEMA 1-15, and a ground (earth) pin. It is rated 15 A at 125 volts. The ground pin is longer than the line and neutral blades, such that an inserted plug connects to ground before power. Both current-carrying blades on grounding plugs are normally narrow, since the ground pin enforces polarity. This socket is recommended in IEC standard 60906-2 for 120-volt 60 Hz installations. The National Electrical Contractors Association's National Electrical Installation Standards (NECA 130-2010) recommends that sockets are mounted with the ground hole up, such that a falling object on a partially inserted connector contacts the ground pin first, however, the inverted orientation (with ground pin downwards) is more commonly used. Tamper-resistant sockets may be required in new residential construction, with shutters on the power blade sockets to prevent contact by objects inserted into the socket. In stage lighting, this connector is sometimes known as PBG for Parallel Blade with Ground, Edison or Hubbell, the name of a common manufacturer. NEMA 5-20 The NEMA 5-20 AP variant has blades perpendicular to each other. The receptacle has a T-slot for the neutral blade which accepts either 15 A parallel-blade plugs or 20 A plugs. Other NEMA types 30- and 50-amp rated sockets are often used for high-current appliances such as clothes dryers and electric stoves. JIS C 8303, Class II unearthed The Japanese plug and socket appear physically identical to NEMA 1-15. The Japanese system incorporates stricter dimensional requirements for the plug housing, different marking requirements—for example, the ground side of the Alexa smart plug is marked with a clearly identifiable "N"—and mandatory testing and approval by METI or