question: Why do some plugs have prongs that are the same size, while some have one prong slightly larger than the other?
comment: The ones that have differently sized prongs are like that because the holes on the socket are different. In some devices, it doesn't matter which prong goes into which hole, and so they have equally sized prongs. Other devices do care how they are plugged in, so they have a larger prong on one side to they can only be plugged in with one specific orientation.
passages: Within each size group, the plugs and sockets are keyed with certain "lugs" protruding on the outside of the plugs. Higher current plugs have more lugs to prevent them from being inserted into a lower current socket outlet, while still allowing them to be inserted into a socket outlet of the same size group rated for a higher current. For example, a 32 A 4-pin plug without neutral can plug into a 50 A 5-pin socket with neutral available.
   Most duplex receptacles have metal tabs connecting the top and bottom receptacles. These tabs are pre-scored so that they can be broken off to allow the top and bottom receptacles to be wired onto separate circuits. This may allow for one switched receptacle for a lamp, or for two separate supply circuits when heavy loads are anticipated. Two branch circuits may optionally share a common neutral wire terminating on duplex receptacles, a condition sometimes referred to as "split-wiring", "split-receptacle", or "half-split".
   Neglecting to break off the tabs when replacing a split receptacle can disable a switched outlet, or cause a short circuit, or accidentally parallel two circuits, depending on how the receptacle had been wired.
  
  Section::::Related standards.
   The contacts in the plug are simple cylinders (sleeves), while the pin contacts in the receptacle have the spring arrangement to hold contact pressure, the reverse of the IEC 60309 type connectors. All contacts are the same diameter. Originally metal construction was used, but now they are also made with plastic shells. Since only keying in the connector shell is used, and since the keys can be damaged in industrial use, it is possible to mis-match worn connectors.
  
  Section::::North America.:Pin and sleeve.:IEC.
   All NEMA 5 devices are three-wire grounding devices (hot-neutral-ground) rated for 125 V maximum, with the 5-15, 5-20 and 5-30 being grounded versions of the 1-15, 1-20 and 1-30, respectively. The addition is a diameter round or U-shaped ground pin, longer than the power blades (so the device is grounded before the power is connected) and located from them by edge-to-edge or center-to-center.
   6-15R and 6-20R receptacles are usually manufactured on the same assembly line as "Industrial" or "Commercial" grade 5-15R and 5-20R receptacles, with all 4 receptacles sharing the same "triple wipe" T contacts behind the varying faceplates. The faceplate bonded onto the receptacle determines the final configuration of the receptacle.
   Plugs generally have one or more pins or prongs that are inserted into openings in the mating socket. The connection between the mating metal parts must be sufficiently tight to make a good electrical connection and complete the circuit. An alternative type of plug and socket connection uses hyperboloid contacts, which makes a more reliable electrical connection. When working with multi-pin connectors, it is helpful to have a pinout diagram to identify the wire or circuit node connected to each pin.
   NEMA non-locking connectors all use blades of various flat and folded shapes (except for the round pins used on grounding connectors). The plugs can be detached from the receptacles by pulling back on the plug body. These connector families have been designed so that connectors of differing types cannot be accidentally intermated.
   Section::::Unusual types.:UK Walsall Gauge plug.
   Section::::Additional safety features.:Tamper-resistant receptacles.
   Ungrounded NEMA 1 plugs are still popularly used by manufacturers of small appliances and electronic devices because of the design's low cost and compact size, and they are upward compatible with modern grounded NEMA 5 receptacles. Standards permit ungrounded plugs where the appliance does not require grounding due to low risk of leakage current, such as on double-insulated devices.
   To reduce the risk of users accidentally touching energized conductors and thereby experiencing electric shock, plug and socket systems often incorporate safety features in addition to the recessed slots or holes of the energized socket. These may include plugs with insulated sleeves, recessed sockets, sockets with blocking shutters, and sockets designed to accept only compatible plugs inserted in the correct orientation.
  
  The term "plug" is in general and technical use in all forms of English, common alternatives being "power plug", 
  
  "electric plug",
  
  and (in the UK) "plug top".
   The mains connectors of the appliance plug are two rounded sockets that accept two rounded pins from the appliance. They are unpolarised. The third connection, earth, is a large metal contact on each side of the plug body which makes contact with the sides of the plug receptacle, grounding the appliance body. Some appliances using these connectors incorporate a spring and plunger mechanism with a temperature-sensitive release system; if the temperature rises significantly above a preset limit - for example, if a kettle boils dry - the spring is released and (if all goes well) the plunger pushes the plug and socket apart. It must then be allowed to cool and reset manually by forcing the connector back into the appliance.
   Section::::Characteristics.:Drawbacks.
  
  A prong setting offers less protection to the gem than inset mounts. In addition, prongs can snag on clothing or fabric. High quality settings are less likely to snag on clothing, as the prongs are firmly attached to the gem and well finished.
   Section::::CEE 7 standards.:CEE 7/1 unearthed socket and CEE 7/2 unearthed plug.
  
  CEE 7/1 unearthed sockets are designed to accept CEE 7/2 round plugs without notches in the body and having pins.
   The small hole near the end of the power (non-ground) blades of some NEMA plugs is used for convenience in manufacturing; if present, it must be of specified diameter and position. Small specialized padlocks are available to fit these holes, allowing "lockout" of hazardous equipment, by physically preventing insertion of locked plugs into a power receptacle.
   SI 32 plugs have no sleeve insulation, so when a plug is partially removed its prongs may still be powered although they can be touched by small fingers, metal objects, etc., with a risk of electric shock.
  
  Sockets have a defined polarity; looking at the front, neutral is to the left, earth at the bottom, and line to the right.
  
  Section::::Types in present use.:Italy (Type L).
   Section::::Non-locking connectors.:NEMA 1.
  
  This "2-prong" design, with two flat parallel non-coplanar blades and slots, is used in most of North America and on the east coast of South America on lamps; consumer electronics such as clocks, radios, and battery chargers; and other double insulated small appliances that don't require grounding (earthing).
  
  All NEMA 1 devices are two-wire non-grounding devices (hot-neutral) rated for 125 V maximum. 1-15P plugs have two parallel flat blades, wide, thick, long, and spaced apart.
   Prong setting
  
  "Merged content from Tiffany mount to here. See ."
   In North America, there are two types of pin and sleeve products that are used:
  
  Section::::North America.:Pin and sleeve.:NEMA.
   Current transformer shapes and sizes vary depending on the end user or switch gear manufacturer. Low-voltage single ratio metering current transformers are either a ring type or plastic molded case.
  
  Split-core current transformers either have a two-part core or a core with a removable section. This allows the transformer to be placed around a conductor without having to disconnect it first. Split-core current transformers are typically used in low current measuring instruments, often portable, battery-operated, and hand-held (see illustration lower right).
  
  Section::::Use.
   Section::::Mechanical features.:Locking mechanisms.
  
  Some connector housings are designed with locking mechanisms to prevent inadvertent disconnection or poor environmental sealing. Locking mechanism designs include locking levers of various sorts, screw locking, and toggle or bayonet systems. Depending on application requirements, housings with locking mechanisms may be tested under various environmental simulations that include physical shock and vibration, water spray, dust, etc. to ensure the integrity of the electrical connection and housing seals.
  
  Section::::Mechanical features.:Backshells.
   In the case of non-permanently connected domestic equipment, a BS 1363-2 socket rated at 13 A is attached to the ring circuit, into which a fused plug may be inserted. (Note, it is not intended that the fuse should protect the appliance itself, for which it is still necessary for the appliance designer to take the necessary precautions.) Multiple socket accessories may be protected with a fuse within the socket assembly.
  
  Section::::Residential wiring.
  
  Section::::Residential wiring.:Cable types.
  
  In domestic wiring, the following cable types are typically used:
  
  Section::::Residential wiring.:Cable types.:Internal wiring.
  
  BULLET::::- Twin and earth polyvinyl chloride-insulated and -sheathed cables, or
   Section::::Obsolete types.
  
  Section::::Obsolete types.:Old Spanish sockets.
  
  Some older industrial buildings in Spain used sockets that took a plug rated for higher current and had two flat contacts and a round earth pin, somewhat similar in design to the ones found on American plugs but larger in size. The two flat contacts are spaced further apart than on an American plug. No domestic appliances were ever sold with these plugs. 
  
  The line and neutral measure , and are apart. All three pins are long, and the earth pin is a cylinder of diameter.
  
  Section::::Obsolete types.:Original American Hubbell plug and receptacle.
   In some sockets conductors 3 and 4 also connect when no plug is present, shorting pair three. Sockets are also available with cams to allow the switching functions to be selectively enabled as required. In some older sockets this could be achieved by simply bending the contacts; The cams merely implement this useful accidental feature in an intentional fashion.
  