question: Why do the prongs on some electrical power plugs have holes while others do not?
comment: I do not find many references to this hole. The NEMA standard does specify this hole as an option and does specify its dimensions. The reason for the hole is to ease manufacturing of the plug. However I am not quite sure how exactly it makes the manufacturing process any easier. It is possible this hole is used for alignment or as a way to hold on to the plugs during the manufacturing process. Another accidental use case for these holes is to secure lock out padlocks to them to prevent the plug from being inserted into a socket. So if you do not want a piece of machinery to be used, maybe you are maintaining it and could be touching its electrical or mechanical parts, then you could threat a padlock through the holes so that nobody without the key to the padlock is able to power up the machine.
passages: 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.
   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.
   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.
   Designs of plugs and sockets have gradually developed to reduce the risk of electric shock and fire. Plugs are shaped to prevent finger contact with live parts, and sockets may be recessed. Some types can also include fuses and switches.
  
  Section::::Application issues.:Shutters.
  
  Shutters on the socket prevents foreign objects from contacting live contacts. The first shuttered socket was introduced by British manufacturer Crompton, in 1893. Electrical insulation of the pin shanks to reduce live contact exposure was added to some designs, as early as 1905. 
  
  Section::::Application issues.:Earthing (grounding).
   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".
   Many plug and socket connectors are attached to a wire or cable by soldering conductors to electrodes on the back of the connector. Soldered joints in connectors are robust and reliable if executed correctly, but are usually slower to make than crimped connections. When wires are to be soldered to the back of a connector, a backshell is often used to protect the connection and add strain relief. Metal "solder buckets" or "solder cups" are provided, which consist of a cylindrical cavity which an installer fills with solder before inserting the wire.
   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.
   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.
   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).
   To reduce the risk of electric shock, plug and socket systems have safety features in addition to the recessed contacts of the energised socket. These may include plugs with insulated sleeves, recessed sockets, or automatic shutters to block socket apertures when a plug is removed.
  
  A socket may be surrounded by a decorative or protective cover which may be integral with the socket.
   The purpose of the plugs is to fill the holes that were made during the casting process, so the foundry could remove the core sand from the coolant passages. Saving the block from cracking in case of a freeze was never the manufacturer's intent for these plugs.
  
  Section::::Welch plug.
   For temporary connections, a split ring-type current transformer can be slipped over a cable without disconnecting it. This type has a laminated iron core, with a hinged section that allows it to be installed over the cable; the core links the magnetic flux produced by the single turn primary winding to a wound secondary with many turns. Because the gaps in the hinged segment introduce inaccuracy, such devices are not normally used for revenue metering.
   Prong setting
  
  "Merged content from Tiffany mount to here. See ."
   A connector whose shell fits inside the shell of the mating connector is called a plug, and the other connector is called a receptacle. The figure shows a plug with pin contacts and a receptacle with socket contacts, but the opposite arrangement is also common.
  
  Section::::How Connectors Fail.
   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.
   One of the more common concerns of child safety is the potential for electrocution or serious injury when an object, such as a key or metal paper clip, is inserted into an electrical outlet. Many childproofing devices exist that block access to the electrical outlets. These devices may be as simple as plastic units that plug into each individual socket but this type could be removed by a toddler or parents might forget to re-insert them after using the outlet. Once removed from the outlet, most plastic plug-in caps become potential choking hazards. Other devices such as sliding outlet covers replace the current outlet plate but still allow parents access to use the outlet.
   Difficulty or ease of core plug replacement depends upon physical accessibility in a crowded engine compartment. In many cases the plug area will be difficult to reach, and using a mallet to perform maintenance or replacement will be nearly impossible without special facilities for partial or complete removal of the engine. Specialized copper or rubber replacement plugs are available which can be expanded by using a wrench when access is a problem, though engine removal may still be required in some cases.
   There are many forms of wall plug, but the most common principle is to use a tapered tube of soft material, such as plastic. This is inserted loosely into a drilled hole, then a screw is tightened into the centre. As the screw enters the plug, the soft material of the plug expands conforming tightly to the wall material. Such anchors can attach one object to another in situations where screws, nails, adhesives, or other simple fasteners are either impractical or ineffective. Different types have different levels of strength and can be used on different types of surfaces.
  
  Section::::History.
   BULLET::::- Adjusting prongs of an electrical cord that are too wide or narrow them with your fingertips while simultaneously plugging the cord into the power mains. The power plug used in the UK and some Commonwealth countries provides limited protection (Type G, with the lower half of the live prongs insulated), but all others do not.
   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.
   Plug board
  
  Plug board may refer to:
  
  BULLET::::- Plugboard, a component of certain encryption machines, unit record equipment and some early computers
  
  BULLET::::- Telephone switchboard, another name for a "manual exchange"
  
  BULLET::::- Power strip a device that plugs into a power socket to increase the number of power sockets available for other devices
  
  Section::::See also.
  
  BULLET::::- Patch panel, a number of circuits, usually of the same or similar type, which appear on jacks for monitoring, interconnecting, and testing circuits in a convenient, flexible manner
  
  BULLET::::- Plug (disambiguation)
   Removing the grounding pin of the appliance’s plug is unsafe, and leaves the appliance without proper grounding even when relocated and plugged into a properly grounded three-prong receptacle. Additionally, since most NEMA 5-15P plugs have both current-carrying prongs the same width and rely on the ground pin for correct orientation, removing it allows insertion of the plug with hot and neutral wires reversed, creating an additional hazard. In most cases it also invalidates the manufacturer's warranty against defects as an unsupported modification of the appliance.
   CEE 7/1 unearthed sockets accept CEE 7/2 round plugs with pins. Because they have no earth connections they have been or are being phased out in most countries. Some countries still permit their use in dry areas. Older sockets are so shallow that it is possible to accidentally touch the live pins of a plug. CEE 7/1 sockets also accept CEE 7/4, CEE 7/6 and CEE 7/7 plugs without providing an earth connection. The earthed CEE 7/3 and CEE 7/5 sockets do not allow insertion of CEE 7/2 unearthed round plugs.
  
  Section::::Types in present use.:CEE 7 standard.:CEE 7/3 socket and CEE 7/4 plug (German "Schuko"; Type F).
   In standard USB, the electrical contacts in a USB connector are protected by an adjacent plastic tongue, and the entire connecting assembly is usually protected by an enclosing metal shell.
  
  The shell on the plug makes contact with the receptacle before any of the internal pins. The shell is typically grounded, to dissipate static electricity and to shield the wires within the connector.
  
  Section::::Connectors.:Connector properties.:Compatibility.
   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.
  