In North America, an appliance plug that has three pins signifies that the appliance is designed to be grounded. Grounding is the function of a 3-pin plug connection in a nutshell, but what does it actually mean?
You've probably heard that it's a safety feature built into residential circuitry, but if grounding is so important for safety, why do some new appliances come with 2-pin plugs instead of 3-pin ones? Spoiler alert: The fact that the pins are different sizes provides a clue to the answer of this question.
Receptacles have changed considerably since the first detachable outlet was introduced by Harvey Hubble in 1903. Before that, there was no practical way to temporarily connect and disconnect a lamp or appliance from an electrical circuit. Hubble's outlet gradually morphed into the NEMA 5-15 outlet, which is the standard 3-pin plug and outlet combination in use today for 120-volt circuits.
Outlets, switches, lamp bases and other common devices are designed for AC circuits because all residential and commercial power in North America – as well as in every other part of the world – comes from induction generators. AC power has different characteristics than DC power, and it has predominated since the day the light bulb was perfected.
The Dawn of the Power Grid
The development of the light bulb began in 1806 and continued through the 19th century until it was more or less perfected by Thomas Edison and his colleagues in 1879.
Demand for incandescent bulbs immediately outstripped anyone's ability to produce electricity for them, and the need for power-generating stations became apparent. Thus began a tug-of-war between proponents of direct current (DC) generating stations and alternating current (AC) stations – a little piece of history known as the War of the Currents.
Edison and his backers were clearly on the side of DC power generation, and on the opposite side was Nikola Tesla, a Serbian engineer who had been an employee of Edison's. Tesla's camp won the day, and one of the first AC generators came online at Niagara Falls in 1892. AC power had proven to be less expensive to produce and more economical to transport than DC power.
Early AC Devices Were Ungrounded and Shocking
The generation of AC power relies on an induction generator, which essentially consists of a spinning coil in a magnetic field. The current running through the conductor reverses itself with every rotation.
This means that the electricity that flows between the coil terminals and all the light bulbs between them doesn't flow directly from one terminal to the other as DC current does, but instead constantly reverses itself, flowing toward one terminal during one half cycle and toward the other during the other half cycle.
Instead of positive and negative terminals, an AC circuit has hot and neutral ones. For any electric device in an AC circuit, the hot terminal is the one connected to the power generator, and the neutral terminal is the one that returns power back to the generator.
If you break the circuit, the hot terminal remains live, but the neutral terminal goes dead. If you touch the hot terminal, you'll get a shock, but you'll feel nothing if you touch the neutral terminal.
As power stations came online, homes throughout North America became electrified, and power washing machines, vacuum cleaners and electric refrigerators became quickly available. Shocks were common, however. Wires, switches and outlets were electrically insulated, but the insulation frequently chipped, cracked or wore off, leaving exposed hot wires in contact with parts of the devices that people touched. Fires were frequent because of worn insulation and loose connections.
How Does Grounding Help?
Suppose a person were to touch a live hot wire, or a switch in contact with a hot wire. If the person were somehow floating in the air or, equivalently, wearing electrically insulated shoes, nothing would happen. If the person were standing on the ground with bare feet, though, the electricity would flow through the person's body to the earth, which is the largest electrical sink available.
It takes only one-tenth of an amp of current (100 mA) to stop a person's heart, so the encounter could very well be fatal.
Now consider if the electricity already has that path available through a conducting wire. The wire provides a lower-impedance path to ground than a human body. (Impedance is to AC circuits what resistance is to DC circuits).
Electricity always chooses the path of least resistance (impedance), so the person touching the hot wire won't get a shock – or at least, not as large of a shock. That's the basic idea behind grounding.
Grounding is also good for electrical equipment. If a short circuit occurs because of worn insulation, loose connections or a broken device, the ground wire provides an alternate path for the electricity so it doesn't burn out the circuit and start a fire. Again, this works because the impedance of the ground path is less than that through the circuit.
The 3-Pin Plug Function
A ground path in the circuitry isn't much good if you don't have a way to connect to it, and that's what the third pin on a 3-pin plug is for. The plug connects to a power cord that in turn connects to the electric apparatus in use, whether it's a vacuum, blender, power saw or work lamp. The circuitry in the apparatus is wired so that everything is connected to its ground terminal.
The ground terminal connects to the ground wire in the building's circuitry via the ground pin on the plug. If an appliance has a 3-pin plug, you should never bypass the third pin by cutting it off or using 3-pin to 2-pin adapter. if you do this, the device you're using isn't grounded and could be dangerous.
The 3-pin plug wire colors aren't the same throughout the world, but they are standardized throughout North America, including Canada, the United States and Mexico. The National Electrical Code (NEC) specifies white as the color of the neutral wire, but it doesn't establish any requirements for colors of the hot wire or ground wire. Nevertheless, there is a closely followed convention to use red or black for the hot wire and green for the ground wire. Ground wires are also commonly left bare.
Why Do Some Appliances Have 2-Pin Plugs?
The NEC began requiring grounded circuitry in laundry rooms in 1947 and extended the requirement to most other locations in 1956. The shift made 2-pin plugs and outlets all but obsolete. The only time you could install a 2-pin outlet was when you were replacing an existing one. All new outlets had to be 3-pin ones.
Yet today, it's common to see new outlets with only two slots and power cords on new appliances with only two prongs. If you look closely at these, though, you'll spot the difference that distinguishes them from obsolete, pre-1947, 2-pin plugs and outlets. One of the prongs is larger than the other one, which means the plug can only fit into the socket one way. These plugs and outlets are polarized. Since you can't reverse the orientation of the plug in the socket, you can't reverse the polarity.
In a polarized lamp or appliance, the hot wire connects to one terminal of the switch, and the internal circuitry connects to the other terminal, which in turn connects to the neutral wire. The switch is insulated from the rest of the circuitry, so when it's open, nothing can come in contact with the hot wire.
If the plug did not have different-sized prongs, you would be able to reverse the polarity by putting it in upside-down. The hot wire would be in contact with the circuitry, and the device could potentially give you a shock. Because you can't reverse the plug or the polarity, grounding isn't a crucial safety feature, and the plug doesn't need a ground pin.
Different Types of Electrical Outlets
The 3-prong plug under discussion so far is designed for 120-volt circuits and to handle up to 15 amps of current. It's the NEMA 5-15 plug and outlet, where NEMA is the National Electrical Manufacturer's Association. This outlet has slots for three pins, but the hot and neutral pin slots are different sizes, so it can be used with a polarized plug.
The NEMA 1-15 is the 2-pin, polarized version of this plug. 3-pin plugs outside North America do not necessarily conform to NEMA standards and usually have different pin configurations.
An interesting feature of the NEMA 5-15 grounded plug is that the ground pin is about 1/8 inches longer than the other two. The logic behind this is that, when you plug something in, the ground pin makes contact first, so you always have ground protection. Many people install the the NEMA 5-15 outlet with the ground pin below the other two, but that's upside down. The ground pin should be on top to prevent anything that falls from above from contacting the conducting pins.
An entire catalogue of NEMA plug configurations exists to handle 120- and 240-volt applications. Some 120-volt circuits have two pins and some have three. Plugs and receptacles for 240-volt circuits usually have four pins, because these circuits have two hot wires, a neutral wire and a ground.
By the way, you often see 120-volt plugs and appliances labeled 125, 115 or 110 volts and 240-volt ones labeled 250, 230 and 220 volts.These all mean essentially the same things. The line voltage in North America is nominally 240 volts, which is split into two 120-volt legs in the residential panel. The various alternate voltages are due to fluctuations in the transmission lines and voltage drops in due to circuit load and distance from the panel.
GFCI Receptacles Provide Ground Fault Protection
Many homes in North America were built before the NEC required circuit grounding, and their ungrounded circuits and obsolete 2-pin outlets are "grandfathered in." That's actually an inconvenience, because most modern devices have either 3-pin plugs or polarized ones. While it's safe to plug a 2-pin plug in a 3-pin socket, the reverse isn't true, and it leaves the device without ground protection.
The easiest workaround is to install ground-fault circuit interruptor (GFCI) outlets in areas of the house that need grounded outlets. A GFCI has an internal breaker that trips whenever the outlet detects an abnormal change in current, such as would be caused by someone touching a live contact while standing in water. A GFCI can prevent electrocution, but it doesn't protect sensitive equipment from current surges and isn't a complete substitute for grounding.
The pins of a GFCI are in the standard NEMA 5-15 configuration, which means two vertical slots, each of different sizes, and a semi-circular ground slot. You usually don't need more than one GFCI per circuit because any GFCI will protect devices wired after it in the circuit. You can therefore protect an entire circuit by changing the first outlet in the circuit with a GFCI.
- The UnMuseum: Who Invented the Lightbulb?
- Stanford University: History of Electricity in the United States
- Pfeiffer Engineering: Principles of Electrical Grounding
- McGarry and Madsen Home Inspection: When Was the Three-Slot (Grounding) Outlet/Receptacle First Required?
- Ohio State University: The Fatal Current
- Hackaday: The Electrical Outlet and How It Got That Way
About the Author
Chris Deziel holds a Bachelor's degree in physics and a Master's degree in Humanities, He has taught science, math and English at the university level, both in his native Canada and in Japan. He began writing online in 2010, offering information in scientific, cultural and practical topics. His writing covers science, math and home improvement and design, as well as religion and the oriental healing arts.