Otis Singleton
The exact year that grounded electrical plugs became standard is unclear, but it appears to have been in the 1950s. One source suggests that equipment with three-prong plugs started to become available in the mid-1950s, with a guess of around 1955. Another source mentions that a house built in 1950 had ungrounded outlets, while a house built in 1969 had three-pronged outlets throughout. Additionally, in 1947, the NEC required laundry outlets in dwelling units to be a 3-pole type designed for grounding. While there may not be a specific year, it seems that the adoption of grounded electrical plugs occurred gradually over time, with varying practices and interpretations of requirements during that period.
| Characteristics | Values |
|---|---|
| Earliest use | 1928 |
| Common use | Mid-1950s |
| Three-prong self-grounding receptacles | Available if the outlet box is metal and connected to a grounding system |
| Grounding alternatives | Ground Fault Circuit Interrupter (GFCI) |
| Hazards | Electric shock, high current, electrocution |
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What You'll Learn
Grounded electrical plugs in the 1950s
The 1950s saw a great expansion in the use of electrical appliances, such as heaters, cookers, hair curlers, and irons. During this time, electrical plugs and sockets were being standardised, with Hubbell's parallel pin configuration being widely adopted despite the company's initial reluctance to standardise.
In the US, the National Electrical Code from 1923 to the 1950s featured a peculiarity: sockets had two different current and voltage ratings, most commonly 10 A 250 V/15 A 125 V. This was because the NEC limited lighting circuits to 10 A until 1923, when the code changed to allow lighting circuits to be fused at 15 A. The higher voltages were rarely used for lighting and appliances. Most sockets with this rating are of the "T-slot" type.
In the 1950s, it was standard practice to separately ground boxes to meet electrical codes. However, the understanding of grounding issues and the consistency of practices and interpretation of requirements were not as advanced as they are today. For example, in a 1950s California house, the kitchen area outlets were all 2-prong, which may have been modified in the late 1960s to install a grounding wire "bus" to properly ground the outlets and convert them to 3-prong GFCI outlets.
In Britain, recognisable two-pin plugs and wall sockets appeared on the market as early as 1885, and British patents for plug and sockets were granted to T.T. Smith in 1883 and W. B. Sayers & G. Hookham in 1884. By the 1950s, Britain had already adopted the standardised plug and socket system, which was becoming increasingly common worldwide.
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Three-pronged plugs
In the United States, for example, the National Electrical Code (NEC) has long required specific grounding methods. As early as 1928, the code mandated some form of grounding. By the 1950s, a mix of grounded and ungrounded outlets was common in homes, with a trend towards three-pronged outlets. A 1969 home inspection report mentions the presence of three-pronged outlets throughout the house.
The exact year when three-pronged outlets became standard is unclear, but it appears to have been a gradual transition. By the mid-1950s, equipment with three-pronged plugs was becoming more widely available. However, practices and interpretations of requirements were inconsistent, and even today, not everyone grounds their outlets correctly.
In some cases, older buildings with two-slot outlets use a cheater plug or adapter to connect three-pronged plugs. This method is potentially hazardous if the grounding tab is not properly connected. A safer alternative is to replace the outlet with a Ground Fault Circuit Interrupter (GFCI) breaker outlet.
While three-pronged plugs are now commonplace, the transition to this standard was a gradual process, and the exact timeline varies depending on the region and the specific electrical code in place.
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Two-pronged plugs
The absence of a ground connection in two-pronged plugs is a notable distinction from their three-pronged counterparts. The ground, typically represented by a round hole below the "neutral" and "hot" slots, is a critical safety feature designed to protect users from electric shock. In the event of a loose wire inside an appliance, the ground ensures that electricity is redirected to the ground instead of passing through a person's body, significantly reducing the risk of electric shock.
Despite the inherent safety advantages of three-pronged plugs, two-pronged plugs are still permitted in certain scenarios. According to the National Electrical Code, two-pronged receptacles are legally allowed to remain in use and can be replaced with another two-pronged receptacle if a ground connection is absent. This exemption primarily applies to appliances without external metal parts, as the absence of conductive materials mitigates the risk of electric shock.
However, it is important to recognise that using a ground plug adapter with a two-pronged outlet compromises safety. By employing an adapter, the safety function provided by the ground prong is negated, introducing potential vulnerabilities for the user, their home, and the plugged-in device. Therefore, while two-pronged plugs may continue to be used in specific circumstances, prioritising safety by transitioning to three-pronged plugs and grounded wiring is strongly recommended whenever possible.
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Grounding boxes correctly
It is important to ground electrical boxes correctly to ensure safety and compliance with electrical codes. Here are some detailed instructions and considerations for properly grounding boxes:
Grounding Metal Boxes
If you are working with a metal outlet box, it is essential to ground it correctly. The National Electrical Code (NEC) and Canadian Electrical Code (CEC) require that all metal boxes be grounded. The ground wire should be attached to the box first, and then a bonding wire is run from the box to the receptacle. This ensures that the box remains grounded even if the receptacle is removed. It is important to note that this applies to surface-mount boxes, and there are different considerations for flush-mount boxes.
Grounding and Bonding
When connecting a ground to an outlet, it is crucial to bond it to its box as well. This can be achieved through the mounting screws, but it is always beneficial to add copper bonding to ensure a strong connection between the outlet and the box. Modern electrical systems prioritise bonding to ground to maximise safety.
Equipment Grounding
Some devices are designed for equipment ground and feature brass squares on the tabs to facilitate a continuous bond. This allows you to ground the box and bond the outlet to the box. However, it is important to keep the outlet replaceable by wiring the box first and then adding the outlet.
Running Ground Wires
When running ground wires, you have some flexibility. You can connect a ground wire to any part of the panel's grounding conductor, such as the big thick braided EGC or the ground screw on the panel. This allows you to establish a proper ground connection without being restricted by wire length.
Grounding Without Ground Wire
In certain cases, you may not need a dedicated ground wire. When installing with conduit, the metal conduit itself provides a path for fault current back to the panel, eliminating the need for a separate ground wire. Similarly, a receptacle within a surface-mount metal cover and box does not require a ground wire if the box is already grounded.
By following these guidelines and referring to the relevant electrical codes, you can ensure that your electrical boxes are correctly grounded, prioritising safety and compliance in your electrical installations.
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Ground Fault Circuit Interrupter (GFCI)
A Ground Fault Circuit Interrupter (GFCI) is a device that protects against ground-fault incidents, which are the most common form of electrical shock hazard. Electricity naturally seeks a path to the ground, and in a ground fault, electricity has found a path to the ground through a person's body. GFCIs are safety devices that can protect human life by sensing the difference in the amount of electricity flowing into and out of a circuit, even in small amounts of current (as little as 4 or 5 milliamps). If a ground fault is detected, the GFCI will quickly (in less than a tenth of a second) trip or shut off the circuit.
GFCIs are particularly important in areas where electrical appliances and power tools are used near water. Water and wet objects are excellent conductors of electricity, increasing the chances of receiving an electric shock. While GFCI protection is now required for some appliances, like hair dryers, it is still not standard for many other appliances.
There are three types of GFCIs: the most common is the "receptacle-type," which is similar to a standard wall outlet. The second type is the circuit breaker GFCI, which replaces a standard circuit breaker and provides GFCI protection to all receptacles on that circuit. The third type is the portable GFCI, which comes in various styles designed for easy transport. Some portable GFCIs can be plugged into existing non-GFCI outlets, while others use a cord and plug arrangement.
While GFCIs provide excellent protection against ground faults, they do not protect against line contact hazards, such as holding two "hot" wires or a hot and neutral wire simultaneously.
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Frequently asked questions
Grounded electrical plugs started to become available in the mid-1950s, with some people recalling seeing them as early as 1952.
It is hard to pinpoint an exact year, but it seems that by the 1960s grounded electrical plugs were becoming standard. A house built in 1969, for example, had three-pronged outlets installed throughout.
In 1947, the NEC required laundry outlets in dwelling units to be a "3-pole type designed for grounding".
In earlier installations, an outlet box could be "grounded" by bonding to other outlet boxes on different circuits and ultimately to something like a cold water pipe.
