Hubbell Twist-Lock®, which provide a separable electrical connection that is locked with a “twist” to prevent unintentional disconnection are one of our longest running product lines since being introduced in the 1930’s. Since their creation, they are responsible for a significant portion of division sales.

Where did the idea come from?

In the late 1800s, one day on a boardwalk, a young man named Harvey Hubbell stopped into an arcade, which featured some new electrically operated attractions. While in the arcade, Harvey had occasion to watch a janitor sweeping the floor. In order to sweep behind the arcade games the janitor had to turn off the power, disconnect the hard wired connection to each attraction, then move the attraction, sweep, re-connect the wires, then push the machine back into place and turn the power back on.

 

Diagram of Harvey’s First Wiring

That day in the arcade spawned an idea in Harvey’s mind. Realizing that electrical appliances and machines were here to stay (and proliferate) Harvey envisioned a design concept for the first ever, separable electrical connection. Harvey’s idea was to create a metal-to-metal connector for each wire feeding an electrical machine that would :

1. Slip together and apart as easily as pushing a pin into a sleeve.
2. Totally insulate the electrically live parts so no contact (shock hazard) would occur to the user.
3. Be so easy to do that even someone with no understanding of electricity could supply power to any machine

With all the new appliances and machines getting plugged in, it was only a matter of time before a unique new situation arose……accidental disconnection.

While a nuisance in the residence, accidental disconnection in the factory is a different story. If power is lost to a radio or table lamp, the consequences are not the great. If however, power is interrupted to an industrial process, lost money, broken machinery or even worse can happen. When examining the cause of the disconnection, two situations were observed; vibration and abrupt cord removal (yanking).

The ease at which a straight blade plug is mated to a receptacle is also in certain situations a drawback. Receptacle and plug connections mounted to heavy equipment would over time, vibrate loose. Electrical cords strung across traffic areas represented trip hazards. Quite often an employee tripping on an electrical cord would result in a disconnected plug. Because of these two occurrences, Harvey would develop a completely new type of product. This new product would “lock” together the various mating devices in an electrical system.

Modifications to Straight Blade

At first, a few modifications were made to the straight blade product line in hopes that the problem could be solved. A hole was added to the plug blade and a positioning nib was added to the receptacle contact. When the blade was inserted into the contact, a small snap- action was created when the nib seated into the hole in the blade. This feature called the “American Detent” would solve the problem slightly. This idea lasted for a while and underwent a few more modifications before being phased out over time.

American Detent Holes

The holes in the plug blades still remain, although not used anymore. Examine your work or living area and look at the variety of plugs utilized. Most of them will have the American Detent hole, but not all.

A new Concept- Push and Turn

In really heavy industrial applications, the availability of a completely new wiring device type would be the only real solution. Additional fixes for the problem included female devices having elongated semi-curved slots with tapered contacts. Once the straight plug was inserted into the curved slot and rotated, the plug blade would bind up in the tapered contact “locking” the mated pieces together. This first attempt at locking devices was called “Twist-Tite “and worked fairly well. Always willing to refine his designs, Harvey went back to the drawing board and created an even better product, one still widely in use today.

Note: Pictured is a page from catalog #31-C-7. Twist-Tite’s big advantage was the fact that all of the two wire plugs in your house or business could be used with standard receptacles OR Twist-Tite receptacles. When an building was wired with Twist-Tite, any lamps, radios, appliances, machines or other devices that were purchased with standard plugs would immediately be able to be plugged in, no plug replacements were required

Enter Twist-Lock

While in design stages, Harvey envisioned a new plug style having “L” shaped curved blades (in addition to the curved slots of the female) which would mate with a receptacle similar to “Twist-Tite“, but that would positively lock the pieces together. By having an “L” shape, once the blade was inserted into the mating slot and turned the “L” shape would lock behind the face of the receptacle.

In this diagram the blades of the plug are just entering the slots of the receptacle. With additional pressure the plug will fully insert. Electrical contact is established at this point, but the two devices are not locked yet. By rotating the plug, the “L” shaped blades will slide behind the receptacle face making withdrawal impossible. To un-mate the devices, the plug must be rotated in the opposite direction then withdrawn.

Sequential Engagement- Additional Safety

This picture shows the blades of a plug. Out of the three blades, one of them is longer. This longer blade is usually reserved for the ground connection. By having the ground longer than the other blades it will be the first to establish a connection with the receptacle. This safety feature ensures that the grounding path of a circuit will be connected before any hot conductors are connected. Upon withdrawing, because of its length, the hot conductors will be broken before the ground path is disconnected. Some devices, as you will see in later modules will have three different blade lengths. The longest blade is the ground path conductors which get established first, followed by the neutral (if there is one) which make contact next and the shortest of the blades are the hots which receive power last.

Usually, the ground blade is longer

 

• In Position 1 there is no contact between the plug and the receptacle.
• In Position 2 the ground blade just enters the ground slot.
• In Position 3 has the ground blade making connecting with the ground contact.
• In Position 4 the hot and neutral blades have established contact and the circuit is complete.
• In Position 5 has the two devices fully mated and locked.

When breaking the connection, the sequence happens in reverse.

Great ideas usually get copied. Harvey’s idea was no different. Before long a host of companies were manufacturing wiring devices. It was only a matter of time before another problem began to appear; circuit mismatches. There are a finite number of combinations that slots, holes, pins and blades can be arranged in before duplication will occur. As long as the blade to slot arrangement of all manufacturers was different everything was fine. If, however, the blade to slot design (called a configuration) was similar or even worse, identical, a potentially dangerous situation could arise. The solution to the problem was a committee or association. This committee would be made up of all the various wiring device manufacturers and be called NEMA or the National Electrical Manufacturers Association.

If we take a closer look at the three slightly different configuration diagrams, we see that they have been reserved for use with three different rating systems. It also appears that there may be some interchangeability between one or more of the configurations. Harvey’s plug uses a key in one of the blades which will prevent it from mating with receptacles from ABC or XYZ. The ABC and XYZ brands plug lacking this key may be able to be force mated into any of the receptacles. A miss-match of voltages between what is being supplied and what is being expected can result in equipment failure or personnel injury.

Configurations

Harvey decided that because there were a variety of different electrical circuits being used in the country he would make each plug and receptacle pair slightly different by positioning the blades/slots in different orientations. Each version would be rated uniquely with respect to amperage, voltage, poles and wires and in some case frequency. This concept is called a Singly Rated Device and promotes safety and interchangeability. Lets examine the Hubbell NEMA configuration chart again to see some of the variations.

Note: The picture above represents one segment of Hubbell’s NEMA Configuration chart. Recapping what this chart indicates we find the products associated with: 2 Pole, 3 Wire grounding, 15, 20 & 30 Amps all rated 125V. Each vertical column is broken into Receptacles and Plugs (or Females and Males). The boxes in yellow denote Standard product while the boxes in white are for Shrouded Watertight.