Whether the appliance is a vacuum cleaner or a floor scrubber, hair dryer or curling iron, microwave or refrigerator, they all have at least one thing in common, the power cord. And, where there is a power cord, there usually is a need to protect it.

Strain reliefs can do just that. They are designed to allow the cord to move without damaging it or the wires inside the cord. They can be used at the plug end, where the cord goes into the wall receptacle, or at the equipment end, where the cord meets the appliance. It is at the two ends of a flexible cord that strain relief is most needed – the transition points where the cord changes from flexible to rigid.

Whether as a separate component or a molded-in feature, a strain relief is designed to protect the cables from damage from abrupt pulls or pushes, and excessive flexing. With stranded conductors, if one wire breaks under the strain, the other wires inside the cord have to carry the additional load. That creates additional heat at the junction where the cord is flexing and could cause catastrophic problems such as a fire, short circuit or electric shock to the user, says Larry Oden, engineering manager of Unicable, a cordset supplier in Bowling Green, Ky.

A variety of Interpower strain reliefs including flex, dome, multi cable and liquid seal.

“If you bend a wire back and forth it will fracture and break,” he says, “that is a metallurgical fact of life with the copper wires.”  When the wire breaks, an internal arcing can occur which is a “real concern for irons and hair dryers and those kinds of devices because that arcing can cause physical injury,” says Oden.

There are several types of strain relief on the market. Many companies such as Interpower Corp. of Oskaloosa, Iowa, offer what are called dome nut and flex style reliefs. Other companies such as Heyco Products of Toms River, N.J., a supplier to cord vendors and some OEMs, offers strain relief bushings, which are offered for flat and round cables and in flexible and locking versions, and Liquid-Tight Cord Grips.

Choosing the correct one often depends on the application, according to Dan Ford, technical support specialist for Interpower Corp., a supplier of cords and cord components. Ford says that the first thing that the cord vendors need to know is the application. For portable appliances, a flex style strain relief might be appropriate. For appliances that are rarely moved or unplugged, a simple dome nut style or strain relief bushing might be a satisfactory approach. In instances, where the power cords are pre-assembled, the locking versions might be the choice, says Vince Giglio, director of engineering for Heyco Products.

Dome nuts are very good for static applications and for a seal to pass a water test. They are less effective for appliances that are moved around during use because they don’t provide for any flex or bend relief.

Heyco’s water resistant products are the Liquid-Tight Cord Grips, which come in standard and flexible versions and are offered with a Buna - N or TPE sealing gland. Heyco Cord Grips have a threaded body that can be either threaded into the appropriate hole or used in a clearance hole with a lock nut. In all cases, they provide a liquid-tight seal that would meet a NEMA 6P or IP 68 application.

Strain relief at the equipment end can protect against damage from excessive pulling and flexing. This Heyco  relief can snap into place.

For appliances that are moved around, twisted and turned, or frequently unplugged, then a flexible type strain relief might be a better choice. This type of strain relief has a series of ridges where the cable meets the plug or connector and gently bends with the movement. An alternative might be the flexible version of Heyco’s cord grips.

Another less common strain relief is a mesh type relief, which acts much like Chinese finger cuffs and become more resistant as the cable is pulled or flexed. These are often used for larger cables.

Within the variety of strain relief types, there are subsets that offer additional features. These features can include electromagnetic interference shielding, fire retardance, multi-cabling capacity, and additional moisture resistance. Additionally, some rigid strain reliefs are available in nickel-plated versions. These can be used for applications in harsh environments where the cord might be subjected to chemicals or other corrosive agents that might other wise harm plastic strain reliefs. These can also be used to improve the product’s aesthetic purposes.

Applications that might have limited space can use a 90 Deg angle strain relief. In some appliances, the cord might be so rigid that it might not bend for two or three inches and space might not allow for that. Oden points to a microwave that is designed to fit into a space above a stove. Often the appliance is pushed back as far as it will go, and if the cord doesn’t have a right angle or some means of protection, the cord coming out of the back of the microwave could be damaged.

Strain reliefs are available in a variety of materials, which is an important factor depending upon the application, says Giglio. He says that Heyco primarily uses Nylon 6/6 for most of its standard products. While Nylon 6/6 resists salt water, weak acids gasoline, alcohol, oil, grease and common solvents, the application may require the use of other materials. For instance, if the application calls for an operating temperature exceeding 105 DegC a Nylon 4/6 can be used. Heyco has also produced products for custom solutions from materials such as: polycarbonate, acetyl, polypropylene, polyethylene, Noryl, and electrically conductive plastics.

An Interpower flex relief at the equipment end. Other strain relief options can be used at either end of the cord.

At Interpower, Ford points to a material that is starting to find uses in strain reliefs called polyvinylidene difluoride. PVDF is a highly non-reactive and pure thermoplastic fluoropolymer that is used in applications requiring the highest purity, strength, and resistance to solvents, acids, and heat and low smoke generation during a fire event.

“It is not very common yet, but some of the strain relief manufacturers make these for the equipment end. And, because they are resistant to chemicals, they tend to push those for the medical applications,” Ford says.

Ford adds, that depending on the types of chemicals that come into contact with a strain relief, certain metal strain reliefs might be better than plastic. Metal can provide EMI shielding if that is a consideration.

In terms of the construction of the strain relief, much more attention is paid to the equipment end than to the plug end, says Oden. “The plug end is not normally as dynamic or frequently moved as the equipment end,” he says, “but you still need a system that provides enough mechanical strength to ensure safety.”

According to Oden, the power supply cord supplier usually makes the decision on how the plugs are constructed and the materials used. Typically, these are off-the-shelf products and the appliance project engineer usually has little input with regards to the plug end of the cord, he says. Proprietary plugs are sometimes requested, especially for use in medical equipment or a special tool plug, but that plug still needs to be compliant with standards developed specifically for power supply cords. The cord suppliers are charged with ensuring that they meet these regulations.

Strain relief can offer damage resistance to cords even when they are sharply bent. Photo from Unicable.

The plug design is controlled by the UL 817 power supply cord standard from Underwriter’s Laboratories, based in Northbrook, Ill. Cord manufacturers submit the plug design to the UL, who do a variety of tests on the plug and the strain relief including jacket retention tests, flex tests and abrupt pull tests. Typically, these tests require the plugs to survive a 30 lb. pull test and what Oden calls a minimal amount of flexing requirements. For instance, the UL appliance standard 60335-1 requires 20,000 cycles. (UL has a series of standards that have been harmonized with Europe. These standards are named 60335-x, and each “x” standard is for a different product type.)

The tests are looking to undercover some of the common types of power cord plug failures found in the field. According to an article by Conductive, Rancho Santa Margarita, Calif, (Appliance Design, Stressing Safety, February 2005, p. 42), these problems, include:

• Plug/Cord Separation. Plug remains in wall outlet and cord is torn out from the plug.
• Ground Pin Pull-Out. Plug is pulled out from the wall outlet, but ground pin simply breaks off the plug and is separated from the plug or remains in wall outlet.
• Plug Blade Pull-Out. Plug is pulled out, but plug blade(s) breaks off or worse yet, remains in wall outlet exposing live current from the wall outlet.

Strain relief tests at the equipment end are much more arduous, and they need to be because even if the cord passes the UL 817 standard, it doesn’t mean that it will pass the particular appliance standard. That is why Greg Ruppert, president of Quail Electronics, Livermore, Calif., which makes plugs, cords and cordsets, suggests that OEMs consider doing their own in-house testing. This includes trying to quantify the force, angle, and number of cycles that the strain relief will be required to endure.

“You can bet that the UL will beat the heck out of the (appliance, such as a) toaster and subject the junction of the toaster’s cord to all kinds of abuse to try and get it to fail,” says Ruppert. “In the long run, it will take much less time and money if that kind of testing can be done in house before UL rejects it and it’s ‘back to the drawing board’.”

Over the years, UL has developed separate standards for specific appliances, but strain relief is a common thread running across many of the standards, says John Drengenberg, an electrical engineer who serves as UL’s consumer affairs manager.

This Interpower graphic shows a dome nut stress relief and how it enters the electronic housing.

“For most appliances, the strain relief requirements that are applied by UL in its safety standards mandate a 35-lb. pull test on the cord,” says Drengenberg. “For the test, a cord is connected to 35-lbs. of force, eased down and held there for a period of one minute.”

The tests checks for:

• Short circuit between the conductors.
• Breakage of more than 10 percent of the strands of any conductor.
• Separation of the conductor from its terminal.
• Loosening of any cord guard.
• Damage to the cord or cord guard.
• Broken strands piercing the insulation.

A variation on this test is written into some standards for low-weight products such as hair dryers. In this case, Drengenberg says, if the hair dryer weighs less than 20 lbs., it would be subjected to a 20-lb. pull test. However, a product like the portable hair dryer might also be subjected to a rotational test. “Hair dryers undergo a flexing action that no lamp would ever see,” says Drengenberg. In this case, the cord goes through numerous twisting cycles to see if it causes a break in the internal wires.”

Many UL appliance standards also require products to undergo a push back test. This test tries to force the cord back into the product’s housing. The cord is pushed into the housing in 1-in. increments, until the pressure exceeds 6 lbs., at which time the cord will likely buckle. This test is important because if the cord is pushed back into the housing while the device is in use, it could come in contact with hot components or with spinning motors.

While UL and CSA are two of the primary places that OEMs should look to for standards, other bodies may have standards of which they need to be aware. Giglio says that if the cord is going to be used in a wet environment in Europe, it may require an IP enclosure rating from the IEC. Or, if it is going into a very hot application environment, the strain relief might need to be made of materials with flame retardant properties as mandated by NEMA.

If selling products outside the U.S., OEMs need to double-check those country’s standards. While many are based on UL standards, or vice versa, many countries have their own rules including Canada, Japan, Mexico and the European Union countries. In some cases, the standards are being harmonized between the agencies so that compliance to one standard means that the strain relief complies with the second country’s standard.

If there is any doubt about what standard needs to be adhered to, the suppliers encourage the OEMs to approach them as early as possible within the product’s design cycle to make sure that the requirements are met.

Oden also suggests meeting with cordset suppliers earlier in the process design to ensure that the strain relief and the product design are designed to best protect the cord. For example, the designer has to design the size of the opening at the equipment end so that it can accommodate the cord and the strain relief.

Or, when considering the strain relief external to the appliance housing, the designer must account for how the cord will bend. He says that a rule of thumb is that a strain or flex relief external to the appliance should be at 90-degrees to the exit axes and the housing should not have any sharp angles. The radius of the arc formed should be at least eight to 10 times the maximum diameter of the cable and the cord should have a gentle bending motion that curves into a gentle arc when it is pulled to its sharpest angle. “The narrow end of the bend should look like a fishing rod with a large Walleye on the hook,” Oden says.

By combining the correct strain relief, with the proper housing design, the cords set will be protected from damage during use. And, if the cord set is protected, then those that use the appliances will be protected as well.

For more information, email:
Conductive: sales@conductivecable.com
Heyco: sales@heyco.com
Interpower: info@interpower.com Quail Electronics: sales@quail.com
Underwriters Laboratories: cec@us.ul.com
Unicable: larry@unicable.com