Appliance manufacturers are constantly challenged to find new ways of reducing cost in components to make their products more competitive. Since power cords are a key cost component in most appliances, buyers and engineers are continually tasked with evaluating power cords to meet cost reduction objectives. One problem often facing such appliance manufacturers is comparing power cords that essentially look the same, but may differ significantly in cost and long term performance. While competitive costs are important in every appliance, subtle differences in power cord construction may be the difference in meeting cost objectives successfully or in causing unexpected problems in the field. Critical risks to product quality and consumer safety can occur if constructional intentions on cord sets are not thoroughly investigated and evaluated.

Static vs. Mobile

Fig. 1. Cord with terminals.

The first fundamental aspect in comparing a power cord set is to identify its service intent. Static appliances such as microwave ovens and refrigerators that are plugged in and unplugged once or twice in their service life are fundamentally different in power cord requirements than mobile appliances such as vacuum cleaners and power tools that are subjected to constant movement and frequent consumer handling.

For static appliances, reducing cost can be as simple as reducing the use of materials to the bare essentials for safety and meeting agency requirements. Minimizing cost this way means using the least materials possible and processing these materials at the minimal scrap rate possible. Buyer and engineering evaluations for static appliance power cords should focus primarily on process capability of the cord set manufacturer. A full control over each process from polymer compounding to cable extrusion and final cord set assembly aids significantly in controlling quality and costs for static appliance cord sets. Attention should be placed on evaluating the cord set manufacturer’s process efficiency and process stability in cable extrusion to maintain minimal thicknesses required in meeting safety codes and standards.

Fig. 2. Terminal positioner.

Evaluation of quality of materials also needs careful attention. Substandard materials can be hard to identify. Commonly overlooked items include conductor copper purity and the performance of insulation plastics in accelerated aging.

The recent spike in copper pricing has brought about the introduction of recycled and substandard copper in the power cord business. The use of recycled copper can cause critical field failure stemming from temperature rise due to conductor breakages, which is why recycled copper should not be used in power cords. Certification of copper purity to above 99.9 percent is critical to ensure quality for use in power cords.

Most insulation materials, including PVC and rubber, can stiffen and lose their flexibility over time. When exposed to the environment, substandard cordage insulation can easily dry out and crack, exposing live conductors and wires. Evaluation must be thorough in manufacturer’s control and testing of PVC and other insulation materials in meeting designed temperature requirements. Accelerated aging tests should be audited regularly to prevent introduction of substandard insulation materials on power cordages.

Evaluation of power cords for mobile appliances need to go one step further in comparing materials and constructional design differences. A 25-ft. SVT power cord for a vacuum cleaner can cost any where from less than a dollar to more than three dollars, depending on the performance and designed quality. It is important to understand the differences in construction of these power cords and how these differences affect reliability and safety performance in actual use.

Double-Insulated Plugs

Most power cord plugs on appliances are single-insulated plugs, meaning that a single layer of plastic is injected over the crimped terminal and conductors.

New developments in power cord design have improved safety of plugs with introduction of double-insulated plugs. The primary insulation in double-insulated plugs acts as the main safety barrier for capturing all live parts within a plug before a secondary injection molding completes the plug’s aesthetic design and provides a secondary protection against electrical shock. This double-insulation barrier greatly reduces the possibility of stray strands that may compromise isolation or fray to the surface of the plug causing electrical shock.

Two common categories of double-insulated plugs are inner-body types and double-injected types. In inner-body types, the primary insulation is achieved by means of a hard plastic assembly that holds each plug blade or pin in a fixed position. This inner-body assembly acts as an insulation barrier to fix and isolate each conductor and terminal. This hard plastic assembly also aids in ensuring symmetric and proper terminal blade alignment, while improving resistance to blade pull-out in event of temperature rise due to high current draw.

In double-injected types, the primary insulation layer is achieved by a low-pressure injection molding process. This primary injection encapsulates all live parts and is essentially a smaller version of the fully functional plug with protection against electrical shock or shorting between the wires. Double-injected types offer similar protection as the inner-body type construction with the exception of the increased blade pull-out resistance.

Cord anchorage is another important consideration in mobile appliances. A high quality cord for mobile devices should also be designed with an internal strain relief or a cord anchorage device within the plug. This internal strain relief serves to eliminate stress to the blade termination from abrupt pulls on the cordage by the user trying to unplug the cordage or from over extending the length of the cordage in use. Examples of cord anchorage devices include hog rings and other clamps that are affixed to the cordage under the plug overmold. These cord anchorage devices are critical in vacuum cleaner and power-tool applications where failures are often seen in plug and cord separation incidents.

Cable design

Fig. 6. Cord with cotton filler.

Cable construction normally presents the most significant difference in power cord cost and quality. Poor judgment in cable construction can lead to premature power cord failures and increased safety issues. Several basic properties of the cable should be reviewed when evaluating cordages for mobile appliances. These properties will affect all aspects of durability from appearance of quality to flex resistance, abrasion resistance, and wrap resistance.

Twist pitch. A conductor or cord with a tight twist pitch, or a more wounded construction, will out-perform a cord with a looser twist pitch, or a straighter twist. A tighter twist will allow the cordage to act like a spring when bent from side to side. Flex bending of a cord with a tightly wound twist will imparts very little direct stress to the conductors. Flex bending a cord with a more relaxed twist, however, will impart more direct stress to the conductors. Varying the twist pitch however, directly affects the cost of the cordage.

The tighter the pitch, the more copper is used in each foot of cable, and the processing time in twisting of the cable is also increased. Comparison of twist pitch, however, is essential as flex durability is essential in a mobile power cord.

Filler vs. no filler. Filler materials such as jute, cotton, or polypropylene are often used in jacketed cables designed for flex durability. Filler materials act as a cushion that help disseminate stress from a flex bend away from the insulated conductors. When processed properly, the presence of fillers in the cable allows the insulated conductors to move freely within a jacketed cable; thus, dispersing the stress point of the flex away from a single point on the insulated conductor. The stranding process of cordages with fillers, however, requires specialized equipment and is significantly slower in processing speeds when compared to cordages without fillers. Cordage with cotton or jute fillers are also more flexible and impart less memory retention when wrapped around an appliance and stored.

Strand elongation. Copper strands, when processed, stiffen and lose their elongation characteristics. A heat annealing process is used to temper and increase the elongation of copper strands up to 20 percent to 25 percent. As optimum elongation properties are reached, copper strands are less brittle and flexible, allowing the cord to be flexed without premature wire breakage. An optimal annealing process adds slight processing time, but is essential in ensuring flex durability of power cords.

Insulation quality. Material quality plays an even more important factor on mobile power cords. Cable insulation and jacketing materials range from manufacturer to manufacturer and major differences do exist. Fillers are commonly used by compound manufacturers to reinforce target characteristics in insulation compounds, but can also be used to reduce cost of compounds. Insulation materials need to be evaluated for flex fatigue and filler content. Specific gravity for PVC, for example, can range from 1.2 to 1.6. Highly filled compounds generally have high specific gravity and are more prone to flex fatigue and poor abrasion resistance. These characteristics should be observed, but more importantly, checked and validated with performance benchmark testing.

Competition in the appliance market continues to pressure manufacturers to find ways to reduce cost of materials. Cost savings on power cords should be evaluated carefully to prevent unexpected field failures. When exercised properly, significant cost savings and productivity can be realized without any adverse effects to intended performance of the appliance. Understanding these fundamentals constructional differences should allow a clearer evaluation of power cords.