The “Simple” Power Cord

The Anatomy of the Common Power Cord, and its Associated Regulatory Issues

A power supply cord is an essential element of all cord connected electrical equipment; it supplies the connection between the equipment and the electrical mains. The power cord can be hard wired to the equipment or it can be detachable. The hard wired (non-detachable) power cord consists of a plug, the cordage, and a strain relief device to secure the cord to the equipment enclosure. The detachable power cord, also known as a power cord set, consists of a plug, cordage and a connector or receptacle. Though seemingly a simple component, when it comes to regulatory approvals, it can become an exceedingly complicated component.

This article shall cover some of the aspects of power cords intended for use in information technology equipment (ITE) and consumer products applications. Other types of power cords, such as those for outdoor use or for hazardous locations, are a topic for another article.

Power supply cords typically have two or three wires. These wires are “line,” “neutral” and “ground.” The International Electrotechnical Commission’s standard IEC 60446 uses the following color codes:

• In North America, the line wire is black in color, while it is either brown or grey in the rest of the world;
• In North America, the neutral wire is white, while it is blue elsewhere; and
• In North America, the ground wire is typically green, while it is green with yellow stripes in the rest of the world.
  

It is crucial to ensure that the electrical rating of the power cord supplied with the product is higher than that of the product being powered. Most product standards require the plug to be rated at least 125 percent of the rated current of the equipment. An under-rated power cord can result in the power cord overheating, and possibly causing an electrical fire. Power cords designed for use with ITE equipment in North America are rated as 125Vac/10A, but other higher ratings are also available.

Non-Detachable Power Cords

The non-detachable power cord is hard wired to the equipment enclosure, and is generally not meant to be removed from the equipment enclosure. The power cord consists of a plug, the cordage and a strain relief connector to secure the cord to the device.

A common means of attaching the stripped end of a non-detachable power cord (see Figure 1) is a lug/ring terminal. One end of the terminal consists of a metallic ring or “loop” that slips over and is attached to a power terminal post or screw. The other end of the lug/ring terminal is generally crimped onto the stripped power cord wire. One lug-ring terminal is attached to one power cord wire. It is important to remember that, when using this form of terminal, the crimp tool must be approved for the terminal for which it is being used. It is good engineering practice to double crimp the lug/ring terminals.

Manufacturers of large equipment that use non-detachable power cords will occasionally ship their power supply cords disassembled from the equipment. This is because the power supply cords are typically very large and very long, and the cords may be damaged during shipping. In such instances, U.S. nationally recognized testing laboratories (NRTLs) do allow for this, provided certain conditions are met. These conditions include:

• Justification as to why the power cord must be disassembled;
• The assembly of the disassembled equipment must be performed by qualified service personnel;
• Detailed assembly instructions must be provided that address all aspects of the re- assembly, including connection of the main earthing terminals;
• Instructions that state that no replacement cord shall be used other than the one shipped with the product; and
• A suitable means of securing the power cord to the enclosure is provided, such as a strain relief grommet.

Flexible cords cannot be used for equipment that requires the cord to be hard wired into the device. Fixed equipment with a flexible cord and plug must be designed to allow for the cord to be detachable for repair and maintenance purposes, per sections 400.7 and 400.8 of the National Electric Code.

Detachable Cord Set

A detachable cord set, sometimes referred to as line cord, mains cord or power cable, consists of a flexible cord with electrical connectors at either end, one male and one female. The male end of the cord set is attached to a molded electrical plug, while the female end typically consists of a molded electrical receptacle. The molded receptacle is meant to prevent the possibility of having an exposed live prong or pin that could cause an electric shock. The male plug connects to the electrical source receptacle or outlet, while the female connector attaches to the piece of equipment. Some cord sets include other components such as an overcurrent protection fuse. An example of this type of cord set is used in the United Kingdom.

There are different requirements for power cords if the cord is packed in the same box with the end product versus when it is shipped separately as a repair or replacement part. Some countries want only the cord that is suitable for their country’s use included with the equipment, and no additional power cords suitable for use in other countries. Some countries allow the provider to include only instructions regarding the selection of the proper power cord, rather than the cord itself, while others countries do not.

In recent decades, power cords have been increasingly impacted by environmental requirements, such as those contained in the European Union’s RoHS (Restriction of Hazardous Substances) or REACH (Registration, Evaluation, Authorization and Restriction of Chemicals) regulations. Most power cords use some form of polyvinyl chloride (PVC) in their manufacture, and there has been growing concern about the environmental and health effects posed by the manufacture and use of PVC materials. Growing world-wide environmental regulations have profoundly impacted its use and exportability. It is therefore important that, when purchasing power cords, to request the RoHS and REACH declarations, or proof of compliance, from the supplier.

Plugs

Each country uses a different plug style. There are 15 types of electrical outlet plugs in use today, each of which has been assigned a letter by the U.S. Department of Commerce’s International Trade Administration (ITA), starting with “A” and moving through the alphabet. The IEC has arranged these plugs into a comprehensive table known as “World Plug Types.” The table categorizes the different plugs by the various countries or regions in which they are used. They start from Type A which is a North American 2-pin plug, and end with Type O, which is a 3-pin plug intended for us in Thailand. All but two of the plug types are 3-pin plugs, with the third pin, usually the center pin, being a ground pin. Type A plugs, generally used in North America, and Type C plugs, generally used in Europe, South America, and parts of Asia, are two pin ungrounded plugs.

An interesting variation of the grounded plug is the Type E plug, commonly used in France, Belgium, Poland, Slovakia, the Czech Republic, and several African nations. It has two male tubular pins (hot and neutral), while the grounding portion of the plug is a female socket which accepts a male ground pin that is permanently attached to the otherwise female socket.

When it comes to plugs, there are two important considerations. The first consideration is the ground pin. Depending on the type of plug and its intended use, it may employ a third pin, or “ground pin.” This third pin ensures a connection to earth that protects the user and product against insulation failure of the connected device. For example, with medical-grade power cords, it is imperative that the ground connection be reliably maintained to protect the patient and medical staff. Typically, equipment that has double insulation, such as cell phones, electric shavers and some white goods (large appliances), may not require a ground pin; hence, they use a 2-pin plug.

The second consideration is the polarization of the plug. This varies from country to country, and depends on their electrical wirings. In order to maintain correct polarization between the plug and socket, typically one of the plug pins is either larger or wider than the other plug pin. For example, in North America, the neutral pin is larger in size, which means the plug can only be inserted into the larger socket opening. This guarantees that the live conductor and the live pole are connected properly.

In North America, the NEMA 5 family of plugs and receptacles is the most common type. The NEMA 5 plug, which does not have a current rating higher than 20 amps, typically has a male plug featuring two flat blades. For higher-rated current, a power cord featuring the NEMA L14-30P plug and the NEMA L6-30R receptacle is rated at 30 amps.

Another type of plug is the hospital-grade plug, which forms part of a “hospital grade power cord.” It is the plug that makes the cord hospital-grade, and nothing else. A hospital grade power cord is evaluated to a higher quality standard than non-hospital grade power cords. Power cords used with North American medical equipment must be hospital-grade.

The hospital-grade plug diameter must conform to NEMA WD-6 and UL 817 standards. The blades must be solid instead of folded brass, the blades are usually nickel-plated, and the plug includes an internal cable retention device, or strain relief, to prevent any stress to the plug’s internal connections. The colors of hospital grade plugs or the cable jacket are produced to the manufacturer’s preference. Although many hospitals prefer that the plug be clear so that internal connections can be inspected visually, UL (Underwriters Laboratories) and CSA (Canadian Standards Association) standards do not mandate clear plugs, nor do they provide any restrictions on color. Most commonly, the cord sets are solid gray, or occasionally clear-blue male plugs.

Hospital-grade power cords and cord sets should carry a “green dot” to signify they have been designed and tested for grounding reliability, assembly integrity, strength, and durability. They are subject to the special requirements contained in the following standards:

• Medical equipment standards: UL 60601-1 and CAN/CSA C22.2 no 21;
• Power supply cord standards: UL 817 and CAN/CSA C22.2 no 21;
• Attachment plug and receptacle standards: UL 498 and CAN/CSA C22.2 no 42.

Cordage

The cord’s conductor size is rated in AWG (American wire gauge) or mm². AWG is predominantly used in North America and mm² is used throughout the rest of the world.

North America uses its own unique nomenclature to identify the cord type. Examples of some of the North America’s most commonly used cables are SVT, SJT, SJTW, ST, SPT-1, and SOW. Each of the letters used in the cable designation represents its use.

The cable nomenclature is:

• S = Service grade (normally rated to 600V)
• J = Junior service (normally rated to 300V)
• V = Vacuum cleaner cord
• P = Parallel cord
• E = Thermoplastic elastomer
• O = Oil resistance
• T = Thermoplastic
• W = Weather resistant
• H = Heater cable
• VW-1 = Flame retardant
• FT2 = Flame retardant

In North America, all flexible power cords are tested to UL 62/CSA C22-2 No 49-14 standard. In Europe, flexible cords are evaluated to standard EN 60779. There are significant differences between the North American and European standards, especially the cable construction requirements.

Cables used in Europe must have a harmonized approval, and carry the “◄HAR►” mark on them. Typically, these cables are certified to European Standard EN 50525. An example of a European harmonized marking is “H03VV-F,” which means the cable is harmonized, 300V/300V, PVC insulated, fine wire (flexible).

Markings

Typically, each country mandates their own approvals or certification requirements. In North America, the body that is responsible for developing standards for power cords is the National Electrical Manufacturer Association (NEMA) and the agencies that test and certify power cords are NRTLs such as UL or CSA. The North American power cords that are typically used for ITE normally carry one of the NRTL’s certification marks such as UL or and c-UL.

Most countries use IEC 60320 which is an international standard for the evaluation and certification of the power cord components. In the United States, the standard used to evaluate power cords is UL 817.

The markings required for each country will differ for agency labels, certificates of compliance, letters of approval, certificates of approval or registration. These marking requirements can be, and often are, different for the plug, cordage, and the connector. In terms of the markings, power cords used in North America should have the following information as a minimum:

Plug

• Manufacturer name or trade mark
• Type number
• Electrical rating
• NRTL file number

Cordage

• Manufacturer name or trade mark
• Type of cable (for example: SVT)
• Voltage and temperature rating
• NRTL file number.
• Number of conductors and wire gauge.

Receptacle

• Manufacturer name or trade mark
• Type number
• Electrical rating
• NRTL file number

Certification requirements differ from country to country, and from one region of the world to another. As an example, the following list shows requirements taken from a single country selected from different continents:

• In the UK, the plug must be certified to BS 1363-1 while the receptacle end must be certified to EN 60320. Lastly, the cordage must be harmonized cable and certified to IEC 60227 and carry the appropriate certification marks.
• In Australia, the plug must be certified to AS/NZS 3112, while the receptacle/coupler must be certified to AS/NZS 60320-1 and the cable to AS/NZS 5000-1.
• In China, the plug must be certified to GB 2099 and GB 1002, while the receptacle must be certified to GB17465.1 and the cordage to GB5023.5.
• In Argentina, the plug is typically certified to IRAM 2063 or 2073.

The examples listed above represent only a sampling of what exists in the global marketplace and are not meant to offer a complete representation of all the applicable power cord standards for each region. For example, aside from China, Asian nations such as Japan, Korea, Taiwan and Singapore each have their own unique standards, while Hong Kong uses the British standard. Though most of these various international standards share much in common and have many similar requirements, there is inevitably at least one or two clauses that are unique. To do business in these countries, the requirements of each standard must be met in their entirety.

Conclusion

As this article has demonstrated, the apparently simple power cord is not so simple after all. Though its construction may only consist of three components, the fact its use is so universal and that it is the essential conduit through which electricity flows from the power outlet to the product makes it the focus of a myriad of international regulations, markings, and standards.

Most products that are bought and sold throughout the world have, at the very least, the option of using a power cord, including many products that are primarily battery operated. The more countries that market a product, the more regulations that come into play. What is acceptable in one country may not be acceptable in another. Therefore, great care must be taken in following and meeting the maze of international regulations and certifications that come with the “simple” power cord.

Homi Ahmadi is the Director of Compliance Engineering at Extron Electronics in Anaheim, CA and has responsibility for Extron global regulatory compliance affairs. He has an extensive background in compliance which includes product safety, EMC and environmental. He is a senior member of the IEEE. He has published numerous articles and conducted seminars in both the U.S. and the UK to aid manufacturers with product design and compliance activities. He received his Bachelor’s Degree in Engineering from the University of Mid-Glamorgan in Wales, UK. He held the position of program chair at the IEEE PSES in Orange County (CA) from 2008-2010, and again from 2013-2014. He is currently the chapter chair for IEEE Product Safety Engineering Society (PSES) in Orange County and can be reached at hahmadi@textron.com.