Even the Simple AC Power Plug Isn’t Simple

I’m always impressed at how much technology and manufacturing expertise is embedded in the apparently simple products we take for granted and about which we don’t give much thought. Consider the AC line-cord plug, which most of us use — and even abuse, by yanking on the cord itself rather than the plug body — without much thinking. It does one thing, usually does it well and safely, and doesn’t cause problems except in rare cases — and even then, it’s just as likely that it’s the socket and not the plug that is to blame.

I realized this again when I saw an advertisement from Interpower Corp., a company that understands and makes these cords in countless versions, configurations, and varieties (I do like no-nonsense, informative ads rather than image-based ads). While I knew that there were different plug styles for various countries or global regions, that seemed to be a minor problem. If you’re traveling, you bring adapters; if you are shipping systems, you provide the appropriate cord for the end-user. Either way, it’s a manageable issue.

But one of the ads pointed out the very subtle difference between plugs used in North America and the plug for Japan, Figure 1. You can sort of use one in place of the other, but there are some critical differences that may result in short- and long-term reliability issues. 

Fig 1: This no-nonsense advertisement highlighted the subtle difference between a North American AC plug and one specified for use in Japan. (Image source: Interpower/author)

It’s not just a matter of meeting the technical standards. The second ad I saw showed the eight cord orientations available for a  standard flat-plug cord to better fit the available space at the outlet or between adjacent outlets, Figure 2. It’s one of those things you don’t think about much until you see it, and then realize how handy it can be in some settings.

Fig 2: Even the basic flat plug and power cable is available in eight orientations to meet unique space and real-estate constraints. (Image source: Interpower/author)

Thus far, these differences seem fairly modest, with just some re-dimensioning of the contact blades or re-arrangement of the wiring arrangement and body molding. But then I spent some time at the Interpower site and learned some more about the intricacies of AC plugs. Sure, they have to meet various UL standards in the U.S. and comparable ones elsewhere — that’s obvious. But a plug for general-purpose use is quite different in internal design and construction than one which is rated as “hospital grade,” even though the voltage and current ratings are the same. Reference 1 explains this in accordance with power supply cord standards UL 817 and C22.2 No. 21-14, including this:

“(1) the blades must be solid instead of folded brass, (2) the blades are usually nickel-plated, (3) the plug includes an internal cable retention device or strain relief to prevent any stress to the plug’s internal connections, and (4) NEMA plug and receptacle are marked ‘Hospital-grade’ and with a green dot.” 

Interpower notes that its retention device uses three stainless steel rings to solidly hold the connections in place, Figure 3. The site also discusses the obvious and subtle differences in standards and tests for hospital-grade cords in various countries.

Fig 3: Among other differences, a hospital-grade AC cord has internal cable-retention as part of the design and fabrication, while a standard molded cord likely has none. (Image source: Interpower)

Another difference between regular and hospital grade is the regulatory test which simulates someone jerking a cord to unplug it. In the “Abrupt Pull Test” for hospital-grade cords, a 10-pound weight is attached and then dropped, repeatedly, while for consumer cords the weight is just 2 ½ pounds (see a video, Reference 2).

Even the ubiquitous power strip has many vagaries. Formally known as a “socket strip” among its other names (UL 1363 refers to them as a relocatable power tap or RPT; other names include power strips, power bars in Canada, power distribution units, and power boards in Australia) it gets considerable attention and exposition in Reference 3. There are also explanations of the different mounting arrangements (table-top versus rack) and circuit breaker/fusing mandates, along with the many country-by-country differences. You’ll never look at a power strip as a <$10 “no big deal” item, that’s for sure. 

Obviously, there isn’t enough time in a designer’s life to become fully knowledgeable about every component in a system or product under development. However, there are many times when the details and relevant standards can affect what you ship and where. It certainly doesn’t hurt to check out various websites and application information from knowledgeable specialists, including component vendors. 

Have you ever been involved in a major product hold-up due to failure to understand or meet a basic standard? How about other regulatory requirements such as for RFI/EMI, or wireless bands and allowed modulation? What about differences between various countries and regions? Have you ever been told (or said) “hey, that’s just a simple XYZ” only to find out there’s no such thing as “simple” in the world of technology, even for mundane components?

References

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