Just a Moment
By Steve Ferguson, Compliance Direction LLC We’ve all heard it – I’ll be with you in moment or I’ll call you back in a moment and it seems as if the moment takes forever. When we say it, the intention indicates a prompt action without committing to an exact time – but promptly is understood. During a recent training session I was presenting on MIL-STD-461 testing, I received a question about this when I mentioned compliance requirements of manual switching transients. The MIl-STD states that “switching transient emissions that result at the moment of operation of manually actuated switching functions are exempt from the requirement of this standard.” My answer indicated that the transient occurred in conjunction with the manual switching operation or that the time different wasn’t discernible. Most in attendance nodded accepting the answer but one student had that look of agreement but a desire for more definitive information. So I tried again to define with guidance that the emission and the manual action happen together which says the same thing as my initial answer. The class accepted my answer and I pressed on to more significant matters for the course, but I sensed that the question remained. I am confident that the author of that part of the standard had this intention, but I also recognize that a few milliseconds is a discernible difference. The target is to allow an emission that could produce susceptibility if you know it was going to happen. Of course, the emission should not cause damage or an extensive recovery process – kind of like hearing a click seeing a flicker when you execute the switching would be acceptable. Since my curiosity was peaked, that evening I did some research hoping to provide better clarification to the class the following day wanting to know if “moment” had a exact time. The general definition coincided with my initial thoughts, a comparatively brief period of time, an instant or the present time (as in at the moment). Still not much definition. More reading pointed to a moment being a medieval unit of time – 40 moments in a solar hour. A solar hour is one-twelfth of the period between sunrise and sunset. Now we have timing definition, but that time varies based on your location and seasonal changes. Based on this approach, a moment could take a little as 0-seconds in the land of the midnight sun or up to three minutes on the opposite earth’s pole. So, a moment averages about 90-seconds – doubtful that the MIL-STD author had this long in mind. We need to keep in mind that testing needs to be supported by measurable, objective requirements to prevent false assumptions or technically deleting emission compliance if the entire process takes less than an average of 90-seconds to complete from the time we press go or make the RF transmission. We need to quantify requirements and acceptance criteria to achieve standardization. Statements like the test is not applicable to “short” cables, use this method for “low” power devices or at the “moment” should be avoided unless “short”, “low” or “moment” has definition. I believe that most of us have encountered these kinds of things. To avoid confusion, make a point to define completely when preparing your test procedures so all concerned understands your decision – avoid a post-test conflict. As an author, be aware of the reader opinion and if it is important – make it clear. About the Author Steven G. Ferguson, Compliance Direction, LLC – Steve has over 40 years of experience in testing and device evaluation. He has instructed in the area of testing methods for the past 20 or so years focusing on electromagnetic compatibility, product safety and environmental test methods. His comprehensive knowledge of MIL-STD-461, MIL-STD-810, MIL-STD 704/1275/1399 and CE Marking requirements for commercial and industrial equipment provides the ability to discuss the many aspects associated with regulatory compliance. With a background operating test laboratories and manufacturing companies designing products, developing test procedures and performing tests, he has the experience to address a wide variety of regulatory compliance topics. He presents various courses on EMI/EMC compliance including EMC for Nuclear Power Facilities, Architectural Shielding and MIL-STD-461 testing presented on-line and at customer facilities. Contact Steve at 240-401-7177 or by email at Stevef@compliancedirection.com.