Vibration: The Good, the Bad, and the Ugly
Over the past few years of developing and performing a variety of test programs, it became apparent that many of the major EIA test procedures have never been upgraded. Although there have been additions, they were in the form of expanded test conditions and the like. A key test procedure that is in this condition is EIA 364 E TP 28, Vibration. This procedure was initially issued in 1969 and was copied from MIL-STD-202. A basic policy in EIA documents is that they do not contain requirements. The requirements are provided in the product specification. In the case of vibration testing, the requirements generally include a visual for structural integrity and monitoring for a 1.0 microsecond interruption. Occasionally LLCR (Low Level Contact Resistance) or CRRC (contact resistance at rated current) measurements are included. This is the area for review, and change is necessary but somewhat complex. EIA TP 46 Discontinuity Monitoring is the test procedure that defines the details of the 1.0 microsecond measurement process. TP 46 was developed to meet the needs of switch/relay applications, which are the source of the 1.0 microsecond discontinuity. TP 46 also defines some of the features of the detector, including triggering resistance and open circuit voltage. Given the wide variety of detectors in use, these parameters are not adequately controlled. The triggering resistance, which varies from 2 to 199 ohms, is of particular concern. Until recently, these detector details have been provided in the product specification. So what does this mean? Here are the concerns:
- Existing “requirements” are not well defined.
- Detectors currently in use vary from 2 to 199 ohms of triggering resistance.
- There is no linkage between TP 46 and TP 28. Such a linkage would facilitate including appropriate requirements in the product specification.
- There is no requirement that the detector triggering resistance be recorded in the test documentation. This omission complicates the comparison of results between test programs.
Under these conditions, is it possible that several labs testing the same product to the same spec will have different results? The probability is high. It is possible that a lab with a 2-ohm triggering resistance may detect a non-resettable open while another lab, which may be using a 10-, 50-, or 100-ohm detector, will pass with no opens recorded. So is there a solution? Yes, there may be two or three, but the solutions will not address what has been occurring in the last 44 years. From a realistic economic point of view, it is not possible to retest all of the hundreds of products that have been tested to the current procedure. To move forward, consideration should be given to ensure that all labs follow a “common” and agreed-upon practice. Also, one has to be sure that there is a mutually agreed-upon definition of what a discontinuity is. Once the above issues are resolved (which will be the bad and ugly part), I suggest the process of improving the vibration measurement methodology (the good part) can be accomplished by a rollover to include TP 46 as part of TP 28. In addition to defining “discontinuity” as noted above, the change would also require that all labs use the same triggering resistance. This requirement may be considered impractical. In either case, I strongly recommend that it should be mandatory for the parameters of the detector to be included in the test report. These changes, definition of “discontinuity,” and triggering resistance are the major issues to be resolved. There are, however, additional issues of concern, including: a) The number of contacts in a series circuit per detector channel (an excessive number of contacts per channel may result in false indications of “triggering”) b) Review of test durations: Many of the durations cited in TP28 are not used in practice. c) Placement of accelerometers: This issue is very complex and merits detailed consideration. d) New methods and test conditions The good news is that EIA has established a working group to work out the solutions (hopefully). So if any reader has an interest or a subject that should be included in this agenda for change, please feel free to contact the author. One last note: Those who have known me over the years will either cheer or cry to hear that I have stopped smoking cigars. But I continue to enjoy top line brandy.
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