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Steve Colwell wrote:
I assumed (dangerous word) the dual B&C Voltage Regulators with Crowbar
Overvoltage Protection would protect my backup TruTrak ADI with its own
back-up battery and the Garmin 396 also hooked to ships power. How likely
is overvoltage protection to fail? By the way, I really appreciate the
Professional Input on LML as one who has little knowledge of technical
issues. You are right, rather than assuming it is better to look at hard data and engineering analysis. One way to do that is to turn to people with appropriate backgrounds who have already done that and published their findings. RTCA (http://www.rtca.org) has done engineering analysis as well as analyzed real word data to come up with the DO-160 spec. This specification has been evolved over the years and currently stands at rev E.
Section 16 of DO-160E, titled "Power Input," talks about the types of abnormal voltages that a device can encounter on various aircraft electrical system. "Category B" of this section most closely resembles most certified and experimental aircrafts: DC system supplied by an engine driven alternator/generator where a battery of significant capacity is floating on the bus at all times (paraphrased).
Section 16.6.2.4.d specifies that a device to be certified for use in a 28V system should be able to handle a momentary surge of 60V for 0.1 second and 40V for 1.0 second. A device being certified for use in a 14V system has to survive half the voltages for the same duration.
The way I read the big picture is that RTCA has determined that these are the worst cases voltage levels that a device connected to the described aircraft electrical system is likely to encounter due to an abnormal condition in the power system. Can your particular B&C regulator with crowbar over-voltage protection limit the worst case transients to a lower voltage? Maybe, maybe not. How much engineering resources are you willing to commit to do a thorough failure analysis to get a definitive answer that you will be willing to bet your life on?
I do not know how TruTrack or Garmin or any other company has designed their power input, so I can only ask pointed questions without providing any answers. It is possible that the input will handle such voltages, either by design or by luck (most things can survive beyond their specs most of the time). If the voltages do lead to a failure, what will likely fail? What will the failure mode be and how far down the path will the destructive voltage travel before a device fails in such a way that the voltage is not passed on to the next stage? Will that kind of a failure happen before or after the point in the power path where the internal battery jumps in? Will this cascade failure take out the battery charging circuit and the battery pack itself? I do not know the answer to those questions and unless a qualified engineer at the manufacturer has sat down and analyzed all of this, nor does the manufacturer. The marketing person's stock answer of "We have a great product and you do not have to worry about it" does not carry much weight.
However, all that said, your alternator going nuts and putting out 60V for 0.1 seconds is not the worst thing you need to worry about. Section 22 of DO-160E, titled "Lighting Induced Transient Susceptibility" talks about the types of voltage and current surges that power and I/O cables can encounter as a result of a lightning discharge in the vicinity of the aircraft (we are not talking a direct hit here). Once again, devices are categorized based on where they are located, the type of airframe (metal or composite) and how critical the device is. The lowest test levels start at 100V or 4A (whichever limit happens first) for a duration of approximately 150 microseconds and goes up to 1,600V and 5,000A for a duration of close to 1,000 microseconds (the times are square wave approximations of asymmetric double exponential waveforms). Typical TSO'd panel mount devices are designed to handle level "A3" which calls for a 6/69 waveform to 300V or 60A, whichever is reached first. The "6/69" means that the waveform has a 6 microseconds rise to maximum voltage/current followed by an exponential decay with a half life of 69 microseconds.
Regards,
Hamid
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