Aerospace Wiring Tips 
With the exception of a few startup companies that are burdened with an abundance of investor cash, engineers are almost universally pressured to save money with every decision they make.  And sometimes those decisions, coupled with a lack of foresight and a bit of bad luck, can have unintended consequences.  To learn how good decisions go bad, this mini-series looks at a few failure modes and how to avoid them in future designs.

Airplanes use a combination of low-voltage and high-voltage circuitry.  Aircraft DC busses typically run on 14/28 VDC and work with AC generators that output 115/230 VAC@400 Hz.  Modern sensors operate at voltages as low as 1.8 VDC, and 110/220 VAC@60 Hz is often provided for interoperability with consumer electronics.  That’s quite a few power busses to keep track of, and with the specialized knowledge inherent in each subsystem, it is highly unlikely that the engineer who designs a low voltage fuel tank sensor has anything to do with the auxiliary power unit design, the in-flight entertainment network designer, or the air-conditioning unit.  But at some point, the high voltage and the digital signal nets have to leave the safety of the printed circuit board and enter the environment — that’s where trouble can occur.

Spark Hazards
As the potential difference between adjacent conductors increases, so does the electric field gradient.  If the electric field gradient reaches a sufficient level (the level depends on environmental contamination and humidity), a conductive ionized path will spontaneously form through the air or a carbonized path will slowly form along the surface of an insulator.  Each of these short circuit conditions allows charges to move between conductors.  The necessary separation distances are defined by relevant regulatory agencies.

It is absolutely true that all electronics and electrical engineers should know about creepage and clearance, but in practice, digital electronics engineers don’t have to worry about such things as the race to lower board voltages of 3.3 and 1.8 V leaves creepage and clearance guidelines well below the lower fabrication limits of printed circuit boards.  For example, the creepage limit for 25V (RMS) in a clean environment might be 1 mil, but the standard lower manufacturing limit for copper spacing on 0.5 oz copper is currently around 2.5-3 mils.  A 1.8 V or 3.3 V potential difference creates an electric gradient that is more than an order-of-magnitude from ever mattering in the day-to-day life of this fictional digital design engineer.

But once the low voltage circuit leaves the board and enters the environment, it has the potential to interact with high voltage circuits — where certain failure modes can allow high voltages to enter wires designed to carry low voltages.....more....

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