Be liberal in what you accept, and conservative in what you send.
—Postel's Prescription, by Jon Postel
I have a day job as a hardware engineer for a telecommunications
company, and in this capacity I'm often designing equipment to be
installed in phone company Central Offices (COs). Unlike the designers
of consumer electronics or data center hardware, I can rely on having
DC available to power my devices; COs have “rectifier plants” that
convert mains AC to 48-volt DC and distribute this power through the
facility with giant copper or aluminum busbars over the racks.
Most CO equipment uses two-pin Phoenix-style connectors as power
inlets. These come with a pluggable terminal block with screw-down
style connectors that accept bare wire from the rack's fuse and power
distribution panel. Because installers wire this plug on site and
mistakes are easy to make, it's good sense (and company policy) for
the hardware designer to put a bridge rectifier across the input leads
so that this connection can be insensitive to polarity.
This is an application of the first part of Postel's Prescription, “be
liberal in what you accept”, to power engineering. If the second part,
“conservative in what you send”, has an analogue in this field,
it could be this: make sure your power outputs, if any, have clearly
defined polarity. Another possible analogue would be: keep the output
to a tighter voltage range than the telecom standard 36-72 volts.