Postel's Prescription and Power Polarity

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.

We've built products at the request of specific customers before and in that kind of development process it sometimes happens that the customer demands I label one pin as positive and one as negative, probably because of some internal documentation requirement. This used to annoy me—it doesn't matter people!—and I'd only grudgingly label the pins in whatever arbitrary way to silence the customer. But I've now realized I was wrong to resist this request; after all, what happens when some random technician comes across this hardware in the future? He'll look at an unlabeled power inlet, scratch his head, and maybe take the risk of plugging it in any which way. Or, if he's a more careful worker, he'll have to go back to his desk and search the Internet for a datasheet or installation manual or something. Given the very long lifetime of CO technology (but that's another blog post!) it may well be that documents have become unavailable. I've worked on systems that have been running longer than I have, and when we design hardware to enter this kind of environment, we should plan on it outlasting our company itself!

So, to help out that future technician, I've decided it really is better just to lie a little bit, and combine being liberal in what you accept with letting the user believe you're being conservative. I suspect this can be generalized to other areas of technology.

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