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Comment by mccoyc | Hacker News Reader

Agreed. 48V (actually -48V) has been used across telco central offices for decades.

It's a really nice voltage with lots of support for batteries and up/dn conversion hardware.

It's also right at the edge of what is human safe. You can burn yourself and blow up cables, but it's very difficult to electrocute yourself (afib or muscle seize) without lots of wet contact.

https://incompliancemag.com/article/experiments-of-dc-human-...

Indeed, I'm aware of only one recorded death by electrocution at 48V, iirc it was a Swiss radio amateur that had done a bunch of gardening sat down sweaty in a metallic chair and reached for the one switch of his set. Probably there were other contributory causes as well, I've been zapped multiple times from much higher voltage sources (that could have easily supplied the power required) and lived.

I can't find a reference for that Swiss case though. I'll keep looking.

I assume if you didn’t live from those zaps you wouldn’t be here making this comment.

Haha, fair point, yes, indeed. The most clear form of survivorship bias, thank you for pointing it out.

How exactly do you define a negative voltage unless you are using some other voltage as a reference?

Label the power pin+ GND and the power pin- becomes -48V

Voltages are all relative. It's like saying 'How do you get a height difference of 10 feet by digging?'

Well, you dig and then label the initial level as +10 feet, and redefine the bottom of your hole to be ground.

No. In telco, the -48V is referenced against ground, like the physical ground. If you're isolated, you can do this. but they would still need to be referencing the 'ground' to something ... likely the negative side of the main battery pack.

The reason why -48V is used is because it is provided as a bias voltage to give wiring cathodic protection, to prevent corrosion of telecom infrastructure. If you used 48V, it would not work. You need a negative voltage referenced against ground.

Ground positive terminal of battery string instead of grounding negative terminal.

I see this more often on European stuff

Generally with respect to ground. There are many good reasons to connect your power system to ground and so this is commonly done. (there are pros and cons to connecting to ground, but it gets complex fast)

It is with respect to ground, the positive pole of the battery is connected to ground.

The telegraph system figured this out very quickly. Most water in nature has at least a bit of salt in it, which is present as positive sodium ions and negative chloride ions. By making the outdoor wiring negative with respect to ground, the chloride ions are repelled, and such wires corrode much more slowly than those that're positive with respect to ground.

Since most of the telegraph network, later the telephone network, is outdoors, this is a pretty big deal.

> that’re

First time I’ve ever seen this typed

What’re you talking about?

I guess he refers to the shortening of "that are" into that're

I think you're missing your parent's joke.

Now I see it, thanks :-)

It's a matter of perspective.

You tie one of the leads to earth (literally grounding it)[1], leaving the other non-grounded. Depending on if you tie the negative or the positive lead to ground, you get 48V or -48V with respect to ground. As long as the potential between the most positive lead and the least positive lead is 48V, the circuit itself doesn't care.

As mentioned here[2], the reason for grounding the positive lead is to prevent galvanic corrosion[3] destroying the buried copper.

[1]: https://www.bicsi.org/docs/default-source/conference-present...

[2]: https://www.poweringthenetwork.com/uncategorized/negative-48...

[3]: https://en.wikipedia.org/wiki/Galvanic_corrosion

Apparently in cars it’s weirder. Wire it one way and the wiring corrodes. Go the other way and the body corrodes.

Voltage is a measure of charge difference so there must always be a reference, usually the reference is 0 V.

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