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.

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)

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

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.