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.
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