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

by mccoyc 3y ago

kurthr 3y ago
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-body-resistance-i/ https://incompliancemag.com/article/experiments-of-dc-human-...
- jacquesm 3y ago
  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.
  - IgorPartola 3y ago
    I assume if you didn’t live from those zaps you wouldn’t be here making this comment.
    - jacquesm 3y ago
      Haha, fair point, yes, indeed. The most clear form of survivorship bias, thank you for pointing it out.
firebat45 3y ago
How exactly do you define a negative voltage unless you are using some other voltage as a reference?
- dragontamer 3y ago
  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.
  - Kirby64 3y ago
    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.
- applied_heat 3y ago
  Ground positive terminal of battery string instead of grounding negative terminal. I see this more often on European stuff
- bluGill 3y ago
  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)
- myself248 3y ago
  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.

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

by mccoyc 3y ago

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

jacquesm 3y ago
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.
- IgorPartola 3y ago
  I assume if you didn’t live from those zaps you wouldn’t be here making this comment.
  - jacquesm 3y ago
    Haha, fair point, yes, indeed. The most clear form of survivorship bias, thank you for pointing it out.

firebat45 3y ago

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

dragontamer 3y ago
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.
- Kirby64 3y ago
  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.
applied_heat 3y ago
Ground positive terminal of battery string instead of grounding negative terminal. I see this more often on European stuff
bluGill 3y ago
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)
myself248 3y ago
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.