Does that mean when you have electronics and use multiple dc-dc converters all the inputs and outputs share the same ground, it's not just the values for that pair of wires?
And if I want to use a telephone on an incorrectly wired 48dc circuit, I could switch the positive and negative wires, as long as the circuit in the telephone is isolated and never touches ground?
Thanks. Somehow I got in my head that all circuits were just about the delta from neutral and therefore nothing outside them mattered.
No, it depends on the converter. There are converters that leave 160V on the DC power rail for a 110V AC input, and 155V on the DC "ground" rail.
They are economic and you could find then when galvanic isolation is at least in theory not important, but they're terribly unsafe when used on PCBs that people might muck with.
If you have some "normal" converters and some of this kind, sharing the ground would be quite dangerous.
I think a circuit should mostly care about the deltas, but when you’re talking about things like phone lines, the earth becomes part of your circuit. You can’t influence its potential (it’s almost exactly neutral because any charge imbalance gets removed by interaction with the interplanetary medium) so everything else is going to end up being determined by what you need for their relative potential to that.
With DC systems you generally think about the issues - which is why modern cars are negative ground. However other than cars most people never encounter power systems of any size - inside a computer the voltages and distances are usually small enough that it doesn't matter what ground is. Not to mention most computers don't even have a chassis ground plane (there are circuit board ground planes but they conceptually different), and with non-conductive (plastic) cases ground doesn't even make sense.
And because that problem of galvanic corrosion the GGP talked about, and the mirror one of material aggregation don't happen. And it also makes switches more reliable.
Both are less dangerous on telephone lines. But are very important on electricity ones.
1 - It won't break your posts, but can easily short small contacts.
With AC it's about where the ground is attached along the length of the transformer secondary. In the EU they ground one of the ends of the secondary, in the US we ground the center point.
I don't get to say this very often ... but the US way is objectively safer with no downside: 99% of human shocks are via ground, and it halves the voltage to ground (120V vs 240V). A neutral isn't required if there aren't 120V loads.
- uninsulated metal pins make contact with supply while partially exposed - much smaller distance between metal pins and the edge of the plug
But there's no inherent power tradeoff: you can have 240V outlets in the US, with the two prongs both 120V to ground. They're just really uncommon in residences.
Yes, but you only get the safety benefit on three phase equipment.
In the US there aren't a lot of 240V plugs, but if you get some installed you can get the safety benefits with plain old consumer goods.