Because the chassis is connected to ground (as in, a literal grounding rod hammered into the soil) and by definition your 0V reference point.
The crucial difference is the direction in which the current is flowing: is it going "in to", or "out of" a hot wire? This becomes rather important when those wires are leaving the building and are buried underground for miles, where they will inevitably develop minor faults.
With +48V corrosion will attack all those individual telephone wires, which will rapidly become a huge maintenance nightmare as you have to chase the precise location of each, dig it up, and patch it.
With -48V corrosion will attack the grounding rod at your exchange. Still not ideal, but monitoring it isn't too bad and replacing a corroded grounding rod isn't that difficult. Telephone wires will still develop minor faults, but it'll just cause some additional load rather than inevitably corroding away.
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.
There is a true zero potential. You can detect this because two charged objects with zero delta between them will still repel each other.
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.
Voltage is _always_ relative; you only ever measure differences between two points. Voltmeters have two probes. Ground is just a convenient reference, not a universal zero. And while Earth can act as a reference or return path in some systems, it’s not perfectly neutral or fixed...it’s just large enough to approximate that in practice. (The interplanetary medium aka aether doesn't exist)
Objects don’t repel because they’re at the same potential. Electrostatic force comes from electric fields due to charge. If two objects truly have zero potential difference and no field between them, there’s no force.
You’re correct that circuits care about voltage differences. After all, all work requires a force gradient of some kind.
Can't you define zero as the limit of the potential at infinity? And neutral charge bodies in empty space are generally close to that.
The interplanetary medium absolutely exists. I'm not talking about aether. I'm talking about the soup of dust, gas, and particles that fills space in the solar system. It contains a lot of charged particles, which is what keeps Earth extremely close to neutrally charged. Any deviation from neutral starts attracting positively charged particles and repelling negative, or vice versa, which equalizes the charge.
I didn't say objects repel because they're at the same potential. I said that objects at the same potential will still repel each other if that potential isn't zero.
Seriously, what is this reply? Aether? Objects repelling because they're at the same potential? You seem to have read a comment very different from what I wrote.
Yeah, I did mistake what you were trying to say. We’re just coming at this from two different sides... physics does have the convention that absolute zero potential is defined at infinity. I was looking at it strictly from a practical circuit perspective, where 'zero' is just wherever we stick the ground probe, and we only ever calculate for the relative deltas. And we generally don't have wires that can make it to the interplanetary medium or the edge of the infinite ground plane :)
> all the inputs and outputs share the same ground, it's not just the values for that pair of wires?
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 have done some projects that needed some generic dc-dc converters from aliexpress (eg stepping down 12v to 5 or 3.3) I alway treated the output of each step down as a pair of wires that share no ground. It sounds like that would be overkill if they were reputable but it's probably best to not try tying the grounds together.
I figured any happenstance from the multimeter that the grounds match was transitory and not to be trusted.
Most large scale systems are AC because transformers are relatively cheap, low maintenance, and efficient. When the system is AC ground makes no difference.
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.
> When the system is AC ground makes no difference.
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.
I agree that the US voltage is safer (with the tradeoff of lower output powers available at your outlets). However, I suspect this is more than negated by the US plug design, which carries a much larger risk of shocks than almost all EU plug designs (Schuko, British/Type G, etc...)
- 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.
In the EU it is quite common for houses to have three-phase power. If you squint a bit, the grounded neutral of the Y transformer isn't entirely unlike the grounded center tap in the US. The voltage is a lot higher, of course!
> because transformers are relatively cheap, low maintenance, and efficient
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.
I ran into a guy at a hardware store who ran just such a power supply attached our city's water (or was it natural gas?) infrastructure. I was incredulous, but the idea that it helped prevent corrosion did make sense.
The crucial difference is the direction in which the current is flowing: is it going "in to", or "out of" a hot wire? This becomes rather important when those wires are leaving the building and are buried underground for miles, where they will inevitably develop minor faults.
With +48V corrosion will attack all those individual telephone wires, which will rapidly become a huge maintenance nightmare as you have to chase the precise location of each, dig it up, and patch it.
With -48V corrosion will attack the grounding rod at your exchange. Still not ideal, but monitoring it isn't too bad and replacing a corroded grounding rod isn't that difficult. Telephone wires will still develop minor faults, but it'll just cause some additional load rather than inevitably corroding away.