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Comment by bluGill | Hacker News Reader

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.

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> 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

100% agree the US plug designs are terrible.

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.

Dryers and Stoves/range outlets are very common in US houses. Of course they are generally hidden behind the device and so most people are not sure if they have them at not. They are also reasonably common in garages (welders, air compressors, table saws... - if your hobby needs them you install them, otherwise you won't have them).

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!

> If you squint a bit, the grounded neutral of the Y transformer isn't entirely unlike the grounded center tap in the US.

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.

> but you only get the safety benefit on three phase equipment.

The wye ground is beneficial even if there are no three phase loads, because the line-ground voltage is 1/sqrt(3) and you can use cheaper switchgear.

The 3-phase alternative to a wye ground is a corner ground (grounding a live phase of a delta secondary), which isn't done in modern installations because ground faults are full line-line voltage and you need more expensive switchgear.

Line-line faults are always interrupted by two breakers on a wye grounded 3 phase system. But with a corner ground, one breaker potentially has to break the full line voltage for a line-ground fault (you can't fuse the grounded phase).

I'm not talking about the safety of different ways of doing three phase. I'm taking it as a given that three phase is wye ground and optimal for safety.

My point is that in the context of a 230V single phase circuit, your ground is no longer in the middle. The ground is on one end of your single phase. If you want a safer single phase, you need to rebalance it to +115 and -115.

I thought they meant a "two phase" service where the two phases are 120 degrees out of phase instead of 180, like an apartment fed with two poles of a 208V wye with 120V to neutral/ground, which is balanced. If it's just a single phase, you're right.

EU power is ~230V on a single phase and ~400V between phases. So a house usually gets three phases, but the only appliance where the voltage to ground is less than the total voltage is probably the oven.

> 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.

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