Utility poles • Hacker News Board

Re: Utility poles

#61

Post by rdiddly » Wed, Feb 14, 2018, 10:41 PM UTC

wlesieutre wrote: post #52

trevyn wrote: post #48

Earlier quoted context omitted.

Now: For two phase in your home, why is one side considered "hot" and the other side considered "neutral", resulting in keyed plug blades?

The two hot wires are perfectly out of phase, so connecting to both of them gives you 240V outlets (used for large appliances like clothes dryers). Your standard 120V appliance outlets are only using a single phase and neutral. IIRC the smaller prong is hot and the larger prong is neutral.

Correct, and more conventions in the US that are mostly true so don't bet your life on them:

hot has black insulation, neutral white

hot wire insulation will be smooth, neutral ribbed

hot terminal screws will be brass, neutral silver (chrome)

Re: Utility poles

#62

Post by twtw » Wed, Feb 14, 2018, 10:43 PM UTC

trevyn wrote: post #48

s_tec wrote: post #41

Regarding three-phase power: The electrical outlets in your home have two power-carrying prongs. The third prong is just for safety (it connects to the ground), and should never carry power unless something is going horribly wrong. In that case, the power should hopefully prefer to reach the ground through that third wire than through your human body. Anyhow, the voltage in those two power-carrying wires is constantl…

Now: For two phase in your home, why is one side considered "hot" and the other side considered "neutral", resulting in keyed plug blades?

That is single phase. The "hot" (or "live") wire has a nonzero voltage relative to ground, and the neutral wire is usually around the same voltage as ground (do not count on this).

The one-way plug ensures that the "hot" voltage stays in the wall when the device isn't on, so if you shove your wand into a toaster that's off you won't get electrocuted. Devices are always connected to neutral, and the switch connects them to hot.

Re: Utility poles

#63

Post by dnm » Wed, Feb 14, 2018, 10:44 PM UTC

s_tec wrote: post #41

Regarding three-phase power: The electrical outlets in your home have two power-carrying prongs. The third prong is just for safety (it connects to the ground), and should never carry power unless something is going horribly wrong. In that case, the power should hopefully prefer to reach the ground through that third wire than through your human body. Anyhow, the voltage in those two power-carrying wires is constantl…

> the voltage in those two power-carrying wires is constantly switching directions

I'm not buying this (in the USA). I've been in my breaker box. For a typical 110 volt outlet, the black (hot) wire is connected to the breaker, which is connected to one of the wires coming into the house from the street. The white (neutral) wire is connected to the same ground bracket that the bare (ground) wire is connected to.

240 volt connections (like my dryer and range) are taking a hot line from one of the lines coming into the house and another hot line from the other line coming into the house. You can see it on the bus bars in the breaker box. That's why dual breakers are used. Adjacent breakers pull for different bus bars.

*edit typo

Re: Utility poles

#64

Post by wl » Wed, Feb 14, 2018, 10:44 PM UTC

trevyn wrote: post #48

s_tec wrote: post #41

Regarding three-phase power: The electrical outlets in your home have two power-carrying prongs. The third prong is just for safety (it connects to the ground), and should never carry power unless something is going horribly wrong. In that case, the power should hopefully prefer to reach the ground through that third wire than through your human body. Anyhow, the voltage in those two power-carrying wires is constantl…

Now: For two phase in your home, why is one side considered "hot" and the other side considered "neutral", resulting in keyed plug blades?

Neutral is tied to earth ground at some point.

Re: Utility poles

#65

Post by blattimwind » Wed, Feb 14, 2018, 10:45 PM UTC

rdiddly wrote: post #61

wlesieutre wrote: post #52

Earlier quoted context omitted.

The two hot wires are perfectly out of phase, so connecting to both of them gives you 240V outlets (used for large appliances like clothes dryers). Your standard 120V appliance outlets are only using a single phase and neutral. IIRC the smaller prong is hot and the larger prong is neutral.

Correct, and more conventions in the US that are mostly true so don't bet your life on them: hot has black insulation, neutral white hot wire insulation will be smooth, neutral ribbed hot terminal screws will be brass, neutral silver (chrome)

Screw terminals in a house installation?!

Re: Utility poles

#66

Post by s_tec » Wed, Feb 14, 2018, 10:49 PM UTC

tripa wrote: post #45

s_tec wrote: post #41

Regarding three-phase power: The electrical outlets in your home have two power-carrying prongs. The third prong is just for safety (it connects to the ground), and should never carry power unless something is going horribly wrong. In that case, the power should hopefully prefer to reach the ground through that third wire than through your human body. Anyhow, the voltage in those two power-carrying wires is constantl…

This is the weirdest rationalization of three-phase power I've ever read. By your reasoning and disregarding cos phi issues, monophase power is also constant-zero power, since it's two wires anyway and one is always the opposite of the other. The best explanation of why three I was ever given hinged on "it's the number-of-phases argmin for the needed volume of wiring".

You are right. Three-phase needs less copper wire for the same amount of power delivery, which is the real reason everyone uses it. It needs less copper because it never has "dead time" where the copper isn't delivering power (hence the desire for constant instantaneous power).

I tried to word my explanation as accurately as I could, but I had to leave out a lot of tricky details for people who aren't electrical engineers. You have to be careful not to mix up regular averages, RMS averages, and instantaneous values when you deal with AC. If you get them wrong, you come to incorrect conclusions, such as the idea that single-phase electricity has zero power. The average voltage is indeed zero, but you can't use regular averages when you calculate power delivery, only instantaneous voltage differences and currents. You get RMS averages when you integrate those instantaneous values over time, and that brings in the the cos(phi) issues you mention.

Re: Utility poles

#67

Post by blattimwind » Wed, Feb 14, 2018, 10:51 PM UTC

ThrustVectoring wrote: post #60

tripa wrote: post #45

Earlier quoted context omitted.

This is the weirdest rationalization of three-phase power I've ever read. By your reasoning and disregarding cos phi issues, monophase power is also constant-zero power, since it's two wires anyway and one is always the opposite of the other. The best explanation of why three I was ever given hinged on "it's the number-of-phases argmin for the needed volume of wiring".

Three phase power is extraordinarily useful for electric motors. Mount three electromagnets radially and wire each up to a different phase, and you get a rotating magnetic field that spins on the axis. Electric generators produce three-phase power pretty naturally through the inverse process of spinning a magnet through the resistance provided by three groups of windings.

You can have a two phase power system[1], but two phase generators are less efficient and more expensive relative to three phase – for the exact same reason why power distribution is less efficient and more expensive relative to three phase.

[1] two phases in quadrature, which is very different from split phase, which is not two phase at all.

Re: Utility poles

#68

Post by ars » Wed, Feb 14, 2018, 10:52 PM UTC

s_tec wrote: post #41

Regarding three-phase power: The electrical outlets in your home have two power-carrying prongs. The third prong is just for safety (it connects to the ground), and should never carry power unless something is going horribly wrong. In that case, the power should hopefully prefer to reach the ground through that third wire than through your human body. Anyhow, the voltage in those two power-carrying wires is constantl…

> The electrical outlets in your home have two power-carrying prongs.

Only one of the prongs carries power. The other is attached to the earth, and is always at zero volts.

> and then they will slowly switch places over the next 1/120 of a second so the right wire is at +170 volt

No. The power prong (the smaller one) will switch from -170 to +170. The other one (the neutral) is always at zero (measured relative to you).

> Turing three-phase voltage into two phase voltage is pretty easy. Just pick any two of the three wires and hook them into the home.

This is not what they do. If they did they would not be able to have a neutral, and you would also not have an option of 240v. Homes don't have two phases, they have split phase.

Instead they take a single phase, and attach it to a center tapped transformer, which I described here: https://news.ycombinator.com/item?id=16380133

> Just pick any two of the three wires and hook them into the home.

In actuality the phases have 240 volts between them, not 120. And from any phase to neutral is 208 volts (which some devices make use of). (Those are voltages to customers. Internally they use other voltages - in particular when they use a single phase, power a street with it - they care about the voltage to neutral, not the voltage relative to another phase.)

Re: Utility poles

#69

Post by blattimwind » Wed, Feb 14, 2018, 10:55 PM UTC

pan69 wrote: post #6

I have always wondered about utility poles, they always seem very fragile. Why not just stick everything in the ground?

Most of Europe buried its LV power cabling decades ago. The answer is probably a mixture of "nobody bombed US cities", "no funding" and "lower population density". Although the latter doesn't apply to metro areas, obviously.

(Also, residential three phase power is fairly common except for the brits).

Re: Utility poles

#70

Post by twtw » Wed, Feb 14, 2018, 10:56 PM UTC

dnm wrote: post #63

s_tec wrote: post #41

Regarding three-phase power: The electrical outlets in your home have two power-carrying prongs. The third prong is just for safety (it connects to the ground), and should never carry power unless something is going horribly wrong. In that case, the power should hopefully prefer to reach the ground through that third wire than through your human body. Anyhow, the voltage in those two power-carrying wires is constantl…

> the voltage in those two power-carrying wires is constantly switching directions I'm not buying this (in the USA). I've been in my breaker box. For a typical 110 volt outlet, the black (hot) wire is connected to the breaker, which is connected to one of the wires coming into the house from the street. The white (neutral) wire is connected to the same ground bracket that the bare (ground) wire is connected to. 240 v…

The explanation in that paragraph is not necessarily incorrect, just confusing because it talks about voltage "in" wires and isn't clear about what two terminals are being referenced.

It is true that the voltage between live and neutral changes sign, so neutral is sometimes 170V above live. But neutral is almost always at ground, and live goes to -170V relative to ground.

That last paragraph though seems wrong, so OP may have been confused himself when writing.