Why doesn't everything use high voltage power?

[Demon-Xanth]

The upsides of 220 is more power can be transferred given the same size wire. Arcing happens whenever two conductors get close enough to make an open a short for their potential difference.

A good illustration of how the higher voltage helps is if you look at say, a Core i5 750. It burns about 95W@1.25V. This ends up being about 80A. To move that much current you would need a 6 AWG wire. (chassis wiring spec) However at 12V, you only need about 8A, which can be fed with a 20AWG wire.

There are other limiting things that explain why we don't have a single small wire going to our MB (mainly the contact's capacities). But if the PSU dropped the voltage itself we'd have battery cables running to our MBs. And yes, I've worked on systems with 5V 150A power supplies that fed VME backplanes. They had inch thick cables feeding them.

But as stated earlier, the flip side is a higher insulation requirement. For uncoated external traces on a PCB, IPC-2221 states that a 15V (peak) circuit needs just 0.1mm of space, while 150V needs 0.6V, and 250V 1.25mm. So you run into a situation where you're balancing a higher voltage to use smaller conductors or a lower voltage to use smaller insulation.

 

[Gibson486]

Is arcing still possible if the line is grounded, i.e., everything is installed properly? Again, I know it's probably a dumb question, but wouldn't a thick copper grounding wire almost always have a lower resistance than your body?

And what are the upsides of ~220V over 120V if not efficiency? I thought the main benefit of high voltage was to minimize the transmission loss. Or is it just that when talking about voltages so low already, the main benefit of 220V is packing more power into a transmission line?

Yes, it is still possible. Arcing happens when you create a short.

What you described is under steady state. Under instantaneous changes, lots of the stuff does not work that way because of electric fields. When you have something get 0 mA current to 100 A current in a split second, you are gonna create a pretty damaging electric field. Yes, it will take the path of least resistance to ground, but that path will also make an electric field that gets conducted by anything that wants to conduct it. In yoru case, the path to ground/least resistance would be the arc.

As for high voltage being more efficiency... A voltage drop is a voltage drop. Just because you have more voltage does not mean you decrease the voltage drop. It's still the same voltage drop, but under high voltages, that voltage means less. In other words, a 10V drop on a 120V line has more impact than a 10V loss on a 240V line. There are lots of ways to compensate for voltage drops (wye vs delta), but for consumers, all they need to know is that higher voltage lead to to less current draw, so breakers can be sized smaller. That's why ovens get 220V. That way, they do not need a monster breaker, which would probably be 50% of the max amperage of the breaker box.

 

[Mark R]

But would you be dead if you were shocked with 1200 volts?

1200 V when supplied from a low-resistance source, like mains power, would almost certainly kill under most 'shock' circumstances.

Between about 60-300 V, it's a bit hit-and miss. If you are shocked with wet hands, or a big contact area - then the shock is likely to be serious. However, with dry hands, or a circuit only completed through clothing/shoes, etc. - or only a glancing touch - there is a reasonable chance of survival. Obviously, chance of survivial goes up as voltage goes down.

Additionally, below about 400 V, there is a relatively low risk of severe arc formation. Above about 400 V, a fault (e.g. short circuit) can cause ionisation of the air, and cause a short circuit through the air - this can be very destructive, causing massive injury to anyone within several feet, and setting fires. Below about 400V arcs can still occur but tend to be severely limited in size and power (they can still cause fires - hence the use of AFCIs) but it would be unusual for someone to get fatal injuries from a 200 V arc (unless they were extremely close).

So there are significant disadvantages going to much higher voltages, in terms of safety -with only modest gains in efficiency, so unless you are using a very large amount of electricity, it generally isn't worth using.

There is also the historical issue. The US is firmly entrenched in using 110 V power - as historically, this was the optimal voltage for the first Edison electric light bulbs which used a Carbon filament. Higher voltages meant the filament was too fragile to make a useful bulb. At the same time 110 V was the preferred voltage in much of the rest of the world.

Then, the tungsten filament bulb was invented. This didn't need such a low voltage - so the electricity companies could change to a much more efficient voltage. Most chose 2x 110 - this allowed 2 carbon filament bulbs to be connected in series - or one tungsten bulb on its own - giving a degree of backwards compatibility. The US chose to stay with 110 V.

The advantages of a higher voltage are several:
1. More power can be delivered over the same power line
2. Higher efficiency of power delivery
3. Better voltage stability. A 2000 W load at 100 V needs 20 A - that might cause a 10 V droop in supply voltage - this means the load only actually gets about 1700 W, and the voltage sags to about 90 V (10% droop - causing significant dimming of light bulbs). At 200 V, the same load needs 10 A - this would cause a 5 V droop (droop is proportional to current) - so the load would actually get about 1950 W at 195 V. (2.5% voltage droop - barely noticable flicker in light intensity).

 

[Gibson486]

If it's not more efficient, then why is it more efficient? e.g. AT PSU tests show 230v tests getting higher efficiencies than the 120v tests.

Because they have the same voltage losses. You have to compensate for the voltage loss. To compensate under 120, you need to have more current to have the same power as the 220/230 counterpart.

 

[Mark R]

If it's not more efficient, then why is it more efficient? e.g. AT PSU tests show 230v tests getting higher efficiencies than the 120v tests.

Some types of load are more efficient at higher voltages. Electronic 'universal voltage' or aPFC PSUs are one example of this.

In this type of supply, a significant loss is from the resistance in the series boost inductor. At higher voltages, the PSU needs lower current, so the current through the boos inductor is reduced, dramatically reducing losses.

Similar effects are seen in other designs of electronic PSU - but they are less marked.

 

[Perknose]

I've been shocked by 110v at least a dozen times. I never died any of those times.
I've been shocked by 220 once and I didn't die then, either.

Did you at least get an erection?

 

[PaperclipGod]

Ahh... this thread has really helped me clear up some of my confusion. Thanks to everyone; especially Mark and Gibson!

 

[ObscureCaucasian]

What gets you is the overall power dissipated into you, that's why static electricity can be incredibly high voltage but harmless. The static electricity has a fixed amount of charge built up, and one that charge is gone no more current can flow. This is the same reason why high voltage tasers and capacitors aren't particularly lethal.

 

[olds]

Did you at least get an erection?

What do you think I was sticking in the socket...

I am surprised that the 220 didn't kill me.
I was drunk and stoned at a pool party. The electrical service panel there didn't have a cover on it. I was showing off and I tripped the breaker. At the time I was wet from swimming and I was standing on a hose that was coiled up under the service panel. I then misread line and load on the panel and put one finger from my right had one one terminal and one finger from my left hand on the other terminal. Of the line side.
I knew about it right away.

 

[TheLonelyPhoenix]

Obligatory "Google is your friend":

http://wiki.answers.com/Q/Why_was_1...of_homes_in_the_US_and_not_some_other_voltage

Yes, high voltages are more dangerous because of the possibility of arcing (edit: and, of course, the increased current for a given resistance in your body), but you can still kill yourself quite handily with 120 Vrms. ~10mA across your heart is more than enough to stop it, easily achievable depending on insulation / skin conductance / etc.

 

[Iron Woode]

What makes high voltage so much more dangerous? Aren't 120V devices just as capable of killing you? Isn't it like saying getting shot in the head with a bullet is "safer" than getting shot with a cruise missile?

Do you mean 220-240 for household use?

that is used for dryers, table-saws, air-conditioners, vacuums, etc...

220 is used in high-load equipment because it reduces current draw. Its not any more dangerous than 120v. Amperage kills you, not voltage.

 

[Evadman]

0.02 amps though your chest for a little while will stop your heart, but 0.06 amps will stop your heart before you can let go of the wire. You will 'feel' a shock if it is 0.005 amps or greater. one of those little christmas tree lights (7.5 watts) is 0.060 amps.

AC or DC doesn't matter (lightning is DC and that kills people just fine), what matters is that the current goes across your chest (such as holding a live wire in one hand, and being grounded on the other side).

Depending on conditions of your body and how good the ground is, your body doesn't have as much resistance as you may think. a 12v battery can kill you if you try hard enough. Remember, 9 volts can travel though you (you can shock your tongue with a 9v battery) but over a long enough distance 9v will be well under 5 miliamps so you won't feel it.

 

[Ika]

What do you think I was sticking in the socket...

I am surprised that the 220 didn't kill me.
I was drunk and stoned at a pool party. The electrical service panel there didn't have a cover on it. I was showing off and I tripped the breaker. At the time I was wet from swimming and I was standing on a hose that was coiled up under the service panel. I then misread line and load on the panel and put one finger from my right had one one terminal and one finger from my left hand on the other terminal. Of the line side.
I knew about it right away.

wow, how old were you? sounds a great thing to do while saying "hey guys, watch this!"

 

[Suspicious-Teach8788]

No, it's not more efficient. There are lots of upside to going 220/230/240 as opposed to 120, but efficiency is not one of them.

That said, we do transmit everything at high voltage. It, however, gets stepped down when it goes to service.

And, you are correct. 120V is still an arc flash hazard...however, the arc will not be as big as, say 460. AT 120, you are more worried about touching live wires, not the arcing itself.

It is more efficient in terms of power dissipated due to resistance in your wires. If the resistance is R, then you lose i^2 * R over that distance. The higher your V, the lower the I. this is why we use HV lines to transmit power. Otherwise you'd be losing too much power in the wires themselves...

 

[Special K]

On a related note, I have heard that we use AC power because less power is lost transmitting AC as opposed to DC. How is this the case? If P = VI = I^2R = V^2/R, how does AC vs. DC factor into that?

I understand that it's easier to step up an AC voltage than a DC voltage.

 

[Special K]

It is more efficient in terms of power dissipated due to resistance in your wires. If the resistance is R, then you lose i^2 * R over that distance. The higher your V, the lower the I. this is why we use HV lines to transmit power. Otherwise you'd be losing too much power in the wires themselves...

Right, but doesn't power also equal V^2/R? I guess that's what I was asking in my post above.

 

[Suspicious-Teach8788]

Right, but doesn't power also equal V^2/R? I guess that's what I was asking in my post above.

If you treat the wire as a resistor, that V refers to voltage drop across the wire, not the voltage being sent.... So yes you could use that V, but it's not the 220V we're talking about. It's a little harder to determine that I suppose?

 

[dighn]

On a related note, I have heard that we use AC power because less power is lost transmitting AC as opposed to DC. How is this the case? If P = VI = I^2R = V^2/R, how does AC vs. DC factor into that?

I understand that it's easier to step up an AC voltage than a DC voltage.

all things equal I don't think AC is more efficient than DC. In fact AC has its own unique modes of losses like capacitive and radiative losses. The efficiency comes from the ease of stepping up the voltage for long distance transmission and stepping down for actual use.

 

[PieIsAwesome]

On a related note, I have heard that we use AC power because less power is lost transmitting AC as opposed to DC. How is this the case? If P = VI = I^2R = V^2/R, how does AC vs. DC factor into that?

I understand that it's easier to step up an AC voltage than a DC voltage.

We use AC because transformers can be used to step the voltage up or down with AC, but not with DC. High voltage can be used in power lines, where it makes more sense to use high voltage for reasons already discussed, and then stepped down to 120v/240v inside the house.

 

[obamanation]

If they really wanted to improve efficiency without affecting safety, they could upgrade from 60hz to something higher like 120hz or even higher than that. I mean just comparing 240v 50hz and 120v 60hz tends to be around the same efficiency, which should say quite a lot about how frequency affects efficiency.

 

[Gibson486]

It is more efficient in terms of power dissipated due to resistance in your wires. If the resistance is R, then you lose i^2 * R over that distance. The higher your V, the lower the I. this is why we use HV lines to transmit power. Otherwise you'd be losing too much power in the wires themselves...

That is a misunderstanding that every one has. The higher V does not automatically give you lower i. Infact, your equation is power dissipated at the load, not in the line and plugging into the standard voltage equation (V=IR), you can easily see that it is indeed wrong. You could argue that you want to include the line resistance in your load calc, but doing so would be useless since you need to compute the power going into the load, not the power from the source.

Given a fixed resistance, you are correct that you lose something like i^2 * R over a given distance...that is however is not the equation you use to compute it. In reality, yes, it is more effecient, but not because you lose less power due to less current. It has more to do with tolerating the power losses. Losing 40 watts has a bigger effect on a 120V line than a 240V line. Also, when you claim 480 V is more effecient than 120V lines, they are referring to a wye vs delta arrangement, which is a whole course in power systems.

 

[Possessed Freak]

I've been shocked by 110v at least a dozen times. I never died any of those times.
I've been shocked by 220 once and I didn't die then, either.

110 tickles. 220 was a bit more enlightening when I felt it.

 

[Gibson486]

On a related note, I have heard that we use AC power because less power is lost transmitting AC as opposed to DC. How is this the case? If P = VI = I^2R = V^2/R, how does AC vs. DC factor into that?

I understand that it's easier to step up an AC voltage than a DC voltage.

because under AC, you also have inductance and capacitance in the line. Under DC, capacitance and inductance either block or short, so all we have is just resistance.

 

[ussfletcher]

No, it's not more efficient. There are lots of upside to going 220/230/240 as opposed to 120, but efficiency is not one of them.

That said, we do transmit everything at high voltage. It, however, gets stepped down when it goes to service.

And, you are correct. 120V is still an arc flash hazard...however, the arc will not be as big as, say 460. AT 120, you are more worried about touching live wires, not the arcing itself.

Using a higher voltage means you can use a lower amperage. The lower the current flow, the less energy is lost to heat in a resistive substance (copper). Thus, this is why transmission lines are several tens of thousands to hundreds of thousands of volts (and miliamps of current). Over a long stretch the savings are very, very pronounced.

Lets not get into the specifics of tri-phase power systems though.

 

[Gibson486]

Using a higher voltage means you can use a lower amperage. The lower the current flow, the less energy is lost to heat in a resistive substance (copper). Thus, this is why transmission lines are several tens of thousands to hundreds of thousands of volts (and miliamps of current). Over a long stretch the savings are very, very pronounced.

Lets not get into the specifics of tri-phase power systems though.

very true!