Why are electrical outlets grounded?

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Goemon
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Why are electrical outlets grounded?

Postby Goemon » Sun Feb 22, 2009 6:22 pm UTC

Anybody have any idea why one side of the electrical power supply coming into your house is connected to ground?

It might save money if it meant needing to run only one wire to appliances (if they used a local ground as a return). But instead, we now have to run THREE wires to provide saftey shielding for appliances and such, which would be unnecessary if one side wasn't grounded in the first place. It doesn't seem to have any cost, saftey, or functional benefits. In fact, it makes things more dangerous and more expensive.

So what's the purpose of grounding one side?

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Re: Why are electrical outlets grounded?

Postby justaman » Sun Feb 22, 2009 7:36 pm UTC

I am not any sort of electrician, but isn't it in case of power surge or lightning strike, to prevent fires from the local grounding?
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Re: Why are electrical outlets grounded?

Postby Govalant » Sun Feb 22, 2009 7:59 pm UTC

Because they misbehaved.

No, really. I see the advantage of not having a big ass-cable that needs to transport three times the current of the other fases. Also I think the generator doesn't have a ground cable, so any one will do. There is no actual zero.

But I dunno.
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Re: Why are electrical outlets grounded?

Postby You, sir, name? » Sun Feb 22, 2009 8:25 pm UTC

Well, it's pretty important to have a common ground when connecting different appliances with each other. If you don't, you will get undesired current that may (worst case scenario) destroy sensitive electronics, cause resistive heating that starts fires, give people unexpected jolts, etc.

A more harmless case is connecting an ungrounded TV to a grounded computer's TV-out jack. You will get noise in the image on the TV because what the computer considers ground and what the TV considers ground are different.
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Re: Why are electrical outlets grounded?

Postby idobox » Mon Feb 23, 2009 12:07 pm UTC

It is also a major security feature:
differential circuit breaker compare the current between neutral and signal (I'm not sure of the English for this). Normally, there is no current to the ground, so the neutral and signal current have the same magnitude and opposite sign. If there a short circuit, for example between the signal and the floor, the current returned by the neutral will be lower than the one fed to the signal, and the circuit breaker will cut the power.
Also, having a common ground avoids static accumulation and greatly lowers electric noise.
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Re: Why are electrical outlets grounded?

Postby zekone » Mon Feb 23, 2009 1:36 pm UTC

idobox wrote:It is also a major security feature:
differential circuit breaker compare the current between neutral and signal (I'm not sure of the English for this). Normally, there is no current to the ground, so the neutral and signal current have the same magnitude and opposite sign. If there a short circuit, for example between the signal and the floor, the current returned by the neutral will be lower than the one fed to the signal, and the circuit breaker will cut the power.
Also, having a common ground avoids static accumulation and greatly lowers electric noise.


What you're talking about are RCDs - [url=Residual Current Disconnects/Devices]http://en.wikipedia.org/wiki/Residual-current_device[/url]. They look at the incoming and outgoing currents, and if there is any difference, then electricity is somehow leaking out of the closed system, usually through something it shouldn't be (wood, fabrics, people, etc.). If electricity is leaking, that's a bad thing, and the RCD cuts off the power.

I'm not sure if I've completely understood the question, but one pin is for the live/hot/phase and is held at +230V (this is UK, it's obviously +110V in the US, for instance), another pin is the neutral/ground held at +0v, and the third is a direct to earth. The live and neutral are supplied into your house so that the difference between the two can always be maintained at the correct voltage. Don't forget that's all voltage is - potential difference.

The earth pin can be locally connected to earth, for example with mobile generators they will supply a live and a neutral, and for the each connection you just stick a large spike into the ground. The earth is used when things go wrong, and current escapes down it rather than where it's meant to be going, and this trips breakers/fuses, as the earth will provide a greater potential difference than the neutral, allowing the live to supply more current, and hence causing safety features to step-in and stop the supply.

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Re: Why are electrical outlets grounded?

Postby eternauta3k » Mon Feb 23, 2009 3:31 pm UTC

zekone wrote:I'm not sure if I've completely understood the question, but one pin is for the live/hot/phase and is held at +230V (this is UK, it's obviously +110V in the US, for instance), another pin is the neutral/ground held at +0v, and the third is a direct to earth. The live and neutral are supplied into your house so that the difference between the two can always be maintained at the correct voltage. Don't forget that's all voltage is - potential difference.
Maybe you know this, but I can't tell from your post. The voltage between live and neutral is a sine wave, it oscillates between positive and negative (I think the peak voltage equals Vrms * 1.414, where Vrms is 220V, 110V, etc). Live and neutral could be at any voltage with respect to ground.
zekone wrote:The earth is used when things go wrong, and current escapes down it rather than where it's meant to be going, and this trips breakers/fuses, as the earth will provide a greater potential difference than the neutral, allowing the live to supply more current, and hence causing safety features to step-in and stop the supply.
I don't know where you read this. A few mA leaking to the ground is enough to trip the RCD. Also, I see no reason why earth should have a greater potential difference with respect to the live wire.
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Re: Why are electrical outlets grounded?

Postby zekone » Mon Feb 23, 2009 4:31 pm UTC

Maybe you know this, but I can't tell from your post. The voltage between live and neutral is a sine wave, it oscillates between positive and negative (I think the peak voltage equals Vrms * 1.414, where Vrms is 220V, 110V, etc). Live and neutral could be at any voltage with respect to ground.


Yeah, I'm vaguely aware of the more complicated physics behind electrical power distribution, but for practical applications, I think of it in more simplified terms. For example, the voltage between N and P1 is 230, and the voltage between P1 and P2 is 415 - that kind of thing.

I don't know where you read this. A few mA leaking to the ground is enough to trip the RCD. Also, I see no reason why earth should have a greater potential difference with respect to the live wire.


Earthing has nothing to do with RCDs, it's only in the case of short circuiting (live-metal casing) when the earth is needed, when it it replaces the neutral and so what I said about it providing greater potential difference than the neutral is frankly absurd (I have no idea where I got that from). When things go wrong, it's better that current travels down the earth than out through a person.

Also, the international ambiguity of hot/phase/live/ground/earth/neutral is very annoying...

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Re: Why are electrical outlets grounded?

Postby Goemon » Tue Feb 24, 2009 6:12 am UTC

zekone wrote:What you're talking about are RCDs - [url=Residual Current Disconnects/Devices]http://en.wikipedia.org/wiki/Residual-current_device[/url]. They look at the incoming and outgoing currents, and if there is any difference, then electricity is somehow leaking out of the closed system, usually through something it shouldn't be (wood, fabrics, people, etc.). If electricity is leaking, that's a bad thing, and the RCD cuts off the power.


Part of my point in the OP is that the only reason you need an RCD (aka GFI) in the first place is because one side of the power system in your home is connected to earth ground. That means that if you're touching the "live" wire with your left hand, you can be killed by touching any grounded surface (pipes, puddles, the ground...) with your right hand. But if the so-called "neutral" wire is NOT connected to ground in the first place, you could lick a pipe while holding a "live" wire in your hand and nothing would happen. We only need RCDs BECAUSE one of the wires is grounded. If only it were floating, there wouldn't be any danger.

So why do we ground one side, and then spend extra money to install RCDs and case grounds? We wouldn't need either of them if the power lines were left floating.
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Re: Why are electrical outlets grounded?

Postby zekone » Tue Feb 24, 2009 10:15 am UTC

Goemon wrote:
zekone wrote:What you're talking about are RCDs - [url=Residual Current Disconnects/Devices]http://en.wikipedia.org/wiki/Residual-current_device[/url]. They look at the incoming and outgoing currents, and if there is any difference, then electricity is somehow leaking out of the closed system, usually through something it shouldn't be (wood, fabrics, people, etc.). If electricity is leaking, that's a bad thing, and the RCD cuts off the power.


Part of my point in the OP is that the only reason you need an RCD (aka GFI) in the first place is because one side of the power system in your home is connected to earth ground. That means that if you're touching the "live" wire with your left hand, you can be killed by touching any grounded surface (pipes, puddles, the ground...) with your right hand. But if the so-called "neutral" wire is NOT connected to ground in the first place, you could lick a pipe while holding a "live" wire in your hand and nothing would happen. We only need RCDs BECAUSE one of the wires is grounded. If only it were floating, there wouldn't be any danger.

So why do we ground one side, and then spend extra money to install RCDs and case grounds? We wouldn't need either of them if the power lines were left floating.


Can you please expand on your use of the word "floating"?

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Re: Why are electrical outlets grounded?

Postby Goemon » Wed Feb 25, 2009 5:19 am UTC

zekone wrote:Can you please expand on your use of the word "floating"?


Well... not grounded :)

In my neighborhood (USA), there's a 2000V power distribution line comng from the electrical substation. Every few houses or so, there's a transformer which steps the voltage down to 240V plus a center tap line. These three lines enter the fuse panel in my house, which in turn distributes it to all the outlets, appliances, etc. You get 120V on each side of center, or 240V all the way across.

The center tap line - for reasons which mystify me - is connected to earth through a rod sunk into the ground (this is standard practise everywhere, of course). If there was no ground connection, the output of the transfromer would be "floating": no voltage between any of the wires and anything else in the universe except for one of the other wires. No voltage relative to pipes or puddles or people sitting in bathtubs.

If I were to cut the ground connection, it would be perfectly safe for me to grasp any one of the three power lines coming out of the transformer while standing naked in a bathtub half full of water. Because one of the lines is connected to ground, and the ground is connected to the bathtub, now there's suddenly a potentially deadly voltage potential (!) between the power lines and the bathtub. So now we need ground fault interruptors, shield grounds, etc. to protect against this manufactured hazard.

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Re: Why are electrical outlets grounded?

Postby ThomasS » Wed Feb 25, 2009 5:46 am UTC

In response to the original question:

First of all, some appliances, extension cords, and older wall plugs do only use two wires. Suppose that, say, a metal lamp is wired with two wires. It has been standardized for a while which wire is neutral (and yes, both neutral and "ground" wires normally attach to the same connection block in the power box), so you have a choice... do you wire that pin to the metal lamp casing, or do you insulate both wires from the metal casing?

If you wire that pin the metal casing, then, among other things, there is a danger that the plug will be jammed in backward and the user will be electrocuted. Or, even if there is no user error, electricians are people and the possibility of reversing the polarity when wiring a house exists. It is a common enough wiring failure that standard plug testers have a light pattern indicating when this occurs. Note that such plug testers can only make this determination on three pronged outlets. Alternatively, suppose that a faulty connection develops along this return wire, then no current flows and the voltage on both sides of the light bulb will be the same. So once again the user is electrocuted. For these reasons, connecting the pin to the metal casing is a bad idea.

On the other hand, suppose that you do not wire the pin to the metal casing. Then if anything - water, frayed insulation, anything - creates a short between the hot side and the casing, the casing becomes hot. For this reason, new lamps and such in the US which operate with only 2 pins must be double insulated.

Responding to Goemon:
If I were to cut the ground connection, it would be perfectly safe for me to grasp any one of the three power lines coming out of the transformer while standing naked in a bathtub half full of water. Because one of the lines is connected to ground, and the ground is connected to the bathtub, now there's suddenly a potentially deadly voltage potential (!) between the power lines and the bathtub. So now we need ground fault interruptors, shield grounds, etc. to protect against this manufactured hazard.


Probably the biggest problem with making them all float is that then you have no way to tell when things are going wrong. One short circuit to ground from any one wire in undetectable, and then a second short circuit has a 2/3 chance of making something dangerous to touch. Also, when you start trying to make electrical equipment talk to each other, ground loops become real issues.

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Re: Why are electrical outlets grounded?

Postby idobox » Wed Feb 25, 2009 3:04 pm UTC

Probably the biggest problem with making them all float is that then you have no way to tell when things are going wrong. One short circuit to ground from any one wire in undetectable, and then a second short circuit has a 2/3 chance of making something dangerous to touch. Also, when you start trying to make electrical equipment talk to each other, ground loops become real issues.

One way or the other, you have to connect your electrical installation to the ground, otherwise static will build up, and you'll have arcing. Normally, no current is supposed to pass though the ground, and since every casing is linked to the ground, most malfunctions will cause some current to go through the ground. So you just have to monitor the ground to know if something does not work properly.
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Re: Why are electrical outlets grounded?

Postby mosc » Wed Feb 25, 2009 9:03 pm UTC

There is a shit ton of misinformation in this thread.

The main reason for electrical grounds in homes (outlets) is electrostatic shock which used to be much more common back when people directly touched potentiometers on the front of electrical equipment to adjust settings. Lightning protection is a small part of it. You ground a house to help with that. Grounding your electronics doesn't prevent much. Lighting is, well, powerful.

Electrical grounding is NOT required for electronics in general. If fact, they are often the source of tremendous headache in sensitive devices. Almost any amplifying device you buy will come WITHOUT a ground. Ground loops get magnified by high gain amplifiers and can generate noise.

Is there some static discharge risk if you don't ground your equipment? FUCK no. We don't have people reaching into active elements of devices anymore to adjust things while they're on. The volume knob on your stereo is not hooked in line with the amplifier anymore. It's a digital device.

Almost every ground plug in my house is clipped. There are very few devices that actually need grounding. Most that do are hooked up by 220V lines. I've been to people's houses who are nuts about grounding everything and then when I pop open their outlets, I find that the grounds from the outlet aren't even connected anywhere! This is common. They are excessively over-used.

Grounds were not used in homes for years. Most older homes, even ones that have 3-pronged outlets, do not have the ground plugs attached to anything. They are a relic from an era in which they were an important safety feature. Today, they just serve to annoy.

idobox wrote:One way or the other, you have to connect your electrical installation to the ground, otherwise static will build up, and you'll have arcing.
Sorry, but no. You fail to understand how AC power works. Alternating current means that you have a waveform of voltage and current. At one part of the cycle it's positive on one side, and on the other part of the cycle it's positive on the other side. Static does not accumulate. It CAN accumulate if there is a short or some other physical malfunction. However, today's electronics are not exposed. Generally, this type of failure results in "the magic smoke" and not electrocuting somebody turning the volume knob.

Remember too that electronics these days is primarily low voltage DC, not 110V AC. Transformers, pretty much by definition, isolate one side from the other independent of ground. Now, you can ground both sides of a transformer but in general principle, it creates relative voltages that are not directly related to "earth" ground.

Goemon wrote:Anybody have any idea why one side of the electrical power supply coming into your house is connected to ground?
Bulk power is 3-phase. Your house does not (generally) get hooked up on three phase. You get one phase. The easiest transform for this is running one phase to your house and connecting the other end to ground. You get a "line to neutral" voltage AC line this way. Your house becomes a resistor between that phase and ground. Load. We call this a "single phase tap" and it is the most common way of hooking up residential customers.

Goemon wrote:If I were to cut the ground connection, it would be perfectly safe for me to grasp any one of the three power lines coming out of the transformer while standing naked in a bathtub half full of water.

No it wouldn't be safe. You would die. First off that's probably FAR from the only ground wire. Second, a transformer creates it's own relative neutral even when not hooked up to an earth ground. You could ground a phase, yes, but there is SIGNIFICANT CHARGING going on. The line, the transformer, the equipment it's attached to, all have charging. You want to swing that phase to ground, it's going to discharge through you like a bolt of lightning. Do not try this at home ;)

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Re: Why are electrical outlets grounded?

Postby idobox » Wed Feb 25, 2009 9:34 pm UTC

One way or the other, you have to connect your electrical installation to the ground, otherwise static will build up, and you'll have arcing.

Sorry, but no. You fail to understand how AC power works. Alternating current means that you have a waveform of voltage and current. At one part of the cycle it's positive on one side, and on the other part of the cycle it's positive on the other side. Static does not accumulate. It CAN accumulate if there is a short or some other physical malfunction. However, today's electronics are not exposed. Generally, this type of failure results in "the magic smoke" and not electrocuting somebody turning the volume knob.

If the insulation between a conductor and the metal casing wears off, and the device has no grounding, the user is in danger of shock. Also high power sensitive equipment often use galvanic insulation.
I know what is AC, I design RF systems. The power grid is essentially a large conductor with no significant DC potential, but there are significant potential differences in the ground between different places, that's why you need a ground, and that's why large buildings need many connections to the ground. When I talk about arcing, it's not necessarily between a knob and your finger, but between anywhere on your power system and the ground.
Also, I cannot use my 3 years old laptop if it is in contact with my skin and plugged, because I get shocks. Static discharges do not need to be powerful lightnings.
Finally, in France we use 220V and grounding is very frequent for high power appliances.
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Re: Why are electrical outlets grounded?

Postby Goemon » Thu Feb 26, 2009 5:33 am UTC

Note that this thread has nothing whatever to do with the third pin now common in electrical outlets which supplies a "case ground". It's about grounding one of the actual power pins.

mosc wrote:Bulk power is 3-phase. Your house does not (generally) get hooked up on three phase. You get one phase. The easiest transform for this is running one phase to your house and connecting the other end to ground. You get a "line to neutral" voltage AC line this way. Your house becomes a resistor between that phase and ground. Load. We call this a "single phase tap" and it is the most common way of hooking up residential customers.


Three power lines exit the substation down the street from my house, carrying three phase power at approximately 2400V. None of these lines enter my house. One phase of this enters the transformer on the pole outside my home. The return line for the transformer primary is a ground connection - this ground connection makes perfect sense, since it saves the power company the trouble of stringing a fourth wire from the substation.

The transformer's secondary is wound to output 240V, plus it has a center tap to provide two separate 120V feeds, 180 degrees out of phase. This is what enters my house, yes?

The original question is: why is the center tap of the transformer output connected to ground?

mosc wrote:No it wouldn't be safe. You would die. First off that's probably FAR from the only ground wire. Second, a transformer creates it's own relative neutral even when not hooked up to an earth ground. You could ground a phase, yes, but there is SIGNIFICANT CHARGING going on. The line, the transformer, the equipment it's attached to, all have charging. You want to swing that phase to ground, it's going to discharge through you like a bolt of lightning.


What exactly does "significant charging" mean?

Suppose you plug a one-to-one transformer into your bathroom wall. The output of the transformer is 120V, but it's not grounded - it's floating. Are you saying that if I touch one of these wires while standing in the bathtub, I'll die?
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Re: Why are electrical outlets grounded?

Postby Tass » Thu Feb 26, 2009 9:37 am UTC

Depends on the size and therefore capacitance of your transformer, but usually no.

In bigger systems it is going to cause all kinds of problems though.

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Re: Why are electrical outlets grounded?

Postby mosc » Thu Feb 26, 2009 9:34 pm UTC

The tiny transformer attached to the outlet is not going to hold a large charge. Could it electrocute you? Hell yes. I doubt it would kill you but if the transformer is big enough it could do some damage. The one outside your house though is more than capable of spontaneously combusting you.

Transformers work by field effect remember. They are two adjacent coils where the field in one induces a field in the other. Not to dig into transformers too deeply, it's important to note that they hold some charge proportional to their voltage. So do the power lines themselves. When you get into high enough voltages (doesn't take much), it's plenty of Coulombs to fry a human.

Note that the ground wire from the transformer is probably carrying some current as well. "Cutting it" would also likely cause electrocution.
The transformer's secondary is wound to output 240V, plus it has a center tap to provide two separate 120V feeds, 180 degrees out of phase. This is what enters my house, yes?
The specifics vary greatly based on who built it. There are a variety of ways to do it but what you're describing is common. Not sure on the phase separation across the tipple tapped transformer is 180 degrees or zero degrees though. Forgetting my math some as I grow old.
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Re: Why are electrical outlets grounded?

Postby ThomasS » Thu Feb 26, 2009 9:57 pm UTC

The transformer's secondary is wound to output 240V, plus it has a center tap to provide two separate 120V feeds, 180 degrees out of phase. This is what enters my house, yes?
The specifics vary greatly based on who built it. There are a variety of ways to do it but what you're describing is common. Not sure on the phase separation across the tipple tapped transformer is 180 degrees or zero degrees though. Forgetting my math some as I grow old.


This is certainly a common standard for US residential power, so your 240V electric dryer or water heater will run across both 120V feeds to get 240 net, while your regular appliance all run between one of the feeds and neutral. The standard for industrial power is 3 phase. It is considered normal to simply attach a 1 phase device across any two taps of the 3 phase incoming line. Going the other way is harder, probably simplest is to feed power into two taps of a free spinning 3 phase electric motor. Then it acts like partially as a generator and you get a third phase from the third tap on the motor.

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Re: Why are electrical outlets grounded?

Postby Goemon » Fri Feb 27, 2009 5:55 am UTC

mosc wrote:The tiny transformer attached to the outlet is not going to hold a large charge. Could it electrocute you? Hell yes. I doubt it would kill you but if the transformer is big enough it could do some damage. The one outside your house though is more than capable of spontaneously combusting you.

Transformers work by field effect remember. They are two adjacent coils where the field in one induces a field in the otter. Not to dig into transformers too deeply, it's important to note that they hold some charge proportional to their voltage. So do the power lines themselves. When you get into high enough voltages (doesn't take much), it's plenty of Coulombs to fry a human.


Perhaps you're thinking of capacitors? Transformers don't "hold a charge". The floating output of a transformer poses no danger unless you touch both sides.

idobox wrote:If the insulation between a conductor and the metal casing wears off, and the device has no grounding, the user is in danger of shock.


The only reason they're in danger of a shock is because the entire system is grounded, which is my point. If none of the wires coming into your house were connected to ground, then when the insulation wears through on a conductor inside your appliance and it touches the metal casing - nothing happens. The wire touches the case, the case touches you, you touch the earth ground - and that's the end of the line. There's no path from ground back to the power system, so no current, so no danger.

BECAUSE the power line coming into your house is grounded on one side, a wire touching the case is dangerous: the return path through ground completes the circuit. We therefore provide an extra (third pin) ground path for the metal casing, to divert the current around YOU before it gets back to the power line through ground. It would be simpler, cheaper, and safer to simply remove the ground path altogether. So why is it there?
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Re: Why are electrical outlets grounded?

Postby heydonms » Fri Feb 27, 2009 6:39 am UTC

I may be completely wrong here, but I think the confusion is coming from differences between theory and practice.

In theory every wire is a superconductor and a transformer with one side of the secondary completely insulated cannot cause a current flow.

What I think mosc is getting at is that in the real world if you have a transformer the two ends of the secondary coil are have a difference in potential. When you touch one of those ends, even though the other end may be completely insulated there will be a "small" current flow as the potential of your body changes to match the potential of that end of the wire. The value of "small" will vary depending on various factors and may be large enough to be dangerous.

This goes against a lot of stuff I have been taught, and as I said may be completely wrong.

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Re: Why are electrical outlets grounded?

Postby Rentsy » Fri Feb 27, 2009 9:05 am UTC

I'm really wary about posting anything in this thread, because electricity is dangerous.

I want to remind anyone reading this thread of that fact.

Do not do anything dangerous. Circuit breakers won't always save your life, especially not the new kind that doesn't need to be replaced.

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Re: Why are electrical outlets grounded?

Postby Tass » Fri Feb 27, 2009 9:26 am UTC

Goemon wrote:
mosc wrote:The tiny transformer attached to the outlet is not going to hold a large charge. Could it electrocute you? Hell yes. I doubt it would kill you but if the transformer is big enough it could do some damage. The one outside your house though is more than capable of spontaneously combusting you.

Transformers work by field effect remember. They are two adjacent coils where the field in one induces a field in the otter. Not to dig into transformers too deeply, it's important to note that they hold some charge proportional to their voltage. So do the power lines themselves. When you get into high enough voltages (doesn't take much), it's plenty of Coulombs to fry a human.


Perhaps you're thinking of capacitors? Transformers don't "hold a charge". The floating output of a transformer poses no danger unless you touch both sides.


You are wrong here goemon. Yes, only capacitors hold charge in idealized theory. But in practice everything is a capacitor. Every part of a wire is going to have some capacitive coupling to the ground/walls/infinity, these capacitances are small, but all connected it parallel, thus they add up. If you touch ground and hold one cable from a transformer, then you keep that cable at 0V. The other cable thus has to keep changing from +V to -V, this change simply doesn't happen without some charges moving in and out of that wire. Those charges has to come through you. If the system is the size of regular electric supply systems, then that current is going to be dangerous.

Also if they where kept floating then static charge might build up. (So that the wall socket change from say 19880V to 20120V rather than -120V to 120V). Then once something is connected that has an actual ground, there may be trouble even if the capacitance behind the voltage it is rather small.

So my guess would be that the one socket is grounded to prevent things like the above. Since it carries a lot of current it is not a perfect ground however, therefore a third - not meant to carry the bulk of the current and therefore being a more stable zero - is occasionally included for sensitive equipment.

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Re: Why are electrical outlets grounded?

Postby Goemon » Fri Feb 27, 2009 11:55 pm UTC

Sure, I'm aware that any system has some inherent capacitance, and there's some charge movement in and out of a human body when said body touches a "live" wire. After all, that's how antennas work. But at 120 or even 240V and 60Hz, the current flow is for all practical purposes negligible - certainly not enough to be dangerous to a human.

And you can pick up several thousand volts worth of static charge just by scuffing your feet on the carpet - there's no reason why your electrical system would charge up any more than that - certainly not just from power flowing in and out...
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Re: Why are electrical outlets grounded?

Postby idobox » Sat Feb 28, 2009 1:30 am UTC

It's not voltage that kills in a static discharge, it's the total charge Q=CU.
Potential differences of the ground between different places can be of a few hundreds of volts, the capacitance of the ground-electrical system is difficult to estimate, but even if it's not a health hazard, it can still damage sensitive devices.

You need to ground your installation, otherwise circuit breakers will be activated very often, if not continuously.
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Re: Why are electrical outlets grounded?

Postby Goemon » Mon Mar 02, 2009 5:43 am UTC

I agree that static discharge can damage sensitive electronic devices. But the practices and standards date from long before there was such a thing as sensitive electronic devices, and these days, most electronics have built in protection. A good example of an electronic system isolated from ground is your car: lots of electronic modules and electronics, even explosives on board (gasoline, airbags) in a system that has no contact to earth ground. I occasionally get shocks from my car, but they are hardly lethal (and I'd point out that a typical American car has more than a mile of wire - far FAR more than a house, and far higher total capacitance and charge than your house electrical system), and the static shocks don't destroy any of the modules onboard.

idobox wrote:You need to ground your installation, otherwise circuit breakers will be activated very often, if not continuously.


A typical static discharge won't trip a circuit breaker, they require significantly more energy to open. The open time depends on total i^2 * delta T; static discharges might exceed the rated current for the breaker, but don't last long enough to trigger it.
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Re: Why are electrical outlets grounded?

Postby dic_penderyn » Mon Mar 02, 2009 4:33 pm UTC

Grounding or "earthing" fixtures and fittings is a very good safety precaution.
Take a metal light switch or lamp fixing. The external metal that can be touched is earthed.

Now imagine for a moment that the live wire somehow came loose or the cable shorted and touched the metal casing.
If i was holding the lamp, the potential difference between me and the lamp is zero (since we are both grounded)
I will NOT be electrocuted as no current will flow.

Now, imagine if the metal case was NOT earthed. I would be killed. there would be a potential difference between lamp and me and current would flow.

D

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Re: Why are electrical outlets grounded?

Postby Tass » Mon Mar 02, 2009 8:47 pm UTC

dic_penderyn wrote:Now, imagine if the metal case was NOT earthed. I would be killed. there would be a potential difference between lamp and me and current would flow.


Goemons point is that that is only true if the other wire is earthed.

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Re: Why are electrical outlets grounded?

Postby quintopia » Tue Mar 03, 2009 6:59 pm UTC

The reason the center tap is grounded is because the ground wire provides MORE MAGIC. You don't want to cut that wire and see what happens when you lose all that extra magic.

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Re: Why are electrical outlets grounded?

Postby Goemon » Wed Mar 04, 2009 5:51 am UTC

At last, the explanation. Now I can rest easy :)
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Re: Why are electrical outlets grounded?

Postby Goemon » Wed May 22, 2013 12:04 am UTC

Ok, finally came across a reason for this.

As noted above, if the transformer coming into your home has no ground connections (at all), then none of the lines are normally dangerous. You can grab any of the three wires and touch a metal pipe or whatever without harm. If one of the wires shorts to the case of an appliance, it still isn't dangerous because you can touch the appliance, and safely touch any pipe or another appliance - the ONLY thing that's dangerous at that point is to touch both the damaged appliance and one of the other power wires, which shouldn't normally be exposed anyway. No single fault (wire touching something it shouldn't) anywhere would be dangerous.

Double faults on the other hand are a problem. If a wire is nicked and touches a pipe or the case of an appliance, no problem. You would never even notice. But if a second (different) wire touches the case of another appliance, the situation becomes dangerous: anyone who happened to brush both the pipe and the appliance or touched both appliances at the same time could be electrocuted.

With the grounding system normally in use, all pipes, metal conduits, and appliance cases are intentionally connected to the center terminal of the transformer. If either of the other wires is nicked or comes up against a pipe or the housing of an appliance, the result is a short circuit which immediately blows the fuse or trips the circuit breaker, removing power from the circuit and rendering it harmless.

So not grounding anything is marginally safer in the case of any single fault, but the intentional grounding scheme is MUCH safer in the event of two separate faults.
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Re: Why are electrical outlets grounded?

Postby moiraemachy » Wed May 22, 2013 4:23 am UTC

Yeah, you pretty much figured it out. Grounding nowadays is for fault detection, mostly (it also helps when lightning strikes - the charge will find a path to reach the earth). Also, you might find this interesting: in situations in which continuous operation of the equipment is critical, high-impedance grounding is used.

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Re: Why are electrical outlets grounded?

Postby lgw » Thu May 23, 2013 5:21 pm UTC

Goemon wrote:Perhaps you're thinking of capacitors? Transformers don't "hold a charge". The floating output of a transformer poses no danger unless you touch both sides.


No: coils (inductors) and capacitors both store energy, that's why they work. From a DC point of view, a capacitor has (ideally) infinite resistance after it has charged, and a coil has (ideally) no resistance after it has charged, but both will store a given amount of energy, and both will discharge that energy very quickly given the chance. The charge a capacitor can hold is its capacitance, and the charge a coil can hold is its inductance - and both contribute similarly to impedance.

Goemon wrote:As noted above, if the transformer coming into your home has no ground connections (at all), then none of the lines are normally dangerous. You can grab any of the three wires and touch a metal pipe or whatever without harm.


Please do not try this! You can absolutely become a discharge path between the current stored in a coil and ground. How likely this is depends on the voltage of the coil, and the total resistance and impedance through your body to ground. At household voltage, the risk is quite real if you have a clean path through you to ground. At higher voltage, it will go right through you even standing on pavement with work boots and holding the wire with rubber gloves. Many people working on the electrical system of their cars have discovered this - you do not want to be holding a disconnected spark plug wire when the coil fires.

Remember, even with no return path through the ground, there will still be current flowing until the system stabilizes, which takes far too long for safety.
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Re: Why are electrical outlets grounded?

Postby Goemon » Fri May 24, 2013 1:45 am UTC

lgw wrote:You can absolutely become a discharge path between the current stored in a coil and ground.


Current like that in your household electrical system must have a complete closed loop path: it always travel around a complete circuit. *

If your body is the only path between a coil and ground, you are not in any danger. Current cannot pass from the coil through you to ground unless there is a return path for the current to get back from ground into the other side of the coil. So if no part of the coil is grounded to begin with, you can grasp one of the coil while licking a well grounded copper pipe and you won't even feel a tingle.


* Static charge buildup can flow from one isolated reservoir into another (e.g. the ground) to balance the charges, but the total energy that can be stored in your household wires and transformers as static electricity isn't going to hurt you...
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Re: Why are electrical outlets grounded?

Postby lgw » Fri May 24, 2013 8:35 pm UTC

A power source doesn't "know" whether a circuit has a return path until a speed-of-light delay from wherever current can flow. In the case of a "transmission line"--a pair of wires with proper impedance for the apparent frequency of the wave traveling down them--current will flow until the leading edge of the wave makes it all the way to the end of the transmission line, reflects off the impedance mismatch of the "open" where the line ends, and comes all the way back to the source. In the case of just one wire heading off by itself (a reasonable model for you being a path to ground with no return path), the impedance mismatch happens immediately, but it still takes about 1/10th of a wavelength for that to matter. The discharge from a coil would be a lot steeper than the 60hz wave driving it, so current wouldn't flow for long, but could still be trouble. I don't know if it would really be dangerous at 120V (and I'm not eager to discover it), but at higher voltages it can get nasty.

Again, just ask anyone who has been shocked when holding a disconnected sparkplug wire when the coil fired, while holding the insulated cable with gloves and standing on pavement.
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Re: Why are electrical outlets grounded?

Postby Goemon » Sat May 25, 2013 5:39 am UTC

The ignition spark circuit on your car is basically an iron ring with two coils of wire wrapped around it. The primary coil has only a few turns, while the secondary has a LOT of turns - like a thousand times as many as the primary. The primary is connected to the car battery by a switch; the secondary has one side connected to the spark plug "finger" and the other side connected to the engine block.

When the primary side switch is closed, current quickly builds up in the primary, and a magnetic field is generated in the iron ring. When the switch is suddenly opened, the magnetic field begins to collapse, and instantaneously induces a voltage in both primary and secondary: the inductance of the circuit does not like it if you try to change the current, and will generate whatever voltage is necessary to keep the current flowing.

In the case of normal spark plug operation, the voltage instantly climbs to about 30,000V on the secondary side (and 30V on the primary side), at which point the air in the spark plug gap is ionized sufficiently to allow a cascade of electrons to break free and continue carrying the current. (The current on the secondary side necessary to make the magnetic field happy is only 1/1000 what the original current was on the primary side.) The current in the secondary flows in a closed loop from one side of the secondary, through the spark plug finger, across the gap, to the spark plug body, to the engine block, and back to the other side of the secondary.

If you are foolish enough to unplug the spark wire from the vehicle and hold it in your hand while the primary charges and the switch opens, then the current path starts at one side of secondary, through your hand to your feet, through the ground to the tires, along the axle and then to the engine block and finally back to the other side of the secondary. Once again, the collapsing magnetic field will generate whatever voltage is necessary to maintain the current - probably still on the order of 30,000V as this is what's necessary to push the current through the rubber of the car tires and the few feet of dirt between your feet and the car.

In other words, the only reason you get a shock when holding an ignition wire is because the closed loop of the circuit does use part of the ground between you and the tires on its way to getting back to the other side of the coil.

As a side note, if you were to set the car on four blocks of ceramic or glass (an excellent insulator), you could avoid a shock even if you yourself were very well grounded. If the higher resistance of the ceramic in the circular path from the secondary through you and the ground and back to the engine block were high enough to require something on the order of 100,000V to push the necessary current through you, you'd be saved by the primary side. The magnetic loop will force the voltage to increase in both the primary and secondary until sufficient current begins flowing in one or both of them (though with the higher number of loops, the voltage in the secondary is about 1000 times higher than in the primary). When the voltage reaches about 100V on the primary side (and 100kV on the secondary side), the solid state switch on the primary side will fail and allow the current to backwash through it on the primary side, making it unnecessary to push any current through the secondary side.

So if the car's electrical system is VERY well isolated from the ground, then it's safe to grab the ignition wires because the primary side fails first and becomes the path of least resistance for the current surge, rather than the path through you, the ground, the insulation, and back to the engine block and the other side of the coil.
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