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Why we ground electrical systems (2015)

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Re: Why we ground electrical systems (2015)

#31

Post by nosianu »

shreddish wrote: post #16
vvillyd wrote: post #12

Earlier quoted context omitted.

If you do not properly bond the ceiling fan (that is, connect the "ground" wire) then the circuit breaker will not trip if there is a fault to ground on the fan. That is, if the hot wire touches a metal part of the fan which is not supposed to be energized (say the metal housing), and the fan is not properly bonded, then there will be no ground-fault path back to the source to cause the breaker to trip. This could me…

Technically, would it only shock you if your body provided a path to "ground" or lower potential? For example if you were wearing insulted boots and touched the energized piece of metal you would not feel a shock, correct?

Yes. That's why this is possible, here the insulation is the air between helicopter and ground: https://youtu.be/DPNK7bc2qvM?t=125

If that guy was connected to the earth he would get fried. Note that you still have to equalize whatever potential there is between you and the system, like they do in that video. Same reason why you should ground yourself and the hardware before working on your PC. Ever had a tiny spark hit you when you touched something (neither you or that "thing" being connected to the electrical grid, just static electricity e.g. from walking on the carpet)? But without connection to ground the only thing that happens is equalization of the potential between you and the target but no further flow. If you are connected to ground on one side and to a source that keeps creating an electrical potential, like the electrical grid with a power plant somewhere, there will be a continuing flow. If you are not connected to ground there won't be.

Re: Why we ground electrical systems (2015)

#32

Post by throwaway542134 »

shreddish wrote: post #17
throwaway542134 wrote: post #14

Earlier quoted context omitted.

I've always preferred to think of it as pumping water from the bottom of a (practically infinite) reservoir and dumping it at the top of a mountain then doing some work as it flows downhill, back to the reservoir. It provides a nice visual for why current always makes it back to ground, just like water always flow downhill. It also removes the tendency to anthropomorphize electrical current and say things like it "se…

Yes water is a great way to describe electricity in a lot of ways. I'm just struggling with the "returning to where it originates". For instance the electricity didn't originate from the grounding rod it came from the transformer. Earth is providing a very low electrical potential that the electricity is "attracted" to or "falling" to in the reservoir example. Am I wrong in thinking that all electricity is flowing ba…

When you talk about "electricity" you need to distinguish between "current" and "charge carrier." Current is the amount of charge that flows through a point per second. The charge carriers are the electrically charged particles that actually move. Where current goes is a matter of what we're trying to do with a circuit, where charge carriers go is why we need ground.

What a voltage does is pull or push the charge carriers. When lots of them flow, you have a current. Conductors, like a wire or ground rod, are full of free electrons to act like charge carriers (kind of like a pipe filled with water, the voltage is a pump that moves it).

The duality of pulling/pushing charge carriers is why we need a circuit. In order to push charge carriers, we need something to pull them from (a source) and somewhere to dump them (a sink). When we have no source and no sink, charge carriers have nowhere to come from and nowhere to go.

Ground is a convenient source/sink for charge carriers because it's roughly uniform in charge and huge, so pulling tons of charge carriers from it doesn't impact it greatly.

And it's not that charge carriers are always flowing back to earth, but back to their source. That's why ground is sometimes called a "return path." To move a charge carrier, you need to give it potential. It will lose that potential and return to the point of lowest potential difference from its origin - which is its origin.

But that said, for things like AC power, the charge carriers aren't actually moving very far at all and have a net displacement of 0. They vibrate adjacent charge carriers, and we convert that vibration into unidirectional (DC) voltages that can push/pull from local sources/sinks, be it the literal earth (mostly for safety ground) or a small plane of copper on a PCB.

Re: Why we ground electrical systems (2015)

#33

Post by vvillyd »

fsh wrote: post #23

This section is plain wrong: "The ONLY purpose for the EGC (or green wire) is to clear a ground-fault (clearing a ground-fault means tripping a breaker or blowing a fuse) in the 'oh shit moments'. It has absolutely NOTHING to do with the ground or the Earth and will work exactly as it is intended to regardless of whether it is connected to the Earth or not." If the EGC is floating, no current will flow in a ground fa…

I'm not trying to be argumentative but you are misinformed. "Floating" means that the system is not connected to ground. That is, there is no system/main bonding jumper. There is no connection to a grounding electrode conductor. This has nothing to do with the Equipment Grounding Conductor, though. I don't know what you mean by a "floating EGC".

The entire purpose of the EGC is to bond all normally non-current carrying conductive parts together to provide a ground-fault current path back to the source of the circuit. If the EGC is not connected to some non-current carrying conductive part, then it is not installed correctly, is against code, and is a safety hazard. If the EGC is bonded to everything properly and there is a fault (say an ungrounded wire touches a metal box), then the current will flow through the ungrounded conductor, through the metal box, through the EGC (which is bonded to the box), back to the source (usually a transformer), across the windings, and eventually back to the breaker that controls the circuit. This will build up enough current (usually VERY fast, like milliseconds) and the breaker will open. The Earth has nothing to do with this.

Re: Why we ground electrical systems (2015)

#34

Post by jotm »

fsh wrote: post #23

This section is plain wrong: "The ONLY purpose for the EGC (or green wire) is to clear a ground-fault (clearing a ground-fault means tripping a breaker or blowing a fuse) in the 'oh shit moments'. It has absolutely NOTHING to do with the ground or the Earth and will work exactly as it is intended to regardless of whether it is connected to the Earth or not." If the EGC is floating, no current will flow in a ground fa…

I'm confused by that as well. The green wire needs to touch the earth, whether by being bonded to neutral at the distribution point or simply by touching the earth somewhere, even through a concrete wall.

I've seen houses where the earth wiring is completely independent and terminated in a metal rod buried outside. Seemed to be up to local code, breakers worked as they should.

Re: Why we ground electrical systems (2015)

#35

Post by ars »

amriksohata wrote: post #15
vvillyd wrote: post #2

I'm actually the person who wrote this Reddit post. I'd never heard of Hacker News before, but someone messaged me on Reddit telling me this 4 year old post just surfaced over here. Let me know if you have any questions about electrical theory or installation, or anything else!

So in the UK we have 3 wires, red, blue and green, the green is ground or earth, what purpose does this serve? Is it the same as ground as you call it in the US

The reason for having ground is simply because if an electrician makes a mistake and reverses the hot and neutral, and you plug something in, the body of the device will now be connected to hot, and you'll get a shock.

By having a separate ground, it's harder to mess that up, and devices can connect the body of the device to that.

Inside the electrical box (mains) the neutral and ground are connected together, and are at the same potential.

The only reason they run separate wires is in case of of mistakes.

Re: Why we ground electrical systems (2015)

#36

Post by shreddish »

bsder wrote: post #21
shreddish wrote: post #17

Earlier quoted context omitted.

Yes water is a great way to describe electricity in a lot of ways. I'm just struggling with the "returning to where it originates". For instance the electricity didn't originate from the grounding rod it came from the transformer. Earth is providing a very low electrical potential that the electricity is "attracted" to or "falling" to in the reservoir example. Am I wrong in thinking that all electricity is flowing ba…

> Am I wrong in thinking that all electricity is flowing back into earth? Yes, sadly, you are wrong. But it's not a strange error. Electricity always flows in a circle(circuit). And, in fact, in medical devices, transformers are often used to completely isolate devices from the line that they are plugged into. Electrons on the device side of the transformer will NOT try to flow back into the line side or an earth. Th…

Yes instinctivly it wasn't making sense as I knew the circuit needs to be "completed". So if that's the case then what is the point of the ground rod that the neutral bar/wires in your breaker box are connected to?

Re: Why we ground electrical systems (2015)

#37

Post by ars »

vvillyd wrote: post #29
amriksohata wrote: post #15

Earlier quoted context omitted.

So in the UK we have 3 wires, red, blue and green, the green is ground or earth, what purpose does this serve? Is it the same as ground as you call it in the US

I'm from the US, so I'll add the caveat that this is answer is based on my foreign understanding and a British electrician may be better suited to answer this. Your green wire would be what the US calls a ground wire, and what is more accurately called a Equipment Grounding Conductor (EGC). It's purpose is to bond all normally non-current carrying conductive parts of a system together to provide a ground-fault curren…

> However, without the green wire, if either the red or black shorts to something, the breaker will not open and you have an enormous potential to harm someone or burn your house down.

This is not true. First of all only the red is hot, if the black shorts to something nothing will happen.

Second if there is a short strong enough to "burn your house down", the breaker most definitely will open.

Re: Why we ground electrical systems (2015)

#38

Post by vvillyd »

jotm wrote: post #34
fsh wrote: post #23

This section is plain wrong: "The ONLY purpose for the EGC (or green wire) is to clear a ground-fault (clearing a ground-fault means tripping a breaker or blowing a fuse) in the 'oh shit moments'. It has absolutely NOTHING to do with the ground or the Earth and will work exactly as it is intended to regardless of whether it is connected to the Earth or not." If the EGC is floating, no current will flow in a ground fa…

I'm confused by that as well. The green wire needs to touch the earth, whether by being bonded to neutral at the distribution point or simply by touching the earth somewhere, even through a concrete wall. I've seen houses where the earth wiring is completely independent and terminated in a metal rod buried outside. Seemed to be up to local code, breakers worked as they should.

So I think you should define what you mean by "green wire". The NEC allows EGCs, GECs, MBJs, and SBJs to all use green wire. They all serve different purposes, and only the GEC is connected to the Earth. Whether or not the GEC is present the EGC and MBJ/SBJ is what allows breakers to work properly.

The MBJ/SBJ is what bonds the neutral. This doesn't have anything to do with the Earth. This bonding has to be done anywhere between the origination of the circuit and the first overcurrent protective device.

The GEC is what connects to the Earth. This is used to stabilize voltage and to eliminate a difference in potential between metal parts during a high frequency event (like a nearby lightning strike or a transformer blowing up). This has nothing to do with the operation of breakers.

Re: Why we ground electrical systems (2015)

#39

Post by shreddish »

hyperman1 wrote: post #20
shreddish wrote: post #16

Earlier quoted context omitted.

Technically, would it only shock you if your body provided a path to "ground" or lower potential? For example if you were wearing insulted boots and touched the energized piece of metal you would not feel a shock, correct?

Please don't insult your boots.

hahah, they piss me off sometimes though

Re: Why we ground electrical systems (2015)

#40

Post by kmill »

bsder wrote: post #21
shreddish wrote: post #17

Earlier quoted context omitted.

Yes water is a great way to describe electricity in a lot of ways. I'm just struggling with the "returning to where it originates". For instance the electricity didn't originate from the grounding rod it came from the transformer. Earth is providing a very low electrical potential that the electricity is "attracted" to or "falling" to in the reservoir example. Am I wrong in thinking that all electricity is flowing ba…

> Am I wrong in thinking that all electricity is flowing back into earth? Yes, sadly, you are wrong. But it's not a strange error. Electricity always flows in a circle(circuit). And, in fact, in medical devices, transformers are often used to completely isolate devices from the line that they are plugged into. Electrons on the device side of the transformer will NOT try to flow back into the line side or an earth. Th…

When it comes to AC (even discontinuous DC), closed loops are not necessary for current flow. A basic example is a capacitor, which can permit current while it builds up an electric field between is plates. A related example is an antenna, where AC radiates energy that can vibrate electrons in a far away antenna. (Radio telescopes are proof an antenna doesn't need a ground or a closed circuit to receive.)
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