Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Why we ground electrical systems (2015)

reddit.com

Post reply on HN
Print view View original

51–60 of 120 posts

Re: Why we ground electrical systems (2015)

#51

Post by NobodyNada »

shreddish wrote: post #48
throwaway542134 wrote: post #32

Earlier quoted context omitted.

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 car…

Hmm okay starting to make more sense now. Do you know a good visual explanation that shows the "path" electricity takes from the transformer to your home and back out to the ground rod. Note: I fully understand in AC the electrons aren't actually moving along this path. But I guess I don't see how the circuit is ever "complete" or a circle.

It doesn't go "back out to the ground rod," it goes back to the transformer. Note this paragraph in the parent comment:

> 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.

Re: Why we ground electrical systems (2015)

#52

Post by NobodyNada »

shreddish wrote: post #36
bsder wrote: post #21

Earlier quoted context omitted.

> 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?

This article posted in a comment above explains it well: http://amasci.com/amateur/whygnd.html

Essentially, the ground rod acts as an "anchor" holding the neutral wire and the ground at the same electric potential. If there was no ground rod, the earth and the circuit would be "floating" relative to one another, and a dangerously large voltage could develop between them.

Re: Why we ground electrical systems (2015)

#53

Post by kmill »

throwaway542134 wrote: post #14
vvillyd wrote: post #9

Earlier quoted context omitted.

Well, electrical potential is only defined as a relation between two points. So the neutral point on a transformer, for example, doesn't have any electrical potential in and of itself. It has electrical potential with relation to, say, the termination point of one of the phases. The electrical potential between any two given points is dependent on the impedance along the pathway between the two. The lower the impedan…

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…

This analogy doesn't work so well for AC, which involves both pushing and pulling on electric charges.

It's more like pipes full of a gas like air. The power plant has a big reciprocating piston that is pushing and pulling on the gas, creating a pressure wave. One side of the cylinder is "aired," meaning it is in contact with atmospheric air. These pipes make their way to clients, who attach the pipes to equipment of their own, for example a piston that converts this wave back into mechanical energy. Again, one side of this piston is in contact with atmospheric air, which is the reference pressure for the piston.

Sometimes the pipe develops small holes, and if a hapless worker gets too close, they can either get cut from an out-blast or hurt from smashing against the equipment when the air is sucking in (both being from the difference in the pipe's pressure relative to atmospheric pressure). As a safety protection, everything is enclosed in another layer of air-proof material, and when a leak is detected the main air supply is shut off.

Special attention is made to make sure the average pressure in the pipe is the same as atmospheric pressure, since the piston motors depend on this to function.

(In real life, steam plants use direct current since there are a lot of losses due to condensation, and also since a lot of the point is transmitting thermal energy.)

Re: Why we ground electrical systems (2015)

#54

Post by ianburrell »

bardworx wrote: post #22

Why are there three wires in new receptacles? From my understanding, new code has you have to have a 3+ground but why is that safer? Did they just add an extra wire so you can have a hot, lead, and neutral to ground all the receptacle? (Plus the green/bare to bond the receptacle to switch/outlet) Also, for home owners, what’s the best resource to learn common sense basics of electrical work (besides reading the city/…

Are you thinking about 240V circuits which used to use 3-wire circuits but now use 4-wire? The US run split-phase power to homes with +120V and -120V hot legs and neutral connected to center tap of transformer. Each 120V circuit uses one of the hot legs.

For 240V, can use 3-wire circuits by getting 240V between the +120V and -120V legs. There are 3-wire grounded plugs (hot-hot-ground) and non-grounded (hot-hot-neutral). Modern 240V wiring uses 4 wires, hot-hot-neutral-ground, which allows making 120V for low-power electronics from one hot to neutral.

Re: Why we ground electrical systems (2015)

#55

Post by jessaustin »

ars wrote: post #41
vvillyd wrote: post #33

Earlier quoted context omitted.

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 carryi…

He's right and you are not. If you don't bond neutral and ground at the main, it's still bound at the transformer. > then the current will flow through the ungrounded conductor, No, it will flow through the grounded conductor. > back to the source (usually a transformer), across the windings, and eventually back to the breaker that controls the circuit The flow of power is the same via ground as it is via neutral. >…

If you don't bond neutral and ground at the main, it's still bound at the transformer.

That would be a pretty good trick. The transformer is outside my house on a pole. Two wires run to it, the hot and the neutral. The "ground" doesn't leave the house. (Other wiring schemes exist, but this is standard USA residential.)

[EDIT: brainfart, see helpful correction below. still no "ground" at the pole...]

Re: Why we ground electrical systems (2015)

#56

Post by ishigoemon »

shreddish wrote: post #48
throwaway542134 wrote: post #32

Earlier quoted context omitted.

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 car…

Hmm okay starting to make more sense now. Do you know a good visual explanation that shows the "path" electricity takes from the transformer to your home and back out to the ground rod. Note: I fully understand in AC the electrons aren't actually moving along this path. But I guess I don't see how the circuit is ever "complete" or a circle.

This video has a great visual explanation of grounding, imho.

https://www.youtube.com/watch?v=P-W42tk-fWc

Re: Why we ground electrical systems (2015)

#57

Post by jessaustin »

ars wrote: post #43
ryanmarsh wrote: post #4

This blew my mind. So the white wire (neutral) is actually what I thought the ground was (connected to earth) and the “ground” wire (usually bare or green) is a conductor for clearing ground faults and isn’t connected to earth. Marvelous explanation.

> conductor for clearing ground faults and isn’t connected to earth. > Marvelous explanation. It might be Marvelous, but it's not actually true. The ground wire IS connected to the earth! I don't want to be rude, but I think the OP should take this post down, it has a LOT of mistakes.

TFA is a bit more abstract than you realize, and perhaps that abstraction could mislead the unwary. It is technically correct, but makes a distinction you've missed. If a gremlin cuts the (green) wire between my breaker box and my ground rod, the system is no longer "grounded to Earth", so it wouldn't handle lightning strikes well. However, the "EGC" in every house circuit is still bonded to neutral at the box, so breakers will still trip when the wire inside my clothes dryer rubs against the metal body. That is the function of the EGC. The EGC itself does nothing to protect against transient earth potentials like lightning strikes. That is the job of the grounding rod etc. Just because both wires are green, doesn't mean they're doing the same thing.

Re: Why we ground electrical systems (2015)

#58

Post by jhayward »

jessaustin wrote: post #55
ars wrote: post #41

Earlier quoted context omitted.

He's right and you are not. If you don't bond neutral and ground at the main, it's still bound at the transformer. > then the current will flow through the ungrounded conductor, No, it will flow through the grounded conductor. > back to the source (usually a transformer), across the windings, and eventually back to the breaker that controls the circuit The flow of power is the same via ground as it is via neutral. >…

If you don't bond neutral and ground at the main, it's still bound at the transformer. That would be a pretty good trick. The transformer is outside my house on a pole. Two wires run to it, the hot and the neutral. The "ground" doesn't leave the house. (Other wiring schemes exist, but this is standard USA residential.) [EDIT: brainfart, see helpful correction below. still no "ground" at the pole...]

> Two wires run to it, the hot and the neutral

Standard US residential is 240V split phase, with two hot and one neutral conductor from the transformer. So three wires, minimum. Unless you have a very old feed.

Re: Why we ground electrical systems (2015)

#59

Post by watersb »

vvillyd wrote: post #5
ganonm wrote: post #3

"Since the ground-fault current path has inherently much higher resistance than the neutral wire, the amount of current flowing through the circuit jumps enormously during a ground-fault." This is incorrect (probably a typo) - the GFC ensures there is a very low resistance path to source, which ensures a current spike that is sufficient to blow the fuse.

Yeah, that was a typo on my part. I didn't notice it at first, and it's now been so long since I posted it that I would feel disingenuous editing it. I think someone pointed out the typo in the comments.

I encourage you to fix the typo; lots of HN exposure means lots of new readers without context to any time that has elapsed since your (really good) article was first posted.

Re: Why we ground electrical systems (2015)

#60

Post by Unsimplified »

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!

For a machine, is it generally a good idea to distribute a common power bus voltage like 48VDC to independent modules (embedded computers, servo controllers) for them to stepdown inside the application module (12vdc, 24vdc) or should the power be stepped down early at the distributor module? What are the advanced tradeoffs?
Post reply on HN