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Why we ground electrical systems (2015)

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Re: Why we ground electrical systems (2015)

#101

Post by Gibbon1 »

esaym wrote: post #70
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!

Here's a good one: A couple of years ago I borrowed a small camper trailer from my grandparents as I was going on a week camping trip. I towed the camper over to my house and left it hitched to the truck. I then plugged the trailer into an outlet on the side of the house so I could run the air conditioner while I packed it. There was an issue with the brake lights not always working on trailer also. I crawled under t…

> So what's up with that?

Usually 50 VAC means the ground is open.

A thing about most ordinary transformers[1] is they wrap the primary and secondary together. Fast, cheap, and more efficient. The result is there is a large amount of capacitive coupling between the primary and secondary. Since one side of the secondary is grounded it's like connecting 1000-5000pf between hot and chassis ground. If chassis isn't grounded then it floats at 60VAC.

[1] For transformers with more isolation they wrap the primary and secondary on top of each other. Less capacitance that way. Medical grade transformers have physically separate windings.

Re: Why we ground electrical systems (2015)

#102

Post by pkaye »

robocat wrote: post #67
Ives wrote: post #18

The details of this also vary based on where you're at. In the US, neutral is bonded to earth at every house I think. In Europe, neutral is only bonded to earth at a local power station.

The US system is somewhat unique and lacks heaps of safety features: I am guessing mostly because a 110V shock is far less dangerous than a 240V shock (Although higher amperages mean higher risk of fire?). For example for UK, EU and NZ etc: sockets and pins have a variety of features to avoid touching the phase (live) wire with say a screwdriver or knife in hands of a child. For example, all new circuits installed in…

In the US there is tamper proof outlets and GFCI is equivalent to RCD. Its pretty much required for new houses though older houses may vary on what they implement.

Re: Why we ground electrical systems (2015)

#103

Post by Gibbon1 »

js2 wrote: post #98
vvillyd wrote: post #47

Earlier quoted context omitted.

After researching the UK electrical system, I realize I was mistaken on one regard. I thought they used 2-ungrounded conductors, but I was mistaken. The red wire is the only ungrounded. This would be the same as the black wire in a US house. The red wire is their grounded/neutral. This would be the same as a white wire in a US house. I am correct on the EGC, though. It works the same as the EGC in the US. If a ground…

The US NEC now requires arc fault circuit interrupters which are pretty darn expensive circuit breakers with built-in electronics to detect an arcing situation that wouldn't trigger a normal circuit breaker but can cause a fire.

I keep hearing electricians say that they can't detect bad splices which how most residential electrical fires start.

Re: Why we ground electrical systems (2015)

#104

Post by bsder »

kmill wrote: post #71
mindslight wrote: post #64

Earlier quoted context omitted.

Closed loops ("circuits") are part of the paradigm, so yes they are necessary at the level of abstraction we generally analyze electronics in. Sure if you came from Pluto with a metal sphere teeming with excess electrons, they would disburse themselves on our planet and never return home, but that isn't circuit analysis! For example, the current into one terminal of a capacitor does equal the current out of the other…

I only brought up ground (in the Earth ground sense) because I've heard people think that antennas in general work by using the Earth as a second conductor, with EM radiation as the first. Antennas are conductors that are part of an un-closed electrical network, relying on the capacity of a conductor to hold an oscillating non-equilibrium charge (though they can be modeled as an RLC network). My previous comment was…

> The realm of electrodynamics is pretty interesting, and it's where seemingly basic concepts like electric potential begin to fail -- it becomes path-dependent!

Not really. The problem is that we have sort of an "electron abstraction" which is incorrect in the Heaviside-Hertz pedagogy when fields start to store energy. I recommend "Collective Electrodynamics" by Carver Mead as a modern formulation without the silliness of an Aether: https://www.amazon.com/Collective-Electrodynamics-Quantum-Fo...

> I only brought up ground (in the Earth ground sense) because I've heard people think that antennas in general work by using the Earth as a second conductor, with EM radiation as the first.

Really? Most of the time I describe antennas as sort of really long range transformers. And, while you need each side of the transformer to be a circuit, the two sides of the transformer don't have to interact other than through a field.

> My previous comment was written out of a reasonably common conception of a closed circuit being a loop with constant current flow, but capacitors break such a circuit.

Yes and no. Capacitors have the hand wavy notion of "displacement current" in classical electrodynamics--but most of the issue is with the fact that we use the Heaviside-Hertz pedagogy which was formulated back when everybody believed in the Aether.

The real issue is that to deal with capacitors you must deal with fields rather than just the notion of electrons. However, if we kind of squint and wave our hands the "electron formulation" can be kinda sorta made to work. (Side note: Capacitive dividers are even more annoying and you have to be really careful.)

Classical Heaviside-Hertz electrodynamics also has a lot of issues dealing with motors and generators, as well. Again, the key is that an "electron formulation" isn't really enough when fields start holding an appreciable amount of energy.

Re: Why we ground electrical systems (2015)

#105

Post by yrro »

vvillyd wrote: post #47
ars wrote: post #37

Earlier quoted context omitted.

> 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.

After researching the UK electrical system, I realize I was mistaken on one regard. I thought they used 2-ungrounded conductors, but I was mistaken. The red wire is the only ungrounded. This would be the same as the black wire in a US house. The red wire is their grounded/neutral. This would be the same as a white wire in a US house. I am correct on the EGC, though. It works the same as the EGC in the US. If a ground…

Current British electrical standards require RCDs (GFCIs to you) to be fitted to new installations. So the RCD should trip 30 ms after the current starts flowing where it shouldn't. Plenty of installations won't be brought up to spec for decades of ever, however...

Re: Why we ground electrical systems (2015)

#106

Post by yrro »

js2 wrote: post #98
vvillyd wrote: post #47

Earlier quoted context omitted.

After researching the UK electrical system, I realize I was mistaken on one regard. I thought they used 2-ungrounded conductors, but I was mistaken. The red wire is the only ungrounded. This would be the same as the black wire in a US house. The red wire is their grounded/neutral. This would be the same as a white wire in a US house. I am correct on the EGC, though. It works the same as the EGC in the US. If a ground…

The US NEC now requires arc fault circuit interrupters which are pretty darn expensive circuit breakers with built-in electronics to detect an arcing situation that wouldn't trigger a normal circuit breaker but can cause a fire.

I've seen videos of these on YouTube and they seem useless!

Re: Why we ground electrical systems (2015)

#107

Post by nemetroid »

mikestew wrote: post #62

One of the few times I wish I had a reddit account, just so I could give an old post some $REDDIT_KARMA_POINTS (whatever currency they use). That is an outstanding explanation that cleared up some things for me that I do because electrical code, not because "knows the 'why' of what he's doing".

Maybe it offers some solace to know that old posts (older than six months) cannot be voted on.

Re: Why we ground electrical systems (2015)

#108

Post by yesenadam »

Stratoscope wrote: post #11
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!

Here is an entertaining take on some related questions, going into the history of how our modern ground systems evolved: http://amasci.com/amateur/whygnd.html I would be interested in your comments about that article, thanks!

Bill Beaty's site has a load of great articles on electricity (and much else), linked to here:

http://amasci.com/elect/elefaq.html

Also misconceptions about electricity taught in school:

http://amasci.com/miscon/elect.html

Or pages on generally believed science myths:

http://amasci.com/miscon/miscon.html

How transistors really work:

http://amasci.com/amateur/transis.html

How capacitors really work:

http://amasci.com/emotor/cap1.html

There's loads more great stuff. He's a pleasure to read - a guy who thinks for himself, and experiments - tries things out for himself.

Re: Why we ground electrical systems (2015)

#109

Post by taneq »

tomatotomato37 wrote: post #65
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!

There's one thing I don't understand between hot and neutral. How do the electric turbines upstream at the power plant only act on what becomes the hot wire? By my understanding of how motors work the magnet causes a positive current on the whole coiling/wire, meaning a positive current on one side is matched by an equally powerful negative current on the other. As both wires on the consumer end eventually join to th…

> than wouldn't that mean both wires switch between hot and neutral everytime the current changes direction?

The neutral line is connected to Earth at some point. The voltage difference is created by a transformer upstream which just pushes a 240V (or whatever) difference between live and neutral, and since neutral is tied to Earth, it stays at ~0V while the live line stays at ~240VAC.

You're right, though, that any current pulled from the live line goes back through the neutral line and they both end up with alternating current going through them. The voltage only alternates between live (or 'hot') and neutral relative to each other. This voltage difference between the two lines is generated by a transformer and there doesn't have to be any connection between live/neutral and anything else (at which point you can just think of them as live-1 and live-2) but the neutral line is connected to Earth at some point (although depending on load and the distance to the earthing point your neutral will actually have some AC ripple on it).

Re: Why we ground electrical systems (2015)

#110

Post by devwastaken »

mindslight wrote: post #94
wrycoder wrote: post #93

Earlier quoted context omitted.

If that "toaster circuit" is protected by a GFCI, it will indeed trip with only two wires. The GFCI is sensing the imbalance between hot and neutral (the rest went through the bath water). Both currents pass through the same toroid, but they are in opposite directions and normally exactly cancel, so no flux is induced in the toroid. If there is an imbalance, a current is induced in a secondary winding. This will trip…

Except there will be no imbalance if there is no other path for the current to travel by... The geometry of the electric field in water should probably be similar to that in air (mostly contained to within the case), just don't drop both a toaster and a waffle maker in at separate ends.

Iirc gfci's are nec code allowed on non grounded lines because they will still trip, but you have to test with the gfci test button because they're only tripping on current difference, not like a breaker.
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