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Why we ground electrical systems (2015)

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Re: Why we ground electrical systems (2015)

#91

Post by mindslight »

ars wrote: post #35
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

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 ar…

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

Erm no.

1. In case the neutral gets disconnected, so the frame isn't at the potential of the hot (no voltage drop across the device)

2. So the return voltage drop in the white wire isn't on the body of the device. ~20 volts between adjacent high-current appliances on different phases probably wouldn't be terribly dangerous to most people, but it is sloppy.

3. For GFCI/RCD, it separates bona fide return currents from "accidental" return currents. With only two wires, dropping a toaster into a (PVC-drain) bathtub wouldn't trip. (Erm, I just realized most toasters are actually only two wires. Well uh, don't do that).

Re: Why we ground electrical systems (2015)

#92

Post by wrycoder »

ars wrote: post #35
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

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 ar…

There is also the matter of unanticipated leakage current between hot and the equipment enclosure. The EGC/green wire keeps the enclosure close to ground potential.

Re: Why we ground electrical systems (2015)

#93

Post by wrycoder »

mindslight wrote: post #91
ars wrote: post #35

Earlier quoted context omitted.

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 ar…

> The only reason they run separate wires is in case of of mistakes. Erm no. 1. In case the neutral gets disconnected, so the frame isn't at the potential of the hot (no voltage drop across the device) 2. So the return voltage drop in the white wire isn't on the body of the device. ~20 volts between adjacent high-current appliances on different phases probably wouldn't be terribly dangerous to most people, but it is…

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 the GFCI internal breaker. Note that this does not involve the green wire.

Re: Why we ground electrical systems (2015)

#94

Post by mindslight »

wrycoder wrote: post #93
mindslight wrote: post #91

Earlier quoted context omitted.

> The only reason they run separate wires is in case of of mistakes. Erm no. 1. In case the neutral gets disconnected, so the frame isn't at the potential of the hot (no voltage drop across the device) 2. So the return voltage drop in the white wire isn't on the body of the device. ~20 volts between adjacent high-current appliances on different phases probably wouldn't be terribly dangerous to most people, but it is…

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.

Re: Why we ground electrical systems (2015)

#95

Post by wrycoder »

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.

The resistance of slightly salty bathwater is much lower than air. While baths are not often plumbed with metal pipes these days, one can still expect some current flow to ground. It only takes milliamps of stray current to trip a GFCI.

Re: Why we ground electrical systems (2015)

#96

Post by madengr »

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…

Yes, just as you describe, but the addition of a transformer allows you to reference the neutral to ground, therefore the hot swings above and below ground. It’s a little more complicated than what I described, as your breaker box is 240V split phase, but the transformer outside your house allows for a ground reference at its center tap.

If you have ever made the mistake of connecting a diode rectifier directly to AC mains, then grounding the negative output, you’ll blow a breaker. A 1:1 isolation transformer is still needed.

Re: Why we ground electrical systems (2015)

#98

Post by js2 »

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…

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.

Re: Why we ground electrical systems (2015)

#99

Post by muzika »

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…

Inproperly wired outlet? Hot swapped with neutral?

Re: Why we ground electrical systems (2015)

#100

Post by muzika »

muzika wrote: post #99
esaym wrote: post #70

Earlier quoted context omitted.

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…

Inproperly wired outlet? Hot swapped with neutral?

Or floating ground?
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