
Why we ground electrical systems (2015) - terminalhealth
https://www.reddit.com/r/electricians/comments/38qiyy/an_explanation_of_why_we_ground_electrical/
======
vvillyd
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!

~~~
amriksohata
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

~~~
ars
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.

~~~
mindslight
> _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).

~~~
wrycoder
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.

~~~
mindslight
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.

~~~
wrycoder
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.

~~~
mindslight
Milliamps sounds like a tiny amount in the context of home power, but 5mA at
120V is still only 24kOhm. This is actually pretty low to just assume away.
I'd agree there could be some water left over in the drain pipe from the last
bath, splashes making a path to the spigot, or something like that. I just
wouldn't count on it.

------
watersb
I have been studying Grounding vs Bonding these past few weeks, great timing
to see this HN discussion.

Some training videos for pro electricians and US National Electric Code
compliance helped me.

[https://youtu.be/qNZC782SzAQ](https://youtu.be/qNZC782SzAQ)

Basically, _bond_ everything together if it can carry a charge. _Ground_ that
bond at _only one place_ in an electrical service.

More than one "ground" sets up a situation where a potential difference can
develop between the various "grounds". Ground Fault detection may no longer
work in that case. Which is not good.

(The presentation is weirdly sexist in offhand comments, which I often
encounter in trade work of this kind, but I do believe that's changing.
Slowly. The USA needs more people who know how to build things.)

~~~
simonebrunozzi
> The USA needs more people who know how to build things.

USA? The WORLD needs more people who know how to build things.

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olouv
Beware as this explanation does not apply to every country in the world, in
France for residential systems at least, the protection (green/yellow) wire is
never connected to the neutral (blue) wire, only to earth with a metal rod at
the house. You would get a shock if it was somehow disconnected from the
rod/earth. Neutral is also not bounded to earth inside the house. Be safe!

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ganonm
"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.

~~~
vvillyd
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.

~~~
penagwin
It's very common to edit your typo but still leave it there either crossed
out, and/or leave an EDIT: and explain what/why you changed it.

It's perfectly okay to ensure your text is communicating accurate information
- in fact leaving it may undermine your explanation as certain folks will
focus on the error (or others won't notice it and learn something incorrect).

------
Ives
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.

~~~
robocat
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 New Zealand since 209 must have an
Residual Current Detector (the actual rules lead to multiple RCDs per main
power board). This will cut the circuit if someone does manage to touch a live
wire somehow (current from phase to neutral doesn't match, because some of it
is grounded through your body, and the mismatch triggers the breaker.
Wayyyyyyy safer than a fuse!

~~~
pkaye
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.

~~~
robocat
Are tamper proof outlets mandatory? The UK has had shutters on the phase that
is opened by the earth pin forever. Which is why their plugs are caltrops, and
have a plastic earth pin if double insulated (only phase and neutral wired,
with no earth wire).

I don't think I have ever seen a US plug where the pins have a non-conductive
protective covering at the base of the pin (I think all modern UK and NZ male
plugs have that. Although I'll be honest that I think the NZ solution makes
plugs less safe due to risk of pins bending then breaking and leaving a live
metal pin exposed in the socket).

EU sockets are mostly recessed from what I have seen (some NZ sockets are, but
the recess is still is often unusable with old plugs, so is not common yet,
but will become so as more plugs are sold that fit properly).

~~~
Doxin
> The UK has had shutters on the phase that is opened by the earth pin
> forever. Which is why their plugs are caltrops

No, bad design is why they are caltrops. Over here in the Netherlands shutters
are similarly required but none of our plugs need a third pin. The shutter is
a see-saw construction that works in such a way that you need to insert both
prongs at the same time for the shutter to rotate out of the way.

That said our grounded plugs are still caltrops for no real good reason,
though less so than the UK ones. They tend to fall with the prongs to the side
instead of prongs-up.

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ryanmarsh
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.

~~~
alexhutcheson
They are both connected to earth at the same point. The big difference is that
"neutral" has current flowing through it during normal operation, but "ground"
only has current flowing through it in a fault condition.

~~~
chrisdhoover
Right because the hot/black is connected to the neutral/white through the load
completing a circuit so current flows. The “ground”/green is only connected to
the chassis so no current flows in normal operation.

------
js2
See also "Why Three Prongs (1996)" discussed recently:

[https://news.ycombinator.com/item?id=20378852](https://news.ycombinator.com/item?id=20378852)

Also, be aware of grounding requirements when using a generator:

[https://cpower.com/PDF/InfoSheets/44.pdf](https://cpower.com/PDF/InfoSheets/44.pdf)

------
bardworx
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/states codes). Are there classes
for such cases?

~~~
vvillyd
The three wires in any typical household circuit are your Ungrounded Conductor
(hot wire), Grounded Conductor (neutral wire), and Equipment Grounding
Conductor (EGC/Ground Wire). The hot and neutral are what actually makes the
circuit work. The EGC/ground is a safety measure that creates a ground fault
current path which enables overcurrent protective devices (like breakers and
fuses) to operate.

As far as learning common electrical, I'm sure local organizations in your
area will have some kind of classes. Probably local hardware stores? I don't
know. I learned through trade school and on-the-job training. As an
electrician, I will self-servingly tell you that you should always hire a
licensed electrician.

~~~
ars
> The EGC/ground is a safety measure that creates a ground fault current path
> which enables overcurrent protective devices (like breakers and fuses) to
> operate.

This is NOT TRUE!! You've said it so many times in this thread, and it's just
not true.

A lot of people are tying to correct your mistakes, but you're not fixing
them. Please do so, you are misleading a lot of people.

~~~
chrisdhoover
Perhaps you could explain it correctly?

~~~
wtallis
You can trip a breaker without the ground wire being involved. The ground wire
does help with one failure case, but that's not the _only_ way that your
circuit breakers/fuses protect you.

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mikestew
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".

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

------
fsh
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 fault and the breaker
never trips.

~~~
vvillyd
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.

~~~
ars
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.

> This will build up enough current (usually VERY fast, like milliseconds)

There is no such thing as "build up enough current", current does not "build
up".

The milliseconds has to do with built in time delays at breakers, it's not a
function of the electricity.

> The Earth has nothing to do with this

The actual Earth is used as a conductor, so yes, it definitely has something
to do with this.

~~~
jessaustin
_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...]

~~~
jhayward
> _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.

~~~
jessaustin
Ah, yes, well spotted. That was an oversimplification. Two hots, one neutral,
zero "grounds".

