
Why Three Prongs? (1996) - nkurz
http://amasci.com/amateur/whygnd.html
======
ken
The obvious follow up question, then, is: why does Japan use 2-prong outlets
even today? Are Japanese people dying from electrical shock at a much higher
rate than the rest of the world? Is Japan full of only Perfect Electricians?
Or is this not actually the significant problem that this myth (in the
etiological sense, not the fictional sense) would lead us to believe?

The only 2-prong device I own is a (Japanese-made, of course) rice cooker.
You'd think that if any device warranted 3-prong safety, it'd be a metal
container which combines water and mains electricity.

~~~
GuB-42
\- Japan runs at 100V, the lowest and therefore safest voltage in the world.
At least when it comes to electrocution. I suppose that it is also the reason
why in Europe, where the standard voltage goes up to 240V, electrical safety
is much more strict.

\- As it is already mentioned, Japan is backwards in several domains and that
may just be one of these. That's despite their technological advance in
others.

\- I have a feeling that there are more "perfect electricians" in Japan than
anywhere else, because of their culture.

\- Some 2-prong devices are perfectly safe because they are double insulated.
This is also called Class II and the symbol for it is a square inside another
square.

~~~
mikestew
_Japan runs at 100V, the lowest and therefore safest voltage in the world._

100V, 240V, if 15 amps runs through you, you’re having a really bad day either
way. That’s another way of saying, “it ain’t the volts, it’s the amperage.”

~~~
Lio
I was always taught “volts jolt; mills kill”. i.e. high voltage will knock you
across the room but a few milliamps of current across the heart will easily
kill you.

A 12V car battery can kill if you’re not careful.

~~~
eemil
So much misinformation in this thread. Voltage, current, and resistance are
physical properties entirely dependent on each other -- increasing one affects
the value of others. That's like saying that the mass of bullets kills people,
without any regard for velocity.

A 12 V car battery can kill you, _if_ it has a low-resistance path that goes
through your heart. Touching the terminals will do nothing. Maybe if you
applied that voltage to conductive spikes inserted into specific points of the
body, there would be some effect.

A voltage source, like a car battery, does not determine how much current
passes through a circuit. That is determined by the circuit's resistance, and
the source's upper limits (i.e. internal resistance for batteries, rated
current for power supplies).

As voltage increases, so does current. High-voltage supplies will absolutely
kill you, as long as they're able to supply the required current. Which, to
harm a human heart, is quite low (tens to hundreds of milliamps). Aside from
fringe sources like static electricity and tasers, most HV sources are
entirely capable.

------
linsomniac
Short form: Neutral is grounded. The "mouth" part of the plug is the ground
and is isolated from the neutral to prevent inadvertent carrying of potential.

Aside: it is generally considered safer to have the outlets ("receptacles")
installed with the ground at the top, but we don't tend to do this in NA
because ground at the bottom makes it look like a little face and humans like
faces.

Edit: Adding link to EETimes:
[https://www.eetimes.com/document.asp?doc_id=1272972#](https://www.eetimes.com/document.asp?doc_id=1272972#)

~~~
count
I recently remodeled a house (in the US), and installed every receptacle with
the ground at the top, because of that.

Every person who comes over that knows anything about electrical stuff
instantly says 'your plugs are upside down'...and then I have to explain to
them how/why it works :)

The inspector had no issue with it though, thankfully! In health/saftey
critical installs (like hospitals), the ground MUST be on top, according to
our local code. It's optional for residential.

~~~
throw0101a
> _I recently remodeled a house (in the US), and installed every receptacle
> with the ground at the top, because of that._

Have you run into any appliances or devices that are "optimized" for a ground-
down configuration?

~~~
kevin_thibedeau
Low profile plugs intended to have the cord exit downward at 45-degrees don't
work as well with upside down receptacles.

------
komali2
> There is a simple solution to these problems: connect your system to the
> Earth. Drive some long metal rods into the dirt, and connect them to your
> wires. That way, lightning currents will be directed into the Earth rather
> than spreading throughout your power lines.

I have tried and failed many times to understand the concept of "ground." Why
does electricity "want to go to the ground?" What constitutes "ground?" Can I
touch wire to concrete, and that counts as "ground?" No? Must I then sink it?
How deep? Until it touches dirt? What about sand? Where does the electricity
"go" then?

I've read plenty of articles, never got it.

~~~
ZainRiz
There are two different mental models I find really useful when thinking about
electricity. The second one will answer your question more directly, but the
first one helps set the stage better.

Mental Model #1 (useful for most work with electricity):

Think of electricity moving through circuits as water going through pipes.

Electricity = water Voltage = water pressure Battery/Voltage Source = water
pump Wires = Water pipes High/Low resistance = Wide/narrow water pipes

Voltage is like water pressure. You have a pump creating high water pressure
is like a power source with a high voltage. The pump is always on and creating
water pressure, but there's a cap placed on it that blocks the water from
escaping. Similarly, in an open circuit the air acts as this cap and blocks
the electricity from proceeding.

The wires in a circuit act as pipes channeling the water to different areas.
Resistance in a circuit is analogous to the thickness of the pipes (low
resistance == wide pipes, high resistance == thin pipes).

The above analogy sets part of the mental model. The twist is that in reality
electrons want to actually travel from a place of low voltage to a place of
high voltage (classical electrical diagrams show this process in reverse).
Which leads us to...

Mental Model #2

Voltage sources are powerful electron vacuum cleaners trying to to _suck_ up
electrons.

(You can apply this on top of mental model #1, just reverse the direction of
the water flow)

Open circuits generally don't conduct electricity since within a nano-second
the voltage source will suck up all the free electrons in the circuit. The
negative/ground pole in circuits are where the free electrons get supplied
from for the voltage source to "suck in" the electrons from.

The ground (by virtue of it being so large and having lots of
moisture/materials which conduct electricity) happens to have a virtually
limitless source of electrons available for the voltage source to suck into
itself. This provides the circuit with all the electrons required to
successfully operate.

Hope that helps :)

~~~
chronolitus
Here's another mental model, the balls model, which I sometimes find useful,
especially in this case. Here it is in case it helps someone:

Imagine electricity as red balls, which can travel along conductors. Groups of
red balls feel drawn strongly to places which have less balls, and away from
places which have more balls, leading them to equalize quantities among all
points in the circuit (let's assume for now cables can't store the red balls,
only transport them. Points in the circuit are places that can store balls).

if you have a battery, both the sides can store balls, but assuming it's
charged, the plus side might have 12 red balls and the minus side none. The
difference in red balls is 12! A quite strong force pushing red balls from the
+ to the -. (that's the voltage). When the circuit is closed with a wire, the
balls will travel quickly and end up with 6 at both the + and -. How many of
them travel at once (Intensity) depends on how large the tube is (Resistance).

What if instead of 12 and 0 we have 112 at +, and a 100 balls at -? Well the
force is the same as before (12), the whole circuit works the same way. 6
balls go from + to -, over the same amount of time (tubes have the same
throughput), you then end up with 106 at both ends. You just have these
'residual' red balls hanging around, doing nothing special.

However, if you suddenly connect this circuit with 212 balls to the first
circuit with 12, the balls will quickly flow from from the first to the
second, this time with a force of a 200! (The point here is that voltage is a
relative measure, unlike charge.)

You might have an isolated electronic circuit starting with 12 and 0 balls,
and think all is well. You touch the circuit once to start it, the balls are
flowing normally. You leave it working for a few hours and come back, it still
seems to be fine. You touch it, and ZAP, a spark, burnt smell, you're
wondering what happened. Well the thing is that external sources can sometimes
add a red ball or two to your system. Over time, they might add up, and after
a while you end up with hundreds. As soon as you touch the circuit, they all
flow to somewhere that has fewer.

Ok, where does the ground come into this? A 'ground' is like the - side of
your battery. It's an object that can store balls the same way. However,
instead of having one 'storage point' which can be filled with balls, it has a
several. So take a metallic object, and say it has, for example, 11 storage
'points'. If you connect it to a single + point containing 12 electrons, the
balls spread out so that there's the same amount of balls in each point (as we
explained at the beginning). So after a while your 1 and 11 points, end up
with one ball at each point. (Analog to the concept of capacity)

So the natural earth turns out to behave like a huge metallic object. So big
that it basically has an infinite amount of storage 'points'. You can pour red
balls into it all day, and still end up at an average of zero balls per point.

So now you take your 12 to 0 circuit and connect the - side to the earth. The
difference between + and - is the same as before, 12. The flow from + to - is
about the same (same resistance), so the circuit behaves almost the same. If
however an external red ball is randomly added, it flows from the - to the
earth, meaning that you never accumulate balls in the circuit.

This allows you to set a reference amount of 0 balls in every circuit you work
with. Which is great. The amount of red balls at the + can still be set to
whatever you like, and they will flow nicely from + to - along whatever
circuit you create (like before), but on top of that you never run the risk of
having red balls jump from one circuit to another if they accidentally touch,
because there is never a large difference in red balls as they all have the
same reference amount.

Anything big which is conductive enough can store a lot of red balls. So what
qualifies? Soil usually works, water works to an extent
([https://electronics.stackexchange.com/questions/164898/can-t...](https://electronics.stackexchange.com/questions/164898/can-
the-ocean-be-used-as-ground)). When you can't connect to those a big object
will have to do, like a car frame, it will have enough storage that over a
short time it looks infinite, but if you really pushed and kept adding red
balls at some point the reference level would change. With the earth you could
pour red balls your entire life and barely make a difference to the reference
level.

(So now you're thinking, the red balls are electrons, right? Well sort of, but
don't tell a physicist! Otherwise they'll get angry and start talking about
flow direction being this or the other way. Or they'll say something about
non-rigorous representations, wave-particle equivalence, the balls actually
being a 'lack of electrons' and we'll all be confused.)

------
nkurz
> Before you "grounded" your system, the AC voltage in general acted pretty
> safe for your customers. The only way they could get a shock was if they
> touched both wires at the same time. This was a fairly rare occurrence. One
> single wire acted as if it was "safe," and it did not deliver shocks.

I'm confused by the assertion that you could not get a shock from a single
wire in an ungrounded system. If there is 120V AC between the two wires,
doesn't this mean that there has to be at least 60V AC potential between at
least one of the wires and earth ground? Is he depending on the "halved"
voltage for safety, or depending on having an intact circuit so the other
connected wire is always "better" than an earth ground, or is he right in some
way that I'm not seeing?

~~~
nkurz
I've gotten some helpful answers, but it's apparent that I've failed make
clear my actual question, which is about the relative safety of a system with
an isolated ground, such as the "razors only" plugs discussed in the article.
The article seems to claim that such systems are safe for any single point of
contact.

Maybe I can make it clearer with a parallel more specific question. Assume I'm
running an gasoline powered generator which produces 120VAC for a single
outlet. As far as I can tell, this is an ungrounded isolated system analogous
to the isolated transformer. If I'm barefoot and standing on wet concrete, and
stick my wet finger into only one side of the plug, will I receive a dangerous
shock?

The article seems to claim that as long as I touch only one of the wires of an
isolated system, I am "safe". My counter intuition is that that the
capacitance of the earth is such that an AC current will flow through my body
and cause me harm. Am I wrong? Is the article wrong? Or is there some
essential difference between my proposal and the example in the article? If
I'm wrong, I'd love to have a more technical explanation of why this seemingly
unsafe behavior of sticking my finger in a live socket is actually safe.

~~~
lisper
> Assume I'm running an gasoline powered generator which produces 120VAC for a
> single outlet. As far as I can tell, this is an ungrounded isolated system
> analogous to the isolated transformer. If I'm barefoot and standing on wet
> concrete, and stick my wet finger into only one side of the plug, will I
> receive a dangerous shock?

Theoretically, no. Your generator isn't going to pick up large static voltage
from the atmosphere the way a power grid does, so it's theoretically safe to
leave it ungrounded.

However, I would strongly advise against actually doing this experiment unless
you are _absolutely_ certain that your generator is isolated from the ground,
and even then I would advise against it. Some risks are not worth taking even
in the name of science.

~~~
nkurz
> However, I would strongly advise against actually doing this experiment
> unless you are absolutely certain that your generator is isolated from the
> ground, and even then I would advise against it.

A safe version of the experiment would be to take a standard incandescent
lightbulb and run one wire to an earth ground and one wire to one side of the
plug on the generator. My guess is still (although with more doubt based on
how many intelligent people here have opined otherwise) that because of the
enormous capacitance of the earth, the light bulb would still light up even
though no clear circuit is formed.

In my quite possibly flawed mental model, electrons will be pulled and pushed
from the earth across the filament, causing it to heat up. Whether this
actually works (in my mental model) depends on the frequency and the
capacitance of the generator, but I'm guessing that at 60Hz whatever is inside
the generator is enough to get an effect. Unless someone has already done
this, I guess I'll have to get a generator and try it.

~~~
pwg
> My guess is still ... that because of the enormous capacitance of the earth,
> the light bulb would still light up even though no clear circuit is formed.

Your statement implies that you are viewing the earth as a huge, perfect,
capacitor. And by "perfect" I mean no series resistance.

In reality, the earth is a relatively poor electrical conductor (i.e., a high
resistance [1]) and so where your mental model breaks is omitting that
resistance from consideration in your model.

I.e., the equivalent circuit your 'experiment' produces would be:

    
    
       G-----bulb------R--C 
    

The "R" and "C" above are the capacitance and resistance of the earth. Yes,
there is an amount of C present, but there is also a quite large series "R"
present as well. And it is that resistance that results in a very limited
current flow from your theoretical experiment. Too low of a flow to illuminate
the bulb.

[1]
[https://en.wikipedia.org/wiki/Soil_resistivity](https://en.wikipedia.org/wiki/Soil_resistivity)

------
dforrestwilson
Perhaps an ignorant question, but given that when I travel I have to bring a
seemingly endless array of plug adapters, which power standard is the "best"
and how do we get rid of the rest?

~~~
nateguchi
I may be biased, but I think British plugs are great. They have integrated
fuses and are switchable too! The sockets have shutters as well to stop
children sticking paperclips or forks in and electrocuting themselves.

Not great to stand on, however

~~~
ken
BS 1363 plug has some neat safety features, but it's really big. Look at the
size of a British power strip, for example. As Tom Scott observes, to get the
full benefit of the safety features, they would need to make them even bigger.
It's also not even close to round, so pulling it out of a nest of cables looks
terrible.

I'm not convinced that a fuse belongs in a plug, either, especially if it's
invisible, and the plug has to be completely disassembled with a screwdriver
to access it. Put the fuse in the device, which certainly has much more space
for it, and probably a mechanism to make it easy to access.

As far as the physical structure goes, BS 1363 looks like it was designed in
the 1940's ... because it was designed in the 1940's. I'd much rather have
something modern and ergonomic like TRUE1. It's small and round, it locks,
it's basically impossible to shock yourself with it, and it's IP rated.

I'm not alone here. There's many videos on YouTube of people hacking the
Edison plugs off their power tools to replace them with powerCON or TRUE1, but
I have yet to find anyone in the world who wants to replace their Edison plug
with BS 1363!

~~~
jdietrich
_> I'm not convinced that a fuse belongs in a plug, either, especially if it's
invisible, and the plug has to be completely disassembled with a screwdriver
to access it._

Moulded plugs generally use a clip-in fuseholder that can be opened with any
pointed object; rewireable plugs can be opened to replace the fuse by
loosening one captive combination Philips/flat screw. It's usually far easier
than replacing a fuse in the appliance, which is often on a PCB-mounted holder
buried within the guts of the appliance.

------
kazinator
The article isn't not really on point. A two-prong system still has an earthed
neutral. The third prong is purely for redundancy. If neutral happens to be
disconnected, there is still a return path. It also protects against mixed up
neutral and hot. If the chassis of a device is grounded to the third
conductor, and that one is properly routed, then an inadvertent reverse wiring
of hot/neutral is mitigated. A third conductor also makes possible ground
fault detection: situations when a device is not returning all current through
the neutral, because its safety ground has become energized.

How might a neutral end up disconnected? Not just by accident; for instance,
someone would incorrectly wire a cold-side switch instead of hot-side (E.g.
wall panel switch that controls an outlet) so that when the outlet is off, the
neutral has been cut off, but hot is still live. In that situation, the third
conductor would still be grounded. Nobody would mistakenly implement the
switch on the ground line unless they didn't bother testing it, because the
switch then wouldn't work; the ground line is implicitly protected from having
switches accidentally planted on it by the fact that interrupting it does
nothing, functionally.

------
jhallenworld
Three prongs was a good idea before the existence of ground fault
interrupters, but now that they do exist, ground fault interrupters with two
prongs provides a much better solution.

The problem with ground is that it's a conductor. If there's a fault on it,
you don't know exactly what it's connected to, so now you have a situation
where the metal chassis is connected to something. You touch the chassis and
the really-grounded a sink and you have a problem.

Here's a real world example of this. The ground is bonded to neutral at the
panel. But what happens when neutral between the pole and the house breaks?
This happened to my neighbor. There is still 240V with neutral/ground floating
somewhere in the middle. All of his appliances on the less loaded side of the
240V blew up and "ground" was very not at earth ground.

~~~
blattimwind
That's a problem with incorrect installation / code. In a proper TN-C-S system
the combined PEN will be split up at the main panel, and PE will be local
ground and pipework is typically connected to that.

------
dang
A thread from 2012:
[https://news.ycombinator.com/item?id=4058834](https://news.ycombinator.com/item?id=4058834)

------
haberman
If GFI sockets and breakers had been widely available at the time, would the
third prong have been added? Would a GFI along with the "one long slot" plugs
make the ground wire obsolete?

~~~
xyzzyz
GFI outlets are $15+, and normal outlets are $2. This adds up when you're
building a house: a typical house has something on the order of 30-50 outlets,
so you're looking at $400-$700 vs $60-$100. It might not seem like much in the
grand scheme of things, especially to software engineers making lots of money,
but if we had this approach not only for outlets, but also for everything
else, building literally anything would cost 5 times as much (and in fact it
does these days, to a large degree because we can afford spending 5 times as
much on building).

~~~
magduf
>GFI outlets are $15+, and normal outlets are $2.

Where are you getting your prices? Last time I checked, GFCI outlets were
indeed around $15, but the normal ones are less than a dollar when you buy
them in bulk packs (which you would for building a house). Otherwise, your
point is sound, but the price difference is even greater than you said.

Those standard outlets are really dirt-cheap, and it's annoying that builders
are so cheap about installing them. New construction has gotten better though,
but really there should be an outlet every 5-6 feet in my opinion, or maybe
even less.

~~~
xyzzyz
Home Depot non-bulk prices -- the only ones I'm familiar, as I haven't had a
need to buy many of them at once.

Also worth noting is that you can wire your circuit in a way that the first
one is GFCI outlet, and the subsequent ones are daisy-chained to it, so that
all of them are protected by GFCI. Of course, there are reasons to not do it
(e.g. it is then quite confusing when the GFCI trips while using some non-GFCI
outlet, because it might not even occur to people to check the GFCI outlet
which might not even be used), but that's also an option if you want to save
money on outlets.

Also, I think the reason for few outlets in buildings is not the cost of the
device, but rather labor required to put it in. Connecting wires to the outlet
is quick, but you need to also install boxes, route the wire to the boxes,
mark and cut the drywall to get to these boxes etc. It can all add up to
significant labor time, and if you look at what electrical contractors charge,
the cost of the device itself is a small part of it.

~~~
magduf
>Home Depot non-bulk prices Definitely check out the 10-packs; if you own a
house that isn't new, and are already doing some wiring work, it's not a bad
idea to keep some extras around and maybe replace some of the crappy older
outlets when they're so cheap.

>Also worth noting is that you can wire your circuit in a way that the first
one is GFCI outlet, and the subsequent ones are daisy-chained to it,

Yes, this is actually normal for kitchens, where all the counter outlets are
on the same circuit. Just put the GFCI on the outlet nearest the breaker box,
and all the others are then protected by it. It is really annoying, however,
when people retrofit these things into old houses and you get something like a
downstairs bathroom outlet that has no apparent GFCI, but then it trips, and
you have to hunt around the house for the GFCI outlet only to find it in a
bedroom on the upper floor. I rented a house for a while where both bathrooms
and both bedrooms were _all_ on the same circuit (lights too!), and on an old
GFCI outlet that would frequently trip and leave me in the dark. Extremely
unsafe.

>I think the reason for few outlets in buildings is not the cost of the
device, but rather labor required to put it in.

Yep, that's exactly the reason, but it's still annoying because years later,
you have to move furniture around to find an outlet that your device's cord
can reach because there just weren't enough installed.

~~~
haberman
> It is really annoying, however, when people retrofit these things into old
> houses and you get something like a downstairs bathroom outlet that has no
> apparent GFCI, but then it trips, and you have to hunt around the house for
> the GFCI outlet only to find it in a bedroom on the upper floor.

Maybe easiest to just have GFI on the breakers?

~~~
magduf
They do have GFCI breakers. I'm not sure how they compare in cost, but one
disadvantage is that you can't easily test them (who ever bothers going to
their breaker box to test the GFCI occasionally?), nor can you easily reset
them when they do trip.

------
bookofjoe
What I learned from reading every single one of the 184 comments (as of 6:28
pm ET) on this post: call an electrician for anything more complex than
changing a light bulb.

------
nck4222
For some reason, this reminds me of the Laryngeal nerve. It's like a natural
process repeating itself in different systems. Because re-engineering the
solution from the ground up to be more efficient isn't an option, there has to
be small incremental steps to arrive at a solution that works.

[https://en.wikipedia.org/wiki/Recurrent_laryngeal_nerve](https://en.wikipedia.org/wiki/Recurrent_laryngeal_nerve)

~~~
jedimastert
> Because of the inefficiencies of the routing the nerve takes, it's often
> hailed as one of the most striking cases against intelligent design — or at
> least calls into question the "intelligence".

Man, what an obnoxious way to start an article.

EDIT: link was removed, this comment no longer needed.

~~~
nck4222
Oh my bad, didn't mean to suggest it was intelligently designed. I didn't read
that full page and didn't pick up on that context. I'll remove that link.

~~~
jedimastert
My comment was a bit rude as well, I apologize. The website seems to be a
little argumentative and it rubbed me the wrong way.

------
peter_d_sherman
Excerpt:

"The sparks occur because of a little-known fact: all the world is a gigantic
electrostatic generator. There is a flow of charge going on vertically
everwhere on earth. Thunderstorms pump negative charge downwards, and the
charge filters upwards everywhere else on earth. Depending on the height of
your circuitry above the earth's surface, depending on the area covered by
your wires, and depending on whether there was a thunderstorm above you at the
time, there might be a fairly huge DC charge on your electrical distribution
system. This charge might be several hundred volts; enough to zap computers
and delicate electronics. Or... it might be many tens of thousands of volts,
enough to create enormous sparks which jump across switches and leap out of
wall outlets, wall switches, across transformer windings, etc. Your electric
power system is acting like a sort of capacitive "antenna" which intercepts
the feeble current coming from the sky and builds up a huge potential
difference with respect to the earth."

------
phasetransition
For those outside the US, or generally confused how US domestic power
distribution works, let me explain in the context of the 3 prong plug and NFPA
70 NEC (National Electrical Code) for the lay reader. We'll start outside the
house and go all the way to the plug:

0\. Before the transformer secondary on the pole outside the house, the wiring
is dictated by the NESC, which is like the NEC for utilities. We'll leave that
there.

1\. Most US homes have a single phase, "center tapped" transformer secondary.
This gives three voltage potentials to feed the home: (Nominal) 0V - from the
center tap; +120Vrms - from one "end" of the transformer; -120Vrms from the
other end of the transformer. And, yes, I am glossing over the phase
relationship here. All three potentials are connected through the chunky
copper of the transformer, which provides a current path to complete the
circuit.

2\. The three potentials feed the house meter and disconnect. The 0V potential
may, or may not, be insulated on the path back to the transformer secondary,
and often doubles as the mechanical support for the incoming lines (the
"service"). The other two potentials will be insulated wiring.

3\. Inside the main service panel the +120V and -120V inputs are split between
the breakers. Every other breaker is connected to +120V or -120V. Typically a
breaker that spans multiple slots is connected from -120V to +120V. That is
the "240Vrms" for ovens, hot water heaters, dryers, air conditioners, etc.

4\. Adjacent to the metal bits that fan out the +120V and -120V is a strip of
metal that allows multiple connections to 0V potential. Colloquially this the
"neutral bus," but in the NEC this is termed a "grounded service conductor."
All of the white wires from the circuits in the house ("branch circuits") are
tied to the spaces in this metal bus bar. This metal bar is also electrically
connected to the 0V potential entering the house from the transformer center
tap.

5\. Also inside the main service panel there is strip of metal that allows
connection for a number of "grounds." It looks just like the "neutral"
connection bar, but instead has a number of green wires attached to it.
Colloquially these wires a called "ground," but the NEC calls them "Equiment
Grounding Conductor" or EGC.

6\. Between the grounded conductor ("neutral") bar, and the EGC ("ground") bar
there is a removable conductive link. In the main panel this link remains
installed, but in secondary panels it is removed. The link is removed in
"subpanels" to insure correct operation in the event of a fault. Fault
conditions are discussed below.

7\. Also connected to the EGC and grounded conductor is a third wire. The NEC
terms this the "grounding electrode conductor" or GEC. The GEC then goes to a
water pipe or "grounding" rod(s). Thus the GEC is the connection between
physical Earth voltage potential and the power panel. It is unfortunate that
the EGC and GEC are such similar abbreviations.

In the event I'm out of characters, I'll pause and reply to myself now.

~~~
phasetransition
Brief recap: Three different voltage potentials in from the pole on three
different conductors. And in the main panel a link between three other wires:
the EGC ("ground"), GEC("earth"), and grounded service conductor ("neutral").

The next section is where people go awry, even licensed electricians I have
met. But it is also the meat of the safety aspect of the three wire
configuration. We will assume a house with a single electrical panel for
simplicity.

8\. Three wires traverse from the main panel, down a branch circuit to the
three prong "edison" outlet on the wall: One of the three prongs connects to
either +120V or -120V; another connects to the EGC bar inside the panel; the
last connects to the grounded service conductor bar inside the panel.

9\. The short blade on the top row of the Edison connector is +120 or -120V;
the tall blade on the top row connects to the grounded service conductor
(colloquial neutral). The single connector on the bottom row connects to the
EGC (colloquial ground).

10\. If you plug in an item that has a three wire plug, the lower single plug
prong will be longer. This is so that the item is electrically connected (i.e.
"bonded") to the EGC before the other two wires. This insures the "fault
current" path is connected before the item is energized.

11\. Under normal conditions, the current path is through the item plugged in
between either +120 or -120V and 0V volt potentials. Crudely think of current
coming "out" the short plug prong and "in" the tall plug prong. The EGC
(colloquial ground green wire) doesn't do anything under normal operation.

12\. The current path is then back down the branch circuit, via the grounded
service conductor (colloquial neutral). the metal bar that links all the
grounded service conductors together then has a path back to 0V on the
transformer by one of the three incoming conductors.

13\. Finally, the current can flow from the 0V location on the transformer to
the higher potential via the wire of the secondary. Notice that the GEC
(earth) connection to the ground rod was NOT a meaningful component of the
current path.

14\. Returning to #11, and considering abnormal operation. Here current
somehow flows outside of the correct circuitry in the powered item. The ECG is
bonded (connected) to the item's chassis and provides an alternative current
path. The EGC provides this connection back to the panel bar with all the
green ECGs tied to it.

15\. The EGC bus bar in the panel has a conductive link back to the grounded
service conductor bus bar (colloquial neutral) via the removable link that we
discussed previously. The link then "brings" the current over to the grounded
service conductor (0V potential from the street), and provides the current
path back through the transformer secondary.

16\. Let's assume the outlet is a GFCI / RCD. It notices the current coming
back on the grounded service conductor doesn't match the current going "out"
into the device, and opens the circuit. That is because the fault current is
"lost" to the EGC, and goes around the GFCI outlet. The outlet trips, assuming
a human body is the fault current path.

17\. People commonly assume that the GEC (earth) conductor somehow matters for
current path in event of a fault, but this is rarely the case. Usually the
water pipe or grounding rod(s) are high impedance relative to the wire in the
transformer secondary, and so the current divider formed is essentially all
through the grounded service conductor.

18\. The GEC (earth) connector holds the pole transformer secondary center tap
near the same relative 0V potential as the house. The GEC may also provide the
lower impedance path at very high frequencies encountered during a lightning
strike.

If you made it this far, I'm happy to field further questions :-)

~~~
function_seven
Thanks. Really clear write up.

What issues can arise if the link between the EGC and GSC bus bars is left in
place inside a subpanel?

~~~
phasetransition
Excellent question!

Stated rhetorically,

"If a GFCI only cares about the current imbalance, and the traditional style
breaker opens to due to a big spike in current, why care about what path it
takes through the building wiring back to the pole transformer?"

If a sub panel does not have the jumper removed, then the EGC and GSC between
that sub panel and the main panel will form a current divider. Let's assume
they are the same wire gauge, basically the same resistance, and so
essentially split the return current.

This leads to a couple problems:

1\. The EGC may no longer able to carry its full rated current load, due to
the baseline "normal" current that the EGC is also bearing. You want to bear
as much current as possible to quickly trip a conventional breaker in event of
a fault to the EGC. This is real, but relatively minor concern.

2\. All EGCs "upstream" of the bonding point have been "contaminated" by the
normal neutral current that got on to the EGC at the mistaken sub panel bond.
This could "put current" on the surfaces of other equipment, exposed ground
wires, conductive raceway, etc. This is the more major concern, as these
surfaces would have the full voltage potential available to them should there
be yet an additional current path.

In the main panel the EGC, GSC, and GEC are all connected at a single point.
And from this point there is only one path back to the transformer, via the 0V
conductor of the GSC. The transformer side of this 0V potential conductor will
match the local Earth voltage, set by the GEC, and wiggle from 0V nominal by
the fluctuations caused by current (V=IR) in the GSC conductor between the
panel and the pole.

Make sense?

------
js2
Since this article was written, the NEC has also started requiring arc fault
circuit interrupters.

[https://en.wikipedia.org/wiki/Arc-
fault_circuit_interrupter](https://en.wikipedia.org/wiki/Arc-
fault_circuit_interrupter)

BTW, if you have a portable generator for emergency purposes and it's
connected via a transfer switch, take note of the important grounding and
bonding requirements:

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

------
CapitalistCartr
Past posting on HN:

[https://news.ycombinator.com/item?id=4058834](https://news.ycombinator.com/item?id=4058834)

------
pkaye
Why do a majority of electronic devices has only 2 prong?

~~~
MisterTea
These are called CLASS II devices and they are designed with double or
reinforced insulation. In plain terms it means that the current carrying
components are themselves insulated AND surrounded by a second layer of
insulation. That or the current carrying components are protected by
reinforced insulation like a hard plastic case.

A practical example is the humble power brick or wall wart. These are class II
power supplies which are enclosed in a hard plastic case which protects the
user from electrical shock.

Reference: [https://www.cui.com/blog/class-2-vs-class-ii-power-
supplies](https://www.cui.com/blog/class-2-vs-class-ii-power-supplies)

------
jonprobably
Why is the metal chassis wired at all?

