
Why three prongs? - xd
http://amasci.com/amateur/whygnd.html
======
gojomo
Even already knowing the gist of things, I enjoyed the description of the
path-dependent incremental evolution here.

But, this sentence makes me wonder about something: "Also, the clear-weather
sky current can no longer build up a high voltage, if any excess charge
immediately leaks into the earth."

Have our region- and even continent-wide grounded electrical networks changed
the expression of lightning storms, such that non-electrified areas are
noticeably different?

~~~
throwaway64
all manner of things can cause a build up of charge in what is essentially a
massive network of thousand KM+ antennas: radio waves, the earth's magnetic
field, cosmic rays, wind interacting with dust particles causing static
charges, etc.

A bit of an interesting side note: it is possible to build a radio set
passively powered by radio waves, requiring no external power:

<http://en.wikipedia.org/wiki/Crystal_radio>

~~~
stephengillie
<http://en.wikipedia.org/wiki/Radio-frequency_identification>

------
zdw
Ever wonder why, in some buildings (mainly commercial properties), the plugs
are installed so that the ground is at the top of the outlet?

I asked an older electrician/installer about this once, and the reason he gave
was that if a wire fell in between the gap between the plug and outlet it
could short easily.

Having the ground on the top makes it so that if something does land in this
way, it will be at an angle and more likely to fall off the shorting outlet.

Plugs in other countries, specifically european or british plugs aren't
susceptible to this because they're designed not to expose hot wires during
insertion.

~~~
TazeTSchnitzel
British plugs[1] have a pretty clever design. There's 3 prongs arranged in a
triangle, each of equal length, but they are rectangular prisms, not cuboids,
so you can only plug it in one way up. The lower 2 "live" holes only open if
there's a prong in the top earth hole. Because of this, double-insulated
appliances still have a plastic prong on the plug.

And yes, we have insulation on both "live" prongs.

Plus, it's the only kind of plug that's both Grounded, Polarised, Fused and
has Insulated Pins[2]

[1] <http://en.wikipedia.org/wiki/BS_1363>

[2]
[http://en.wikipedia.org/wiki/AC_power_plugs_and_sockets#Comp...](http://en.wikipedia.org/wiki/AC_power_plugs_and_sockets#Comparison_of_sockets)

~~~
magic_haze
After an hour-long detour through Wikipedia's various power plugs articles
(shocking how many of them there are), I have to ask this: historical reasons
aside, isn't there any kind of consensus on which is the "best" plug/socket
system (in purely objective terms)? The comparison table in [2] would have us
believe BS1363 has the most features, but surely, there must have been more
improvements in plug design since that huge lump was designed in the 1940s?

~~~
ricardobeat
The IEC 60906[1] standard was designed to replace BS1363 too, but the british
requirement for fuses prevents it's adoption.

[1] <http://en.wikipedia.org/wiki/IEC_60906-1>

~~~
ginko
Couldn't they just install the fuses in the sockets?

It seems terribly inefficient to require fuses in every plug even when the
electrical wiring may not need them.

~~~
tomfakes
In the UK, until fairly recently, it was illegal to sell a product with a plug
attached. Consumers had to wire their own plugs to their electrical devices
before use.

Now that is inefficient!

~~~
jrockway
Citation needed.

~~~
tomfakes
I may have had it a little wrong. I can't find older legislation about this
subject that confirms my supposition.

This 1994 legislation made it the law that plugs _must_ be fitted to certain
consumer devices: <http://www.legislation.gov.uk/uksi/1994/1768/contents/made>

Before that time, almost everything (certainly everything I purchased) came
without a plug and you had to buy a plug and wire it yourself. Probably
because these things are expensive, and costs could be reduced by leaving them
out.

~~~
shabble
I have a suspicion you have it exactly backwards. It's now a requirement for
some|many devices to have non-removable moulded plugs (although they still
have replaceable fuses).

I'm not certain on the exact motivation, but I think it's a mix of preventing
people doing it wrong, and better mechanical properties (especially strain-
relief) of moulded plugs.

It's also required now that the live/neutral pins be partially insulated (the
1/3rd closest to the plug or so) to prevent shock hazards to people curling
their fingers around the plug when inserting or removing.

~~~
FuzzyDunlop
It wasn't uncommon for us to have spare cord and plugs either, although the
change of law to require moulded plugs obviated the need for any of it.

I remember us having to wire up kettles, lamps, and irons when I was a kid.
And you'd sometimes go to antique (or second hand) shops and come across old
appliances that had no cord attached. Anecdotally, we bought an old style
telephone that we then had to wire back up.

That being said, I can't corroborate any claim that this was backed by
legislation.

------
thought_alarm
Prior to the Ground Fault Interruptor sockets found in modern bathrooms, an
isolating transformer was often used to protect bathroom outlets in the 60s
and 70s. These are two-pin unpolarized sockets, usually with "Razors Only"
stamped on the front (because if you plug in a 1000 W hair dryer you blow out
the transformer).

~~~
texec
Current installations use Residual-Current Devices instead of individual
breakers, at least in Europe. This requires three way wiring up to the
distribution. In Germany RCDs are required for all outlets in new
installations.

~~~
brudgers
In the US, it is relatively common to use circuit breakers in new construction
but in no way universal - protection at the receptacle is still very common
and can be significantly cheaper since only the first receptacle in a series
needs to be GFCI type...of course this creates an issue of finding the tripped
device for a person unfamiliar with the wiring scheme.

------
bpowah
Now would somebody please tell me what the little holes are for in the end of
the plug prongs!? <http://www.worldofstock.com/slides/BEN2097.jpg>

~~~
pndmnm
Many sockets have little bent-wire brackets that pop into those holes when the
plug is fully inserted in order to help hold it in place.

------
lcargill99
Never mind that yes, A/C can count on polarity and that you are isolated from
the grid by one or more transformers.

The third prong is the "safety" ground, in case neutral fails. Go back to old
guitar amps with "the cap of death" or a "ground" switch for one market in
which this evolved. It's a fairly simple mod to make an old amp fully safe.
Yes, they cover that in the "bad outlet" story, but it's easy to fix if you
know how to run a meter and can find a ground.

The lead guitarist from Badfinger died from this.

I'd never heard about DC buildup on the grid - free power, maybe? :)

------
danbmil99
Been waiting 40 years for this explanation. Electricians have tried to explain
it but their answers always ended up sounding circular and ill-founded.

~~~
tjic
An electrician explaining electricity is like a cop explaining the law.

They're hired to follow an instruction book. That doesn't mean that they have
the least idea of the whys and wherefores of what's in the book.

Look at any of 1,000 videos of cops making up laws, falsely citing either the
Patriot Act of 9/11, or explaining that parts of the Constitution and Bill of
Rights don't apply in certain circumstances.

I'd rather listen to a cattle farmer talk about bovine genetics - he's likely
to have some clue!

~~~
TazeTSchnitzel
Yep. The correct person to ask about electricity is a Physicist, or in your
analogy's case, a lawyer.

~~~
DougBTX
As a physicist*, I would suggest speaking to an electrical engineer.

~~~
K2h
As an EE, I say ask one specific branch of the Electrical Engineering
profession.

see [1], here there are 3 specific sub disciplines you can test in, and all
are considered "EE"

computer

electronics

power

you want to talk to an expert in the power arena, but from what I remember
when studying for the exam, the content was mostly things way above
residential (which is what this discussion has been about) and more of power
gen and conversion. Y, delta configurations and conversions, power angles and
factors, etc. the kind of thing you come out really knowlegable in one very
small area and still clueless on the NEMA power plug.

[1] <http://www.pels.ca.gov/applicants/refs.shtml>

------
nroman
In college I lived in an old apartment building that had been updated to have
3-prong outlets. One day I was wiring up some audio equipment for a party. As
I hooked my computer to a a receiver in another room the audio cable
immediately started sparking and the rubber insulation melted off. I yanked it
out of my computer. Luckily the only real damage done was the sound card for
my computer and the destroyed cable.

Later I did some investigation. First I bought one of those standard outlet
testers. It seemed to suggest that everything was fine with the outlet. Then I
did some more investigation. I got a volt-meter and started measuring the
voltage from the different prongs. Finally I measured the voltage compared to
a copper pipe in the house.

Turns out that both the neutral and ground wires were actually hot (as
measured against the grounded pipe), and the "hot" wire was the ground. The
outlet in the other room had been wired correctly. The shielding on the cable
was grounded so suddenly it sent 120 volts down the audio cable when I plugged
it in—destroying it in seconds.

The electrician didn't believe me when he came in to fix it, but was finally
convinced when he checked against a pipe.

Just goes to prove that you can't make any system fully fool-proof.

------
colanderman
I thought the primary reason for a separate ground was because neutral was a
current return path and thus could develop a voltage on it due to resistance
of wires, whereas a true ground carries no current and thus will not develop a
voltage?

~~~
DenisM
I remember there are two reasons for the third prong, and neither is mentioned
in the article.

The first reason is the one you have outlined - the larger the current, the
bigger the differential between the actual ground via and the neutral wire in
the appliance.

The second reason is that if you're using a transformer to change the voltage,
the output of the transformer is not connected to the input, and thus can have
arbitrary large voltage differential with the ground due to charge
accumulation, and so it needs to be grounded. Now I'm getting fuzzy here, but
I think there is a reason why you can't connect one of the transformers inputs
to one of it outputs (the neutral one) to provide the "grounding" this way, so
the third wire is used. I've noticed that devices which do not need to provide
voltage conversion usually lack the ground prong, e.g. heaters.

------
davidw
As an aside, one of the things that's sorely missing from the day-to-day
integration of the European Union is a standard for electrical plugs:

<http://en.wikipedia.org/wiki/AC_power_plugs_and_sockets>

Here in Italy there are at least three kinds that are fairly common. Go north
to Austria, and things change and you have to buy more adapters.

~~~
gioele
> Here in Italy there are at least three kinds that are fairly common.

Well, one of them is the Europlug (the "small" 2-pin variant) and that works
everywhere in EU except UK.

The problem is that the two grounded versions are peculiar to Italy and not
the standard ISO ones. Italy lost its chance to switch to the ISO sockets
(compatible with the Europlug) sometimes in the 90s, when the EU forced the
adoption of 230V (+-10V) from the previous 220V. The legislator failed to
force the use of the ISO grounded plug and now it is not uncommon to see in
Italy sockets that are compatible with Europlug, the 2 Italian grounded kinds
of plugs, Schuko and type C. It is obvious that these sockets are both clumsy
and expensive.

~~~
davidw
As somewhat of a neutral observer, I think the 3 pin Italian sockets are
actually quite nice: it's easy to put a lot of them in a power strip, and they
are a lot less big and clunky than the Schuko ones.

------
HarrietTubgirl
This is an awesome article. Here's an example of the usual non-explanation:

[http://electronics.howstuffworks.com/everyday-
tech/question1...](http://electronics.howstuffworks.com/everyday-
tech/question110.htm)

This is how it was always explained to me. Yes, neutral and ground are the
same thing. Why can't you just connect the case to neutral? Because you
connect it to the ground plug. Huhh???

------
anologwintermut
Can anyone confirm the articles assertion that " 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?"

It seems that if I stick my finger in the hot socket, there is still a path
via me to 0 voltage (the ground). I will still get shocked.

~~~
DanBC2
> _It seems that if I stick my finger in the hot socket, there is still a path
> via me to 0 voltage (the ground)_

Over-simplified, but you need a complete circuit. In your example you say that
voltage is across you - live at a finger and earth at your feet. But earth is
not connected to the circuit, so there's no complete path. That's why the
article talks about sticking rods into the Earth to create a ground
connection. Now there is a complete circuit. (And this would be across your
heart - the current flows through a finger, your arm, your heart, down your
leg, and to Earth. This is especially dangerous, and is the reason you see
test engineers working with one hand in their pocket. Not having two hands
connected to live equipment means it's harder for a voltage across the heart.)

There's another thing to mention though: Imagine wearing rubber boots, on a
dry wooden floor, and jabbing a metal rod into the socket. That's still risky,
because your body will act like a capacitor and the charge can be fatal.

------
bpowah
Wonderfully entertaining to read. I was once told by a certified electrician
(and physicist) that you don't even need a neutral wire and many rural
services only have one "hot wire" and use the earth as the neutral "return"
circuit (which really just need to have capacitance. I think the author could
have started with a "one slot" age. More info:
<http://en.wikipedia.org/wiki/Single-wire_earth_return>

~~~
cynicalkane
You still need two slots, unless all of your appliances happen to rest on
copper stakes hammered into the ground.

~~~
bpowah
or have some capacitance of their own.

------
bartwe
Why don't Schuko plugs have this orientational bias ?

~~~
bmelton
Schuko plugs expose some material on the sides to ground to their chassis. I
personally think they're the better design, but if you look at this picture:
[http://upload.wikimedia.org/wikipedia/commons/2/23/Schuko_pl...](http://upload.wikimedia.org/wikipedia/commons/2/23/Schuko_plug_and_socket_annotated.png)

Number '2' is grooved out, and there is a receiving prong that sticks out to
make contact to the groove-exposed metal, similar to how the contact chargers
you're probably used to seeing on handheld phones (not cellular, but cell
batteries are often similar) -- The important thing to note is that length-
wise, 2 comes in contact to ground before either hot or neutral wires do
(length-wise), so it's safer on paper than the traditional design.

~~~
kalleboo
That solves a different problem though - the problem of touching the prongs by
mistake when fumbling with a plug.

I imagine there's no polarity in european plugs because the "use a third lead
for grounding" solution was chosen right away instead of trying to patch
things with forced polarity first (possibly with the prior knowledge of
incidents from the U.S. - kind of like how PAL could avoid the issues inherent
in NTSC)

edit: looks like my assumptions may be wrong - the Schuko patent predates the
grounded NEMA plug patent by 2 years. But who knows how ideas like this spread
back then... This is the kind of history I'd love to read a book on.

~~~
hollerith
>I imagine there's no polarity in european plugs because the "use a third lead
for grounding" solution was chosen right away instead of trying to patch
things with forced polarity first

I remember when polarized plugs started to appear on appliances in the U.S.
(probably in the 1970s) and by then the third lead was many years old.

------
tjic
> Another problem springs up. At present, some appliance manufacturers
> INTENTIONALLY connect the outside of their metal products to one of the
> power wires. This must be stopped.

I disagree with this.

If one side is tied to ground (and it is), then tying the chassis of a piece
of equipment to that same line is the same as tying it to ground...a ground
that you're already standing on.

Am I missing something?

~~~
Sidnicious
Keep reading. At that point outlets aren’t polarized and the case might end up
connected either to ground or to the hot wire depending on how you plug it in.
Even when outlets are polarized…

> “Occasionally an electrician will accidentally wire an outlet backwards.
> This can't be helped, because Perfect Electricians are far more expensive
> than the normal human variety. And so we cannot intentionally wire appliance
> cases to the Wide Prong of the plug, since it would cause a lethal hazard if
> the appliance was plugged into a miswired wall outlet. Miswired outlets look
> exactly the same as the normal ones.

> “The solution? Why, add a Third Prong! Connect this prong to the neutral
> side of the network, but do it only in one place in the circuit, and run a
> new third wire out to all of the wall-outlets. Give this wire a new color,
> one which is different from the other two. Give this this third prong a very
> different shape as well, so even Highly Imperfect Electricians will rarely
> connect the special prong to the wrong wire. Inside metal-cased appliances,
> insist that manufacturers connect this third wire to the case.”

~~~
excuse-me
And if there is any current flowing in the circuit and you have a finite
resistance in the neutral wire there will be a (small) potential difference
between the case and ground.

------
jrockway
Do we need grounded sockets now that we have GFCIs?

~~~
keenerd
Do you absolutely 100% trust the GFCI breaker? They do require a small
imbalance to trip, so something bad could happen in that brief time. Or maybe
stopped working one day.

But technically yes, ungrounded GFCI will still protect you. Doing it
intentionally is a bit like not buckling up because you have 100% faith in
your air bags.

For the non electrical people: GFCI works by measuring the current in vs the
current out. Normally these should be perfectly equal. But if you drop your
hairdryer in the bathtub, maybe 10 amps comes in (through the plug), 9.9 amps
goes out (through the plug) and 0.1 amps goes out (through the bathtub,
killing you). GFCI notices that 10 != 9.9 and shuts down. The trick is
detecting 0.01% differences and shutting it down in milliseconds, fast enough
to save someone's life.

GCFI is way more complicated than a ground wire, and you'd be a fool to not
have a cheap, passive and redundant safety system.

~~~
ars
Actually, it's totally legal. If you have an outlet without a ground wire, you
can use a three prong GFCI and put a sticker on it that says "No Equipment
Ground" and it's totally legal and to code.

You are over exaggerating how useful the ground is - a GFCI is far more
effective, the ground pin hardly does anything at all.

~~~
keenerd
While I never said it was not legal (though I may have implied as such
considering seat belt laws), code does require properly grounded outlets in
bathrooms and other high risk areas.

------
ajb
This is a great article.

As an example of innovation in this space, some Chinese company has devices a
socket which accepts many international plugs:
<http://www.bbc.co.uk/news/magazine-18266022> This appears to be the company:
<http://www.europlugs.com/>

It's not clear to me that they have managed to fulfil all the safety
requirements. I guess it would take some effort to really be sure, but can
anyone see any obvious bloopers?

------
CUR10US
I love the amasci website (I don't mean the design; I have no idea about what
it looks like in a graphical browser; I only mean what there is to read- lots
of text! I just extract the text for viewing on 80x25). If only all websites
could have this much information density. Long live sites like amasci!

------
no_more_death
I recall moving a water dispenser that leaked, as a child, stepping on the wet
spot barefoot, and plugging in a two-pronged vacuum cleaner. It was a shock.

------
toddh
If there's so much DC energy available can we store it?

------
BigTuna
That site is an absolute gold mine.

------
iveney
Clears the mystery :)

~~~
xd
Yeah, I dug this up when wondering why it mattered that the sensor for the
wireless power usage meter I have could be clamped to either live or neutral
but was told by the building manager that it had to go to live; yet I wasn't
convinced.

Anyway, as you say .. mystery now clear.

------
gcb
heh, in brazil they just finished a mandatory replacement of all the plugs.

previously we had a plug with a flat face like the US, but the connectors were
like "d b" meaning a US flat in the middle with round ones right out. using
the same metal connector for both formats in the middle. clever, cheap, worked
with everything. (US, Japan, Swiz, ...all except UK and australia as far as I
know)

now, they have mandatory round plugs only, slightly larger so they can't fit
the old ones by one or half milimiter, and a recessed insert like the new
Switzerland type J, and an optional ground.

It serve the purpose of solving NONE of the mentioned issues in the article,
but help lots of people that manufacture the new plugs.

~~~
ricardobeat
You could be a little more proud of your country - it's among the first to
implement an international standard that is safer, simpler and more reliable.
The new plug solves real problems (including all those mentioned in the
article) and it does fit the old european-style outlets when groundless - if
it has a ground pin or is the 20A variant it should not be plugged into a
standard outlet anyway.

~~~
gcb
Remember that polarized plugs are not to protect against properly designed
products. And with the vast amount of international electronics in Brazil
(heck, i think i had only a electric shaver that had national plugs,
everything else required adapters) the cheap adapters null all the benefits.

