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'[OT] The third prong in the plug'
2006\07\29@070438 by Lindy Mayfield

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I ordered something from the UK and it came with that strange three-pronged plug and I needed an adapter so I asked this British guy in a pub where I could buy one here in Helsinki.

He asked me, "Do you know what the third prong is for?"  I said, "Yes, it's the ground."

And he went on to explain that the plugs have fuses that keep you from being electrocuted.  He said that there is a fuse in there.

Now I thought he third plug was in essence a fast path to ground so that in case of electrocution instead of the electricity finding a path through me to the ground it would go through the third plug to a ground with less resistance, and I mightn't die as easily.

Who was right, me or the guy in the pub?

-Lindy

2006\07\29@073025 by Philip Pemberton

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Lindy Mayfield wrote:
> And he went on to explain that the plugs have fuses that keep you from being electrocuted.  He said that there is a fuse in there.

Actually, the fuse is there to protect against short-circuits in the wiring.
It'll do precisely nothing if you accidentally put yourself across the live
and neutral (um.. phase and return?) lines.

> Now I thought he third plug was in essence a fast path to ground so that in case of electrocution instead of the electricity finding a path through me to the ground it would go through the third plug to a ground with less resistance, and I mightn't die as easily.

The idea is that with metal cased equipment, the case is grounded. That way if
something goes wrong and the case ends up live, the current will get fed to
ground. Usually you'd have a circuit breaker in the consumer unit (fuse box)
that was designed to pick up the earth leakage and cut off the power.

It's a safety thing.. If the case is plastic, there's no real need for a
ground, so it's usually omitted in that case. But with metal-cased kit, it's
essential.

--
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2006\07\29@074354 by Spehro Pefhany

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At 01:04 PM 7/29/2006 +0200, you wrote:
>I ordered something from the UK and it came with that strange
>three-pronged plug and I needed an adapter so I asked this British guy in
>a pub where I could buy one here in Helsinki.
>
>He asked me, "Do you know what the third prong is for?"  I said, "Yes,
>it's the ground."
>
>And he went on to explain that the plugs have fuses that keep you from
>being electrocuted.  He said that there is a fuse in there.
>
>Now I thought he third plug was in essence a fast path to ground so that
>in case of electrocution instead of the electricity finding a path through
>me to the ground it would go through the third plug to a ground with less
>resistance, and I mightn't die as easily.
>
>Who was right, me or the guy in the pub?
>
>-Lindy

Both of you, sort of.

The ground is for protection against shock or electrocution, by shunting
any leakage currents to earth. Green/yellow striped wire "PE" (protective
earth). It should not carry any appreciable current in normal operation, and
should be essentially at the same voltage as the neutral (blue in the UK).
If there is "double insulation", the third pin may be made of non-conducive
plastic, and just serves to polarize the plug.

The fuse is to protect the power cord etc. from excessive current (typically
ring circuits might be fused at 30A or more in the UK), and has
nothing directly to do with the ground connection. It's in series with
the "hot" side of the mains voltage. Otherwise the line cords might have
to be good for something like 35A rather than a few A which most devices
require (5A is 1.1kW@230VAC resistive), and would be huge and unwieldy.

If there is an extremely high ground fault current, the fuse will blow,
protecting the circuit *and* ensuring that the ground connection does not
fail open first, potentially leaving the device electrically "hot", and
inviting shock, death etc., so there is a shock-safety side to the fuse.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
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2006\07\29@121500 by Peter

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On Sat, 29 Jul 2006, Lindy Mayfield wrote:

> Who was right, me or the guy in the pub?

Both. The brit plugs have both a fuse and a ground prong. You can
replace it with a Schuco plug, European style. The fuse is necessary
because the brits (used to) wire all the plugs in the house to one main
fuse. That provided enough current to start a fire with the power cord,
had it been unfused.

Peter

2006\07\29@143007 by Gerhard Fiedler

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Peter wrote:

> Both. The brit plugs have both a fuse and a ground prong. You can
> replace it with a Schuco plug, European style.

FWIW, that's "Schuko", as in "Schutzkontakt", something like "protective
contact". It's originally German; I'm not sure the other Europeans (or
which of them) would consider it European-style.

http://en.wikipedia.org/wiki/Schuko

Gerhard

2006\07\29@201701 by Hector Martin [PIClist] n/a

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Gerhard Fiedler wrote:
> Peter wrote:
>
>> Both. The brit plugs have both a fuse and a ground prong. You can
>> replace it with a Schuco plug, European style.
>
> FWIW, that's "Schuko", as in "Schutzkontakt", something like "protective
> contact". It's originally German; I'm not sure the other Europeans (or
> which of them) would consider it European-style.
>
> http://en.wikipedia.org/wiki/Schuko
>
> Gerhard
>
Lots of europe uses it now though, including here (Spain). However, the
"europlug" is more universal, which is a semi-standard type with no
earth connection that can mate with Schuko and all other european
sockets, except British. Here in Spain it's common to see both types of
sockets. Earth-requiring appliances use Schuko, which are usually
mounted straight on the walls at a lower level. Some smaller sockets
designed for smaller utilities like lights and chargers are plain
Europlug, no earth, although many of them also accept Schuko (without
connecting Earth, which isn't very safe). This usually happens if the
Europlug sockets aren't recessed (if they are, Schuko won't fit). On
power strips you can many times see both types, with three or four
Schuko plugs and four interspaced recessed Europlugs. The smaller size
of the europlug means many times you can fit a wall wart on the Schuko
and still plug in a cord on the Europlug next to it.

http://en.wikipedia.org/wiki/Europlug

--
Hector Martin (hectorspamKILLspammarcansoft.com)
Public Key: http://www.marcansoft.com/hector.asc


'[OT] The third prong in the plug'
2006\08\01@081024 by Lindy Mayfield
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Thanks all for the sanity check.  By the way, my three prong wall wart that I received had a third prong made of hard plastic.  

Go figure.

{Original Message removed}

2006\08\01@084006 by Alan B. Pearce

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>By the way, my three prong wall wart that I received
>had a third prong made of hard plastic.
>
>Go figure.

That is required on the UK plugs to move the shield in the wall socket that
is designed to stop baby fingers poking things in. The earth pin part of the
shield is designed that it is needed to move the portion over the other
holes, so any double insulated item such as a wall wart need a dummy earth
pin to do this.

2006\08\01@111046 by Howard Winter

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Lindy,

On Tue, 1 Aug 2006 14:10:22 +0200, Lindy Mayfield wrote:

> Thanks all for the sanity check.  By the way, my three prong wall wart that I received had a third prong made of hard plastic.  
>
> Go figure.

What Alan said about the shuttered sockets, plus it polarises the plug so that the live/neutral aren't reversed.  This is important
because with the plugtop fuse in the live line, an overcurrent fault which blows the fuse would leave the live connected (which could have
fatal consequences) if the wires were connected wrong-way-round.

The way our power system is set up is very much an integrated design, and some things have to be the way they are to make sure they don't
compromise something else (eg. you can have thin flex supplying things like radios, table lamps, and so on, because the small-value fuse
in the plug protects it without it having to take the full fault-current needed to trip the breaker, often 32A).

Cheers,


Howard Winter
St.Albans, England


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