Why have an earth when neutral's the same?

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robert.a.hunt

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Dec 31, 2000, 7:08:23 PM12/31/00
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Just wondering if anyone knows why we need an earth terminal in house
mains - why not just use neutral as a protective conductor, seeing as it's
the same conductor anyway!


Sinikka ja Seppo Sauna-aho

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Jan 1, 2001, 7:25:46 AM1/1/01
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Having separate neutral (N) and protective earth
(PE) offers more safety and less disturbances.
This system is called TN-S. A system with common
PE&N = PEN conductor is called TN-C.

TN-S allows the use of a GFCI (Ground Fault
Circuit Interrupter) in the panel. Other names
for a GFCI are GFI (Ground Fault Interrupter), RCD
(Residual Current Device), earth leakage unit
etc.

In a TN-C system you have a common PEN conductor
carrying the load current. That current causes a
small voltage drop in the PEN conductor which
makes potential differences between the housings
of e.g. a PC and a peripheral device. That may
generate relative high currents in the signal
cable between these devices.

Seppo

Randle Bain

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Jan 1, 2001, 10:04:20 AM1/1/01
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> Just wondering if anyone knows why we need an earth terminal in house
> mains - why not just use neutral as a protective conductor, seeing as it's
> the same conductor anyway!

Robert,

You question is very logical and shows that you have given the matter some
thought. First a couple of definitions. The National Electrical Code
defines neutral and ground the following ways ( I paraphrase a bit for
clarity)

Ground doe not normally carry current. It is wired to the case of the
device.

Neutral is a current carrying conductor. It provides the return path for
120 volt devises.

Lets say you have an old metal framed drill motor operating on 120VAC. The
plug has three wires, hot, neutral and ground. The hot wire is run through
the trigger switch and on to the motor. The neutral wired is wired to the
other side of the motor. The ground is wired to the metal case.

In normal operation the operator grabs the metal case. By doing so he makes
a connection through his body to ground. He pulls the trigger supplying
electricity to the motor, the motor turns and the return current goes back
through neutral to the breaker box. Ground has almost no current. All is
right with the world.

Lets say something goes wrong in the drill motor. A wire comes loose, the
motor shorts to ground, whatever. It is possible to have the metal case
become energized. Since the user is holding this case he could become a
conductor of live current. A bad scenario. The ground will provide a path
for the current. The ground path will likely be of much less resistance
than the path through the user. Most of the current will travel to ground.

The new (shows how old I am) double insulated devices often have no third
prong on the plug. These devices are designed in a way as to prevent the
case from ever being energized. There are many ways to do this, once is by
using an insulator (often some form of plastic) as a case. The intent is to
by design prevent the user from becoming a path to ground.

Finally how ground fault protection works. The idea is that if the ground
wire gets current above a certain level the breaker trips out. It is often
thought that if anything goes wrong in a device the molded circuit breaker
will trip off, protecting the user. There are a couple of problems with
this line of thought.

One, the breaker is sized to protect the wire it feeds not the device
connected to it or the person using it. It is designed to operate when
current gets above a certain level. If current remains below this preset
level it will never trip.

Two. It is possible to loose neutral (the return path) but not have a short
circuit. Lets use the drill for example. The neutral comes loose from the
cord and lays against the metal case. The user is now the conductor to
ground. The user experiences a shock, but the circuit break back at the
panel only sees normal operating current of the drill so it does not trip.

A ground fault breaker looks at the world differently. The amount of
current in the hot wire is always equal to the amount of current in the
neutral (under normal conditions). Most ground fault breakers found in the
home look at the difference between the current in the hot wire and the
current in the neutral. Once a difference occurs the breaker trips. This
breaker can be designed to operate on very small amounts of differential
current. These devices are designed to protect the user.

I hope my long winded explanation helped clear things up.

Randle Bain
rtb...@flash.net


Chris

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Jan 3, 2001, 3:59:51 PM1/3/01
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Randle Bain <rtb...@flash.net> wrote in message
news:U%046.32481$bw.20...@news.flash.net...

> > Just wondering if anyone knows why we need an earth terminal in house
> > mains - why not just use neutral as a protective conductor, seeing as
it's
> > the same conductor anyway!
>
> Robert,
>
> You question is very logical and shows that you have given the matter some
> thought. First a couple of definitions. The National Electrical Code
> defines neutral and ground the following ways ( I paraphrase a bit for
> clarity) --Large snip--
>
>
> Randle Bain
> rtb...@flash.net

A great exlpanation, thank you,

Erm, Which National electrical code?? (i.e. Which Counrty)

Chris


davep

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Jan 3, 2001, 8:00:03 PM1/3/01
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Chris wrote:

> Randle Bain <rtb...@flash.net> wrote in message
> news:U%046.32481$bw.20...@news.flash.net...
> > > Just wondering if anyone knows why we need an earth terminal in house
> > > mains - why not just use neutral as a protective conductor, seeing as
>> it's the same conductor anyway!

> > Robert,

> > You question is very logical and shows that you have given the matter some
> > thought. First a couple of definitions. The National Electrical Code
> > defines neutral and ground the following ways ( I paraphrase a bit for
> > clarity) --Large snip--

> A great explanation, thank you,



> Erm, Which National electrical code?? (i.e. Which Counrty)

A reasonable question, on a world wide group.
The phrasing, etc, suggest the US NEC, which is called the National
Electrical Code.

best
dwp
> Chris

Randle Bain

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Jan 4, 2001, 5:33:24 AM1/4/01
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> A great exlpanation, thank you,
>
> Erm, Which National electrical code?? (i.e. Which Counrty)

Oops! My apologies. The United States National Electric Code. And please
remember that I did not give the exact definition as listed in the code. I
paraphrased some to allow non-code readers to be able to understand. The US
NEC gets a bit wordy and difficult to understand at times.

Randle Bain
rtb...@flash.net

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