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From: rex@cs.su.oz (Rex Di Bona)
Newsgroups: sci.electronics
Subject: Re: RMS versus pk-pk (was Re: polarity)
Message-ID: <2399@cluster.cs.su.oz.au>
Date: 8 May 91 13:17:05 GMT
References: <11864@jarthur.Claremont.EDU> <12900002@hpavla.avo.hp.com>
Sender: news@cluster.cs.su.oz.au
Reply-To: rex@cluster.cs.su.oz (Rex Di Bona)
Organization: Basser Dept of Computer Science, University of Sydney, Australia
Lines: 99

In article <12900002@hpavla.avo.hp.com> devlin@hpavla.avo.hp.com (Lee Devlin) writes:
> 
> Cliff Stein writes:
> >
> >What's the purpose of polarized wall outlets with an AC
> >signal?  I don't understand.   Is it really bad to force
> >something to plug in the wrong way?
> 
> >I am serious.  With a real AC signal (sinusoidal waves) I can see no reason
> >at all for it.  How can the electrical equipment tell?

> I came to the conclusion that they are trying to keep the internal fuse
> in the hot line so that, if it blows when there's a short against the
> enclosure, the enclosure won't be sitting at line voltage.  This would be
> the case if the internal fuse were in the neutral line.  That is the only
> reason I can see for using 2-prong polarized plugs.  For this scheme to
> work, the electrician has to have wired the house correctly (left = line,
> right = neutral).
> 
> Lee Devlin

The three wires in an AC General Purpose Outlet (those wall sockets)
are the active (red or brown), neutral (light blue or black), and
earth (green/yellow). Under IEC and CEE regulations yellow/green is
ALWAYS earth, and if there is a neutral it is light blue, and the rest
are available for actives. It is the active conductor that has the current
'flowing' in it, and your appliance uses this by letting the current return
via the neutral conductor. The neutral conductor will provide no
current of its own, and is usually joined to earth. If you only have two
pins in the socket they will be active and neutral (in Australia the
active is on the left, when looking from the plug side).

Now, if a break down occurs it will occur in one of three ways, either
(1) the circuit is broken, ie, a conductor breaks, or (2) part of the
circuit shorts out and extra current flows (to either neutral or earth),
or finally, (3) metallic (well, usually metallic) parts are brought
into contact with one of the conductors, and are accessible to the
user.

If 1 occurs then there is no problem. The current stops flowing, and
you fix the device.  If 2 occurs then the protective device for the
house supply should cut off the current as excessive current is being
drawn, and the conductors may overheat and ignite.  If 3 occurs then
there is a dangerous situation, no apparent fault exists, but current
may be conducted if either a source (unlikely) or sink (very likely, ie
you) comes into contact with the device. This moves the fault into
category 2 and we hope the house device activates.

Now, it is totally possible that the house protective device can supply
a LOT of current. Circuit breakers that allow 30A before breaking are
common, so you can put 240V into anything above 8 ohms and not reach
that 30 amps. (actually thats' RMS, and the peak is higher, giving a
max resistance of about 11 ohms before the breaker will trip). Now, a
person is usually higher than 12 ohms, so an additional protective
device is added to the circuit. This protective device must go into the
active conductor to be effective, ie, the current that is killing you
is going active->device->you->earth and nowhere near the neutral conductor.

This is why the plugs are polarised. To ensure that the additional
protective device is in the right conductor.

Now, I would hope that a metallic case would be earthed. Only with
doubly earthed devices should there be no earth wire. Good audio
equipment is, however, usually double insulated to reduce the hum
formed by earth loops.

In Australia the installation of electric wiring is controlled by a
specified standard, AS3000-1986 SAA WIRING RULES.

0.5.58 Insulation, double:
    A live part shall be regarded as separated from non-current-carrying
    metal parts by double insulation only if two layers of insulating
    material complying with the appropriate requirements of AS 3100
    intervene between the live part and such metal parts.
    
    The layer of insulation adjacent to live parts is referred to as
    'functional insulation' the second layer is referred to as 'protective
    insulation'.  (The symbol for double insulation is a square within a
    square)

5.3.8.1 Double insulation: general.
    Equipment complying with the requirements of AS 3100 for double
    insulation need not be earthed.

5.3.8.3 Internal equipment wiring.
    Conductors within equipment having double insulation shall be protected,
    secured, or insulated so that, if any one conductor becomes detached
    from its termination, neither the conductor nor its functional
    insulation can come into contact with accessible metal. The attachment
    of one conductor to another by tying, lacing, clipping or the like, in
    such a manner as to prevent either conductor coming into contact with
    accessible metal if it becomes detached from its termination, shall be
    deemed to comply with this requirement.

This means that we don't want to allow (3) to happen, and so you will
also see no fuse on a double insulated device.
--------
Rex di Bona (rex@cs.su.oz.au)
Penguin Lust is NOT immoral