Text 9985, 101 rader
Skriven 2010-08-17 01:59:37 av Michiel van der Vlist (2:280/5555)
   Kommentar till text 9981 av Scott Little (3:712/848)
Ärende: Just Curious
====================
Hello Scott,

On Monday August 16 2010 09:51, you wrote to me:

 MV>> The ungrounded socket will accept grounded and ungrounded plugs.
 MV>> A situation that is no longer considered safe. One of the
 MV>> reasons, the ungrounded sockets are phased out. In Germany they
 MV>> were never in use after the mid fifties.

 SL> Who was the genius that put the ground pins on the outside?!

I do not know it probably was a German.

It might have been to do with making the grounded sockets not accept an
ungrounded plug, but not vice versa. A situation diametrically opposed to most
countries where an grounded socket will accept an ungrounded plug but not (if
ungrounded sockets exist) vice versa.

The reasoning stems from the first half of the previous century when houses had
wooden floors and 90% of the electrical installation in an average house were
light fixtures. And perhaps a washing machine in the kitchen. The reasoning was
as follows:

1) It must take two faults to create a dangerous situation.

2) There are wet and dry spaces.

3) Most spaces such as living rooms and bedrooms, qualify as dry spaces. In dry
spaces there is nothing a user can touch that will bring him into contact with
ground. So if an applience has a metal encasing and the insulation fails and
the casing comes into contact with a live wire, nothing much happens, as there
is no grounded metal around that the user can touch. Only if a second applience
also has an insulation faulðt and that fault connects the casing to the null,
will a shock hazard exist. So in dry rooms, there is no need for grounding, and
hence no need for grounded sockets. Appliences, destined for use in dry spaces,
have an ungrounded plug, even if they have a metal encasing. They are also
called class zero appliences.

4) In wet rooms, such as kitchens were there is grounded metal (water taps)
that can be touched or where a wet concrete floor can bring the user into
contact with ground, all equipment must be grounded. All equipment (also
called, class one equipment) must have a grounded plug and all sockets must be
grounded. So that in case the insulation fails, and a live wire comes into
contact with the metal encasing, the fuse blows. Again it takes two faults to
create a situation with shock hazard, The insulatiom must fail AND (the ground
connection must be broken OR the fuse must fail)

5) Class zero equipment must never be used in a wett (class one) area as that
would create a situation where  a single fault (insulation failure) leads to a
shock hazard situation. To prevent class 0 equipment being used in a class one
environmemt, the class zero (ungrounded) plug does not fit into a class one
(grounded) socket.

6) The reverse however is no problem. When a class one piece of equipment is
moved into a dry area (class 0), plugging it in an ungrounded socket should be
no problem, as a class one piece of equipment with an unconnected ground is
electrically equivalent to a class zero (ungrounded) piece of equipment. It
still takes two faults to create a shock hazard situation. So a grounded plug
fits into an ungrounded socket.

7) It was allowed to place grounded sockets in dry areas. However the two types
may not be mixed. If one socket is grounded, all the sockets in the same room
must be grounded. Otherwise the "two faults" rule would be broken.



This is all history, The philosophy folded with the coming of central heating.
From then on there was grounded metal in every room. But the "two fault"
requirement was saved by the invention of the earth leakage switch. Equipment
behind an earth leakage switch could still be used in a central heated dry room
without shock hazard. It would take both the insulation and the earth leakage
switch to fail to create a lethal situation.

In the meantime the "two fault" rule has been replaced by the "three fault
rule" for all new domestic installations and old installation that undergo
major changes. Evertything must be behind an earth leakage switch and all
sockets must be grounded. Ungrounded equipment must be class II, or so called
double insulated.  So for a shock hazard situation to occur now, three thing
must fail.

     The earth leakage switch must fail AND
     (
        ( The insulation must fail AND
             ( the grounding must fail OR the fuse must fail
             )
        )
        OR
        ( Both layers of the double insulation must fail
        )
     )


That is the philosophy now but the ungrounded, class zero, sockets are still
around and there is also some class zero equipment around. It may take another
30 years to fully disappear.


Cheers, Michiel

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