>> >First, to get common names:
>> >Hot = one Phase
>> >Neutral = Protective Ground
>> >Ground = Ground (the starpoint on 3 phases connected to earth at the
>> 'generator')
>
>> That explains the confusion. In the US:
>> Hot is one phase and in single phase systems you can and do have 2 hots.
>> Neutral is what you are refering to as ground. The starpoint for both
>> single and 3 phase.
>> Ground is the protective / frame ground.
Yes. In the UK, "Hot" is usually referred to as "Live", and until 1992 this was
the official term. In 1992 the term was changed to "Line", so that the phrase
"Live parts" could be introduced to mean any conductor connected to the
electrical supply (i.e. not including the protective ground, which is called
"Earth")
>> >Second, do I understand that US appliances got fuses on both wires
>> >and both are switched ?
>> On 240 volt equipment yes.
>
> Ahh ja - we also use fuses in each 'Hot' wire - so if you have a
> 3~ installation, 3 fuses are to be installed _and_ they have to
> be tied (mechanical), so if one blows all 3 will go off.
In the UK it is similar, although I don't think there is a requirement for fuses
to break all three phases for a fault on one. Since the US has many 240V
centre-tapped-to-ground installations, 240V would have fuses in both lines (both
hot) but none in neutral. ISTR the neutral wire in some 240V equipment (cookers
are an example that springs to mind) is used instead of protective ground, which
makes me a little uneasy.
>> >Over here in Germany only one wire is
>> >switched and protected with a fuse, althrough it is not defined
>> >which one is the 'hot' line, since our connector is symetrical
>> >- there is no way to insert the plug in a way where the Hot and
>> >the Ground side is defined - Our plug isn't inherited from the
>> >DC times, but rather a new design when 220V AC became standard
>> >in the 30s, to fit the needs of (and only of) AC Eq.
Hans, I think you've gone some way to answering your own question here. See
below.
>> The US did not used to make a distinction between hot and neutral until
>> about 30 years ago when they added the "protective" ground pin. Now they
>> are keyed. If you look closely at a receptacle, one blade is wider -
>> neutral, The narrower blade is Hot and the round pin is protective ground.
>> Some "equipment" uses only 2 wires. The blades and often one is larger to
>> assure that it is polarized correctly. These polarized plugs have come into
>> common usage in the last 20 years.
Ah. That explains some sockets that I saw when over there last year.
> Strange, but common - I don't know any reason why the system has
> to be keyed, I always assumed the US development of keyed plugs
> was triggered by old DC systems (since in DC Eq. it is necersarry
> to know the orientation), but not a new addition. Strange, we
> dropped all orientationwhen switching to AC (with Protective
> Ground). Could anyone give give a reason why the system has
> to be keyed _IF_ a Protective Ground is included (other than
> define the Protective Ground pin) ? Because, without Protective
> Ground a definition of Neutral is essential, but with, no longer.
Because the fuse and/or switch _in the equipment_ has to be on the hot/live side
of the supply. Otherwise a blown fuse - because of a short circuit - could
leave the equipment with the hot supply still connected to whatever shorted out.
Which could be the chassis...
Also, at least in the UK, some old equipment had neutral solidly bonded to the
chassis and no protective ground. This is common on valved (tubed) radios and
things. ISTR that here the fuse was generally on the chassis side, so that when
the fuse blew, the chassis would no longer be connected. Still not really safe,
since the chassis would be connected to the other side via the heaters of all
the valves, and these typically pass 100mA to 300mA at mains voltage, or more if
they're cold.
Philip.
Received on Tue Apr 20 1999 - 12:17:14 BST
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