| What about touching the circuit will standing on damp concrete on a hot day
| (sweat) and touching the wire. At 120 you might "feel a tingle." At 240,
| if your are lucky, you are knocked on your ass.
Why would it be any different? Both have a 120 volt potential between the
wire and ground. The center tapped transformer sees to that. You get 120
volts and 240 volts from that very same wire, depending on where the other
connection is made.
Or are you referring to the system in Europe and much of Asia where the
ground potential is from 220 to 240 volts?
Or are you referring to the 277 volt circuits in industrial locations in
the US (347 in Canada)?
| Well, electricians who work with more than 150 volts to ground have to be a
| LOT more careful: not only for thier own protection but becuse of the
| spectacular effects of the higher voltage faults. This is both a practical
| and a safety issue. That's why control circuits are usually 120 or 24:
| mistakes just don't do as much damage.
At a given kVA capacity, a transformer with 2 times the voltage will have
only 1/2 the current, and thus 4 times the impedance. That will result in
only half the available fault current if the transformer is the bulk of
the impedance of the circuit.
However, one usually sees higher voltage used where substantially more
power is needed, and so you will see more spectacle if there is a fault.
But that's not because of the voltage specifically, but because of the
I believe control circuits are usually 120 or 24 volts because it is
more practical to build them for a market with a variety of voltages.
If you had control devices for 480 as well as 240 and 120, then things
just get more expensive with too many different versions of things.
| Yes, but a car hitting you at 25 will not kill you (most of the time) and
| that's why urban speed limits are kept that low on many city streets.
|> It's thinking like that that explains why OSHA has seen a disturbing
|> increase in electrical *fatalities* of experienced electrical workers that
|> are working on *low* voltage systems. The *experienced* workers figure
|> lower voltage isn't as dangerous and end up dead.
| Accidents happen. But, at the expense of an occasional mistake,
| electricians can work quickly and efficiently around 120-0-120 systems.
| Much equipment is rated for less than 150 volts above ground. That's why
| 120-0-120 is a GOOD system.
But the 120-0-120 system presents the SAME hazard whether one touches the
black wire OR the red wire. And that's the same whether those wires are
the ones going to a 240 volt air conditioner, or to a 120 light fixture.
But also, for the reason 120-0-120 is a good system, 60-0-60 is a better
system, if your 120 volt equipment doesn't assume one of the hot wires is
a grounded neutral wire. With 60-0-60, touch either the black wire or the
red wire and you only get slapped with 60 volts, versus being smacked with
120 volts if you touch the black wire or the red wire of 120-0-120, or
being whacked with 277 volts if you touch the brown wire or the orange
wire or the yellow wire of 480Y/277.
Or what about 30-0-30?
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
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