[SI-LIST] Re: threshold and 60 Hz
- From: boris.traa@xxxxxxxxxxx
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 1 Aug 2003 10:43:00 +0200
As far as I know DC is more dangerous than AC due to the fact that in case
of DC some unwanted chemical processes will take place in the human body.
In case of AC the polarity is changing all the time so these processes
are probably inverted too each time (I'm not a chemist by the way). It
seems that somewhere between 50 Hz and 60 Hz there is a break even point
above which these processes are reduced significantly. For this reason a/o
the USA seems to have chosen for 60 Hz.
Another interesting 50/60Hz phenomenon is that you can notice this break
even point by watching a TV or a computer monitor and switching the frame
frequency from 50 to 60 Hz. Then you will perceive that the "flicker" will
reduce. At the same time you will observe that the contrast sems to
decrease while this doesn't change when you measure it.
Kind regards
Boris Traa
System design engineer EMC
PDSL/EMC3
Location SFJ 017
PObox 80002, 5600JB
Eindhoven, The Netherlands
Tel: ++ 31 40 27 22249
Fax: ++ 31 40 27 45758
E-mail: boris.traa@xxxxxxxxxxxx Seri: btraa@nlsce1
Wyland <dcwyland@xxxxxxxxxxxxx>
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2003-08-01 02:59 AM
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Subject: [SI-LIST] Re: threshold and 60 Hz
Classification:
A little more trivia. 60 Hz was chosen mostly for motor reasons, perhaps
the most important use of electricity. .
The 60 Hz number yields an 1800 RPM synchronous speed for a 4 pole AC
motor. (60 Hz * 60 seconds/minute * no of poles/2) Higher frequencies
give higher speeds. This speed (actually 1725 under load) is the
standard speed for almost any AC induction motor you find, such as in
washing machines, pumps, etc. 1800 RPM is a good working speed. Most
useful things seem to happen between 120 and 1800 RPM.
60 Hz also yields nominal transformer sizes, in pounds (and dollars) of
iron per volt-ampere. Higher frequencies use less iron. Europe uses 50
Hz. Their motor speed is 1500 RPM nominal, and their transformers are
20% larger than ours for equivalent rating. Parts Canada (and I think
England) used 25 Hz for a while. However, the transormers were large and
the frequency was low enough that you could see the lights flicker -
annoying.
In WW II, airplanes used 400 Hz as their nomnal AC frequency to reduce
the weight of the iron. This would reduce the weight to 60/400 = 15% of
the 60 Hz value. The down side is that the 4 pole motor speed is now
12,000 RPM! Fortunately, the primary use of the AC was for electrical
and electronic equipment, not motors. The only motor use I am aware of
was for small, high speed cooling fans where it could be made to work.
400 Hz would probably not be good for general purpose use on the ground
because you would need a lot of poles on any motor to get the shaft
speed to any reasonable value for most applications. Many poles = higher
cost because of many independent windings to be tied together. Also,
iron hysteresis losses increase with frequency. This would result in
increased losses due to iron heating on all motors and transformers.
So, 60 Hz is a most useful and optimally painful frequency, as would be
predicted by Murphy's Law.
Dave
Jacobson, Karl wrote:
>60 Hz is a particularly dangerous frequency of AC, which is our standard
=
>wall socket value.
>
>Threshold voltages for dry skin vary greatly from wet skin which in turn
=
>is much higher than a direct contact under the skin, as with a needle or
=
>something like that in contact with interstitial or cellular material. =
>The current that can become dangerous is also varies with voltage, and =
>especially location. =20
>
>Skin effect from a lightning strike has been survived many times, you =
>could say.
>
>Karl
>
>-----Original Message-----
>From: Robert Szalapski [mailto:Rob.Szalapski@xxxxxxxxxxxx]
>Sent: Wednesday, July 30, 2003 6:54 AM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: threshold
>
>
>The threshold voltage is very different for AC & DC voltages, as was
>demonstrated by Thomas Edison. He electrocuted many dogs in an effort =
>to
>prove that AC was only good for killing; AC currents are far more lethal
>than DC. (Some claim that these experiments gave birth to the idea of =
>the
>electric chair.) I believe that, for AC, the dangerous level is around
>70-80 V, so typical household wall sockets can be dangerous; 110 V is a
>rather odd choice from that perspective.
>
>-- Rob
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of hariharan
>Sent: Wednesday, July 30, 2003 8:44 AM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] threshold
>
>
>what is the threshold Voltage (V) and current (amps) beyond which human
>beings can get hurt or feel it?
>
>any idea...??
>
>
>
>
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