----------------------------
----------------------------"Gerald Newton" <
[email protected]>
wrote in message
If he isn't using the Code, he could try jacking the voltage up and
down and use the steel wire. Maybe, the electric fence idea would
work. It sure would beat buying enough copper wire to go 3200 feet
with the price of copper being so high.
Cheap fence wire insulators and transformers can be purchased at a
food lot store, the kind farmers use. An ignition transformer might
also work.
A used oil burner ignition transformer could raise the voltage to
10,000 volts then at the other end back feed another transformer to
get 120 volts. Then feed the 120 volts into a common Class 2 50 va
bell transformer to get 12 volts. I have never seen this done, and
do not know if it would work.
At 10,000 volts:
Power at 12 volts:
P= EI =12 x 0.5 = 6 watts
At 10,000 volts:
I = P/E = 6 / 10,000 = 0.0006 amperes
VD = VD=2(95.8)(1640)(0.0006)/16512
VD = 0.01 volts
Percent voltage drop = insignificant
I would place signs around the fence warning people, and tell the
inspector that the fence is electrified to keep the wildlife out.
Electric Fences are not covered by the NEC for obvious reasons. The
NEC has a purpose that is the practical safeguarding of persons and
property from the hazards arising from the use of electricity while an
electric fence has the prupose of shocking. When I was an inspector,
the foreman at the Atigun camp for the Trans Alaska Pipeline repair
job in about 1990 asked me about this. They installed an electric
fence around the camp to keep the grizzlies out and he wanted to know
where the code rules were. There are none! Atigun Pass is in the
Brooks Range in Alaska and is a very beautiful place. It is also the
highest point on the pipeline. I flew in there from Fairbanks in a
Cessna 150 with a bush pilot. He couldn't make it over the pass
because of fog so we landed at Chandalar field and I hitch hiked over
to Atigun. Those were the days, my friend, those were the days.
And what are the voltage regulation and exciting current requirements of
the
two (or 3) (high impedance) transformers needed as well as leakage on the
crappy little fence insulators? They might be such that the scheme still
wouldn't work. In addition, aren't most electric fences pulsed?
At least 24VAC makes sense from a safety point of view.
--
Don Kelly
[email protected]
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24 vac does not make sense because the voltage drop will be too high.
Do the math.
0.5 amperes at 12 volts equals 6 watts.
I for 24 volts is 6/24 = 0.25 amperes
VD=2(95.8)(1640)(0.25)/16512
VD = 4.75 volts
4.75/24 x 100 = 19.8 percent
If an igniton transformer is used the output is not pulsed and losses
are minimum.
I think some electric fence power supplies are electronic and would
not work.
Additonally, 24 volts is not safe in a wet environment. That is why
12 volts is used for bell transformers. Read note 2 to Table 11A and
B in Chapter 9 of the NEC quoted below.
This note is based on finding from the original work by Charles
Dalziel, who, by the way, invented the GFCI.
2. For nonsinusoidal ac, Vmax shall not be greater than 42.4 volts
peak. Where wet contact (immersion not included) is likely to occur,
Class 3 wiring methods shall be used or
Vmax shall not be greater than 15 volts for sinusoidal ac and 21.2
volts peak for nonsinusoidal ac.
-------------------------
Some time ago, I did the math at 12V and I'll generously take your math as
correct even though the numbers that you have used are undefined: 95.8
what? 1640 what? 16512 what? You appear to be using a cookbook expression
that I am not familiar with -without giving the units so I have no way to
check your data/calculation except that the result appears to be reasonable.
Yes the voltage drop is high but I wasn't considering DC and neither were
you. Now using a 24/12V transformer with a 19.2 V input at the receiving end
and converting to DC gives a peak voltage of 13.5V and a fat capacitor will
leave you close enough to 12VDC average. There will be some voltage drop in
the transformer impedances but this should be fairly small as the smallest
12/24 or 120/24 transformer that one can get will be rated at a fair amount
more than 6 Watts. In any case dropping from 19+VAC to 12VDC at 0.5A for DC
is not a big deal, even using a resistor, is actually cheaper than the
warning signs .
If 24V (42.4V peak) is not safe in wet environments and the "safe" limit is
21.2 peak( the peak voltage of a 15V sinusoid)_- then what about 10KV?
Sure- we both know fencing units are can be safe ( not according to the
code that you quote) but that is due to their high impedance and that
impedance along with wet condition leakage can have a considerable effect on
your scenario-which is not according to any code. There is no use in
considering code in one situation and ignoring it in another. If code is to
be ignored- then I would rather go with the 24V setup.
In particular, the 24V can be floating with respect to ground so contact
between both wires is needed for a hazard and this can be limited by some
planning as to which wires are "hot" and which are grounded "shield" wires.
It isn't damnfoolproof but neither are signs- in either case, the hope is
that there are no damnfools around .
I note that you did not consider the impedance of the transformers in your
proposal. It will be high- by design.
Actually, there are other alternatives and these are based on location,
purpose, etc. Paralleling conductors in a 3/4 grouping as suggested by
others will bring the voltage drop down to less than 6% so the problem,
after a 24/12V transformer is too high a voltage at the load.
My point is that 12V wont do the job so what is the lowest standard voltage
that will do the job. 24V appears to be OK.
