First, I assume we would always call the local utility company and
ask them if the lines are service drops if we don't know how to
identify them.
> OK, I've watched this thread long enuf.
> 1st my qualifications: 21yrs. with the Board of Public Utilities in Kansas
> City, Kansas.
> One rule to remember: ALL LINES CAN BE DEADLY!!!
That rule includes extension cords, and the cords on our electric
drills, saws, razors, VCR's and TV sets. That's true with almost
ANYTHING in life.
'> Now, lets assume the triplex (or concentric neutral line, or open
wire
> service, whichever it maybe be), is fed from a 37.5 KVA transformer. A
> single 120 leg of the feed is capable of 312.5 AMPS and NOT exceed the
> capabilities of the xfrmr. That is NORMAL FULL LOAD. Not a short or low
> impedance route to ground or thru you. Then it is ALMOST unlimited in
> amperage.
While this is true for direct connections from any conductor to the
other conductors in the triplex, it is not true for paths through a
person or other ground paths. Current is limited by the resistance
of the path and the voltage driving that resistance...not by the
available source current.
That's why primary lines (in the dozens of kilovolt range) are such a
hazard, and we should stay well clear of them. One mistake and
we are toast. Leather gloves and tennis shoes won't hold off 10 kV
peaks or higher. The danger is proportional to voltage, not the
current.
Current can be an issue for fire damage, but not for electrical shock
hazards involved with accidental contact with a line. It takes only
about a dozen milliamperes of well-placed current to kill someone.
Transistor radio batteries can supply more than enough current to
electrocute someone.
Fortunately humans generally offer a resistance high in the kilo-
ohm range to external currents...or the world's population would
substantially reduced. There would have been few hams left alive in
the 60's, when Heathkit DX-100's and almost all other rigs had a
hundred volts across the key leads, and those leads could often
supply a few hundred milliamperes of current.
Here is my puzzlement:
Many of us seem to be saying we can't mount an antenna
anywhere near a normal triplex 240 volt drop, even if reasonable
care is used to be sure nothing contacts that line. It seems that
warning is for shock hazards and fire.
With this in mind, wouldn't it also make sense to remove all line
cords from outlets in our houses? Shouldn't we cover the cords
with some sort of non-flamable conduit? After all, we can get about
20 amperes at 120 volts (2400 watts) from an outlet and often for
extended periods. The normal house has dozens of line cords
running across materials with low ignition temperatures, and even a
hundred watts of power will easily produce temperatures in the
thousands of degrees in a short time in a small physical area. We
have fuel, potential heat, and oxygen and it's all inside our homes
while we are sleeping!
There is many times the lethal current available, and the voltage is
just as high as on the service drop.
Clearly we have a much larger hazard inside our homes than would
be caused by a tower 15 feet from an outside drop, or a guyline
crossing above or below but spaced away from that drop. Certainly
we are much more likely to contact 120 volts plugging in a plug on
our toasters or lamps.
And what about all those times we reach up and put our hands on
a metal housing for a table lamp, with only a few thousands of an
inch of cardboard or plastic to save us? We need to install isolation
transformers on all out table lamps, or at minimum three conductor
cords.
The odds of a lamp tipping and bulb breaking, allowing contact with
the same voltage as the drop, are substantially more than an
ungrounded tower (it would have to be ungrounded or have a poor
ground or the current would bypass us) falling while we are hanging
on the tower barefoot in the rain.
The voltage is the same and current can easily exceed the few
milliamperes required to kill us, so the human hazard with contact
is exactly is the same as with contact to a service drop.
The insulation is thinner and less rugged, so it is easier to damage
the cord or lamp socket. The cord or lamp (or other appliance) is
right where we can touch it. They have no bare ground, so we can
cut the "hot" conductor's insulation without a direct path to ground.
If we are going to bury a less hazardous drop, what should we do
about these more common hazards?
So why not eliminate more likely problems? Why not spend $800
for isolation transformers for our lamps, or install GFI breakers at
every outlet?
73, Tom W8JI
W8JI@contesting.com
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