One frequently hears news accounts of persons electrocuted while
holding something - a ladder for instance - that touches an overhead
electric power line.  Years ago, I attended a safety seminar where it
was stated that the covering of overhead power lines is
weather-proofing, not insulation - hence do not touch power lines with
anything, even if they appear insulated - since they're not insulated.
 Yet as I walk through neighborhoods with overhead lines, I can see
innumerable trees with branches touching power lines, with no apparent
ill effect, i.e., the trees aren't sparking as they conduct
electricity to ground.

How does one reconcile the two observations.  My specific reason for
asking, is that my wife allowed her son (my stepson) to partially take
down a tree where the limbs are close to interfering with the power
lines.  He survived the experience - no shocks, etc.  I'm convinced
that the residual trunk and branches will come in contact with the
residential power lines if he continues.  I'm very willing to pay
someone who knows what they're doing - a tree service - to take down
the tree.  I don't think I have convinced them of the danger.  Or
possibly I overestimate the danger.

So my question is something like - if the limbs of a tree are touching
a power line without obvious problems, is it safe to cut down the tree
with no particular precautions regarding the tree limbs touching the
power line in the process?  If yes, then how does one distinguish the
lines that electrocute, vs. the ones that don't.  (Maybe I don't want
to know this - a little learning is a dangerous thing...)  If no, when
why aren't power lines shorting out everywhere as tree branches touch
them?

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Clarification of Question by aleck-ga on 14 Sep 2005 19:24 PDT

Thank you, formica34-ga, for a very informative and understandable
answer.  I consider my question answered.  If there is any way you can
receive payment for the answer, I'd be pleased for you to do so.

Also thanks to nelson-ga.  I don't think wood conducts very well,
either.  However, I had a tree struck by lightning in my back yard
years ago, and it practically blew apart.  I think the moisture in the
tree, turned almost instantaneously to steam, probably is what
exploded.  I presume it was high resistance (the wood) meeting high
voltage (the lightning strike) resulting in some electric current and
a lot of heat.


Aleck

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Request for Question Clarification by maniac-ga on 30 Sep 2005 13:38 PDT

Hello Aleck,

If you are satisfied with the comments, I suggest you close the
question so you cannot be charged for an answer. If instead, you need
some further references (they don't just believe random people...),
please make a clarification request and one of the researchers would
be glad to answer the question.

As a follow up, I have attended a demonstration of high voltage power
by a local electric company (not SCE) and was extremely impressed by
the damage caused by incidental contact with a power line (in this
case, it was 20 K Volt). For example, a hot dog inside an insulating
glove with a pin hole leak was cooked (in parts exploded) in a second
or less.

If you need references, a quick search of
  power line tree safety
  tree removal site:sce.com (replace sce.com with your utility company)
or similar phrases can find good resources. In some cases, the
electric company will trim and/or remove the tree without charge - SCE
for example has a reimbursement program.

  --Maniac

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Clarification of Question by aleck-ga on 30 Sep 2005 16:58 PDT

Thanks, Maniac-Ga.  All very useful responses.  I'm quite happy with
the comments received.  I'll close the questions.

Aleck

Virtually all distribution-level power lines (i.e. the ones on normal
telephone poles) are insulated.  The big HOWEVER though, is that often
this insulation is decades old, and easily flakes off.  It's not easy
to make insulation that lasts for decades exposed to the sun and
weather.  Although insulation today is pretty good, it wasn't so good
30+ years ago.

Consequently, a power line that originally had good insulation when
installed can be very dangerous.  Just touching or jostling it could
flake off some insulation, creating a path to the condutor.  A tree
limb touching the line may be fine, but when jostled, could flake some
off.

Of course connections to transformers on poles, etc. aren't usually
insulated at all.

The other big variable is the path to ground.  Anything can touch a
live wire if  there is no path to ground (or to another wire).  Birds
and squirrels do it all the time.  However, if the bird has a path to
ground or another line, it completes the circuit and gets fried.  This
is a very unpredicable effect.  A tree limb could be touching a live
line, and conducting some current into the ground continuously.  This
wouldn't necessarily be a short circuit, just lost power into the
ground, depending on soil and tree conductivity, etc.  If a person
touches the tree, they will bleed off some current- the amount depends
on their skin conductivity, their path to ground (e.g. are they
barefoot?), how much moisture the tree contains, etc. etc.  There are
way too many variables to predict what will be safe.

Things get much worse if you cut a branch or tree and it falls on the
line.  You could actually bring the line down if the connection to the
transformer is loose, or flake off insulation from an old line,
creating a much better conductive path to you.

The bottom line is to not mess with it - call in professionals!  In
theory the electric utility is supposed to keep the lines clear - you
might start with them.  They are supposed to perform regular "tree
trimming" to make sure falling branches don't cause outages.  If you
can't get them to do it, call in professionals.

I'm an electrical engineer with some experience in distribution-level
power, and would never try to cut down trees or branches that would
touch a line, even with the protective gear I have.

I don't think wood is a very conductive substance.

No problem - I'm not a google answerer, so I can't be paid for the
comment.  I just wanted to make sure no one got electrocuted!

Clean, dry wood is normally not very conductive, but it depends a
great deal on moisture content, and especially the amount of water or
other material on the surface of the tree.  At 7200 volts and above it
doesn't take much conductivity to draw current, or to start an arc
around the tree directly to you.

I would like to add a clarification to formica34's post.  

All distribution lines 7.2kv, 12kv, 24.5kv etc. are insulated, however
very rarely are the wires themselves insulated.  As you will notice
the connections on most poles will have a polymer or ceramic insulator
that insulates the phase from the structure. The higher the voltage,
the larger the insulator.  Wood has been known as a good insulator in
the utility industry for years and has very good resistive properties.
 Wood poles must run a copper or aluminum lead down the poles to
provide adequate grounding in case of fault currents or lighting
strikes.

I would however agree with formics34's conclusion.  I would contact
the local utility and inform them of the problem.  While wood is a
good insulator, water is not, and should the branches be damp you
could short the line.  Secondly using a conductive ladder near a power
line is not a good idea.  The EM field, or corona, can extend outward
around the line and cause conductive items to become electrically
charged.  Most utilities will use an insulated bucket truck for said
procedure.