"bz" wrote in message
98.139...
"N_Cook" wrote in
:

Arfa Daily wrote in message
...
Sorry all. O.T. as a repair issue. Just something I read about today
that
I'm not sure I understand.

Anybody come across the term "step potential" or its effect in regard
to a downed live power line in contact with the ground ?

Arfa



Hollywood versions show the severed end of the cable, in intermittant
contact with the ground, snaking about like an unsupported hosepipe
issuing a water jet, - true, I don't know ?

Seen it. I was at a gas station north of LSU one day when linemen were
working to straighten a power pole.
They were tightening a guy wire when the winch slipped.

The pole snapped back to 'relaxed position' (about a 3 degree lean) and
the
impulse traveled down the power lines.

The power lines started swinging to-and-fro. The linemen ran. I didn't
know
why.

The wires touched on a swing and arced. Bzzzzzt. A loud, rough, 60 Hz
note.

The arc cut the wire.
The hot wire dropped onto the pavement and started jumping around,
sparking
and buzzing at each contact.

I am standing about 50 feet away, putting gas in my car.
Cars on the street are slamming on their brakes and dodging the hot wire.

Finally, the circuit breaker tripped, killing the power.

THEN I had time to be scared.

I suspect the jumping is due to the magnetic field caused by the current
flow.
One section of the wire attracts or repels another, nearby section of the
same wire.


--
bz 73 de N5BZ k

I don't know about that. I think that it might be due to the force of the
mini-explosion - i.e. expanding air blast - each time it touches. The only
reason I say this is because I saw something a couple of weeks back, that in
35 years of service work, I had never seen before. I had a switch-mode power
supply on the bench. An internal one from an LCD TV. The design has a PFC
boost converter stage on the fron end. When working correctly, it shoud
produce 390v across the main filter cap for all input voltages from a
hundred or so, up. The filter cap is laid on its side, soldered to two pins.
When I powered this one, it was going "CRACK - CRACK - CRACK" very loud. It
took me by surprise, but I quickly turned off and checked for any sign of
arcing. There was none, so I covered my eyes, and turned back on. The main
filter cap should be stuck down to the board, but they have always broken
free on these boards, so are secured just by being soldered to the pins. On
this one, when I looked closer, the cap was bouncing with each "CRACK !" It
was in fact arcing internally with enough force to physically move it - and
it's not a small cap. The reason was that the regulation feedback on this
converter had failed, and there was over 700v across the cap ...

Arfa