At 02:20 PM 3/10/98 -0600, you wrote:
>>When you test the insulation of a long spool of cable (one end connected to
>>a power source and the other open), when you disconect it from the power
>>source and leave it open the transient efect can creat a surge at one end
>>of the cable (the one connected with the power source). As the cable is
>>open at both ends this surge travels from one end to the other (as a
>>reflectd wave) until all its energy is exausted by the cable losses. If you
>>have a good quality cable this energy can stay there for quite a while.
>>If we are talking about high voltage cables (energy transportation lines),
>>this efect can kill pepople toutching the cable days after it has been
>tested.
>>The only safe way to avoid this is to conect the cable to a correctly
>>adapted load until all residual energy gets exausted before stocking the
>>cable.
>
>Are you saying that the resistance of the cable is so low that a signal can
>bounce along the unterminated transmission line for _days_ without
>undergoing significant attenuation?  (If so, wouldn't that essentially be a
>superconducting tuned transmission line oscillator?)
>
>
>Isn't it _much_ more likely that the cable (I'm assuming we're discussing
>coax or two conductor cable) is acting simply as a capacitor?  After all,
>what's two conductors seperated by an insulator?
>
>
>newell
>

I find either case (the "superconductivity" or the capacitance theory)
unlikely under normal circumstances. If it did happen, I would attribute it
to the capacitance.
        First off, Scott (or Newell, sorry, still not sure which you prefer) is
right, as far as I know, that no ordinary conductor can possibly support
currents for more than a millisecond or so with out an external source.
(even an ms is an EXTREAMLY long estimate).
         Secondly, lets suppose that there was enough wire in the cable to creat
e
a significant capacitance. The effective circuit would look like a
capacitor with a resistance in parallel (we can ignore the inductance, it
certainly won't make much difference beyond seconds of time). The value of
the effective parallel resistance can be approximated by the resistance of
the cable. Assuming the cable is copper and of a resonable diameter, lets
say 26 ga. 26 ga copper wire has a linear resistivity of .0408 ohm/ft. So,
even if this cable were 10 miles long, the effective resistance would only
be around 2.2k. Now, suppose we had 1 farad of capacitance. The time
constant for the circuit would be 2200 seconds. So, the voltage would drop
to less than 1% of its original value within hours of charging, and this is
for an extream example. The spool of wire holding 10 miles of wire would
probably barely fit on my desk. So, I guess if you had a huge spool, you
could theoretically have it hold charge for days or weeks, but it would
have to be HUGE. Besides, this effect diminishes with wire diameter so
anything which one could rightfully call "cable" instead of wire would
probably not exhibit this effect at all.
        In order for a static charge to naturally build up on the cable, the
average rate of energy transfer to the cable would have to exceed its
discharge rate. This also seems unlikely. However, I have no experiance
dealing with any really large amount of wire or cable, so I certainly could
be wrong.

Sean


+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
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