At 14:20 98.03.10 -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
>

In normal low power aplications, all the energy is lost in fractions of a
second, but in cables for energy transportation lines who work at very high
voltages (several KV) and low freq the insulation testing is above the
double of the nominal working voltage, so if you have 1000 meters of cable
its a pretty good amount of energy we are talking about and in this case
you can get killed days after the testing if that energy isnt properly
drained.
About beeing a capacitive efect, I think you can call it that. We shouldn't
forget that one of the matematical models for a cable (or trnsmition line)
is a RL series circuit with caps in paralel.
                  R            L
        ------/\/\/\/\----@@@@@-----
                        |
                        |
                       ---
                       --- C
                        |
                 R      |    L
        ------/\/\/\/\----@@@@@-----

(hope you can understand this, first time, ASCII line art)


        Jorge F
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cumprimentos / best regards
      Jorge Ferreira          mailto://jorgegf@mail.telepac.pt
------ Make sure brain is in gear before engaging mouth -------
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