Re: [TCML] X-ray cable best practices for feeder cable

["DC Cox" <resonance@xxxxxxxxxxxx>]

Resonance can be quite wonderful, or, as Phil explains in his examples, very
destructive.

Resonance in your HV feeder cable is not something you want, so I avoid
using any feeder cable that is coaxial in nature, ie, has a ground
shield around it.  X-ray cables were designed strictly for DC power
transmission and not for transferring AC power.
Been there, done that ----- and it cost me a pole xmfr!

Dr. Resonance




On Mon, Jan 5, 2009 at 9:25 PM, <FIFTYGUY@xxxxxxx> wrote:

>
>
> In a message dated 1/5/09 10:37:26 P.M. Eastern Standard Time,
> sparktron01@xxxxxxxxxxx writes:
>
> >I am now in the world of "Power Engineering", and I  absolutely
> >concur with DC; I've seen evidence of it in Medium  Voltage
> >(>600V, <15kV) systems.
>
>
>
> Dave, how about repeating your story of the  direct current power cable
> with
> a pinhole burned through the insulation every  foot? :)
>
> >One of our companies clients had a bad substation ground  at
> >2.4kV L-L to a 1000 HP motor.  Interesting to see a 45  deg
> >"crucifix" burned around a bonding bushing on a 600A
> >feeder  conduit entering the MV MCC.  MCC/Motor Ground
> >missing to  substation ground, EXCEPT for MV cable shields
> >(not there from original  installation, not our companies
> >installation); length of "burn/arc"  flashing at least 3" long.
> >So this situation is EXACTLY like using a  shielded X-Ray
> >cable in Tesla service.
>
>
>
> Switching (and other transients) can excite  resonances in shielded cables
> and other stray capacitances which of course can  cause the damage you
> describe.
> Worse is the problem of these resonances being  excited by harmonics from
> non-linear loads.
>
> >On a delta system (no ground) with a "capacitive" single  phase
> >arcing fault to ground, transient over voltages can  exceed
> >8X V applied.  If you doubt, check the IEEE "Buff"  Technical
> >reference for details.  I trust we all understand the  implications
>
>
>
>
> >of applying ~1kVAC to a 120VAC branch  circuit....
>
>
> That's why you stick arrestors and transient  devices all over the place
>    BTW, Dave, do you still have all those  MOV's?
>
> >Proper grounding is important in house wiring, but MUST
> >be  installed correctly at MV; a fire, catastrophic equipment
> >damage (even  collateral to unrelated systems), or electrocution
> >will result if  improperly installed.
> >
> >Imagine what would happen if a plant  technician
> >had his bare hand on 1000HP motor while running, and a  single
> >phase (or capacitive gradient) current occurred.  At a  minimum
> >a severe shock, or more likely death would have  occurred.
> >I have made it a personal rule to never touch high  horsepower
> >medium voltage motors while running for this  reason...
>
>
>
> The root cause is the bad grounding. A 1/20  horsepower motor at 60V, or
> even
> less, can kill you just as dead.
>    The worrisome part is the high-resistance  grounding utilized in many
> medium voltage systems, or the intentional lack of  grounding in many
> systems
> where downtime is more costly than safety. Then  monitoring devices are
> supposed
> to prevent disasters. But I wonder if  facilities with ungrounded systems
> change their procedures when they know they  have a ground fault ("Don't
> touch
> anything until we can shut down next  Saturday!"). Bad Things have happened
> when
> one ground fault a while later  precipitates a second fault, and then a
> line-line fault occurs...
>    Or in an electrolytic cell room, where grounding  is also intentionally
> avoided to prevent accidents. Now *there's* a  low-impedance source for
> you!
>
> -Phil LaBudde
>
> Center for the Advanced Study of Ballistic  Improbabilities
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