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

[FIFTYGUY@xxxxxxx]

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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