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Re: [TCML] X-ray cable best practices for feeder cable
In a message dated 1/5/09 10:37:26 P.M. Eastern Standard Time,
>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
>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!
Center for the Advanced Study of Ballistic Improbabilities
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