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 Post subject: Shielded power cable questions
PostPosted: Fri Nov 01, 2013 3:49 am 
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I keep running into lots of questions about power cables and don't have a lot of good (believable) answers and want to hear how others answer.
  1. Why would voltage appear outside the shield? Doesn't Faraday apply? What effect would multiple shields have?
  2. Why would voltage appear on the jacket at all? Even if it does, isn't the available current well below lethal limits?
  3. Are there any hazards associated with capacitive taps on elbow connectors where again, current is microamps?
  4. Does any of this change with different constructionIf there are hazardous voltages on medium voltage cable at what point is the jacket and thus cable considered insulated and nonhazardous? 120? 480? 600? 1 kv? 2 kv? 5 kv? 25 kv? 35 kv? 69 kv? Note: NEC high voltage cutoff now 1 kv, unshielded cutoff nominally 2 kv, multiple shields above 35 kv.


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PostPosted: Fri Nov 01, 2013 12:26 pm 
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Thoughts on question 1. A shield inhibits the electric field, but the magnetic field penetrates. (CTs sense through shielded cables). The magnetic field could induce voltages outside the cable. (I have a piece of copper shield from a 15kV cable neatly attached to my file cabinet with a magnet).


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PostPosted: Mon Nov 04, 2013 9:59 am 
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The question can be addressed by asking the question what other cables will be used in the same area or tray. What voltages will be encountered.


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PostPosted: Mon Nov 04, 2013 10:22 am 
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Depending on the voltage rating of the conductors and the application, you will see potentially high currents accumulating along the surface of energized cables. In a perfect world Faradays principals would provide 100% shielding. Since the voltage sine wave and the insulating materials contain impurities, you get resistance (impedence) and harmonics. Cable constructions may include semi-conductive layers as well as drain stranding or copper tape around the outside circumference of the cable insulation to "drain" off the capacitive flow of electrons that escape the insulation layer. If this cable shielding is not continuous throughout the length of the conductor, the unshielded portion of the cable will become a weak link, subject to higher operating temperatures, higher corrosion and higher stress on the cable insulation- a sure recipe for cable failure. The jacket of a shielded/jacketed cable is to be considered as energized to the system voltage rating potential. It usually doesn't have the ampacity to cook you but it can definitely ruin your day! Capacitive test points are just that- capacitive- meaning they build up a charge depending on the available energy around them. Some analog meters and the old wiggies were used to sense energized elbows- but if the load wasn't steady or high enough to keep the test point "hot", the test equipment could drain the test point with the first contact. Following contacts made to the test point could give a false de-energized "dead" signal. New electronic voltage sensors do not drain the test points and are much safer to use for this purpose. MSHA has good library references for this hazard since it is the #1 biggest concern with surface-laid mining cables. Take some time to learn the different duty ratings for cables used within your industry.


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PostPosted: Tue Nov 05, 2013 7:52 pm 
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considering the outer jacket live seem a be to far...most of the time it`s deposit on a cable tray that are grounded, ended with cable connector that are grounded

that is probably the same theory that some people say you must check voltage on the high voltage transfomer casing before touching it....a bit more and you will have to check the outside perimeter fence for voltage in a substation

but yes on the ungrounded side of a shield and very long cable voltage can build


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PostPosted: Wed Nov 06, 2013 10:25 am 
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>Why would voltage appear outside the shield? Doesn't Faraday apply? What effect would multiple shields have? -

One common answer is that some times that "shield" isn't grounded. That makes it just part of a capacative divider.


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PostPosted: Tue Nov 12, 2013 7:51 am 
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A shielded cable, if installed properly, should have not imposed voltage/current. Reality is a different animal. Like any "ungrounded system" there is still some "grounding" because of capacitance be phase conductors and ground. Like wise there is some capacitance between the conductor and the grounded shield in a shielded cable. Properly installed shielded cable should be grounded only on one end. With voltage on the ungrounded end and zero volts on the other there is a potential difference, a potential difference on a conductor provides a path for current, current across and impedance give voltage.

I think the question is "is there enough voltage or current on the shield to be a problem?" - for the electrical system no, would I touch it with bare hands hell no.

For the comment "CT's sense through shielded cables", they do but the shield must not pass through the CT unless it brought back through to cancel out the noise.

BTW, I have a insulator(sheet of paper) stuck to my filing cabinet with a magnet, pretty sure it will not conduct electric or magnetic.


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PostPosted: Tue Nov 12, 2013 9:31 am 
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Somewhere between thinking we are all going to die, and the fact that utilities run these shielded cables up barely protected riser poles in public locations, I don't think this thread has found the total answer yet. Perhaps a few more experts will add comment.

Concentric neutral shield wire will constitute the return path for single phase distribution and may acquire a few volts potential rise. If it is a tape shield, it is not intended to return current, so a separate neutral conductor is installed and sometimes the tape sheild is only grounded at one end to prevent neutral current flow.

If I see a secure ground connection nearby I have taken clamp-on load readings around the shielded cable (in a position where the ground strap doubles back through the meter) and felt no threat.


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PostPosted: Wed Nov 13, 2013 9:54 am 
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Engrick said"Properly installed shielded cable should be grounded only on one end - "

That depends on the purpose of the cable. For a communications or data cable that would be true. For a power cable, it isn't. An for that matter a power cable may have more than two "ENDs".
Properly installed it should have the shield bonded to ground at each "end" and each splice, or tap, in the "middle". This provides a lower impedance fault path for when the insulation system fails, and a certain degree of redundancy in the grounding connections. This (usually) results in shorter clearing times, and lower impressed voltage on the shield system.
Any shield you don't ground at a given location would need to be treated as if it were an energized conductor.

Not grounding one end, also leads to the situation where: You don't ground "your" end, because you assume I grounded "mine", and I don't ground "mine" because I assume you grounded "yours".


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PostPosted: Wed Nov 13, 2013 10:06 am 
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Sparksmore said" Capacitive test points are just that- capacitive- meaning they build up a charge depending on the available energy around them. Some analog meters and the old wiggies were used to sense energized elbows- but if the load wasn't steady or high enough to keep the test point "hot", the test equipment could drain the test point with the first contact. Following contacts made to the test point could give a false de-energized "dead" signal. "

Not really. Or not at all, except for DC.
A capacitive test point is meant to allow detection of AC, and its potential will not "go away for a time after measurement" as the above implies. Any one trying to use a capacative tap to measure DC is foolish, using it to check a circuit 'dead" even more so.

And in ANY case it has NOTHING at all to do with the load.
High current, low current, no current, AC current, DC current. A capacative tap will act the same regardless. It could care less about current.


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PostPosted: Mon Nov 18, 2013 11:37 am 
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If the cable jacket can carry a potential, then why wouldn't the PVC conduit it is installed in also carry a potential?


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PostPosted: Mon Nov 18, 2013 6:53 pm 
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If you ground both ends of the shield, you form a transformer with the ground as the primary and the phase conductors as the secondary. This violates the concept of equipotential grounding because it creates induced voltages.


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PostPosted: Tue Nov 19, 2013 1:42 pm 
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For single phase shielded conductors, 13.8kV typical, each end has a stress cone applied. At each cable end the stress cone has a braid to tie the cable shield to ground. As stated above, the single phase shielded conductors are permeable to the magnetic field. Permeability of copper, aluminum and air are almost identical and frequency independent.

Multi conductor AC drives triad cables, 690V typical, like GGC, but with a shield. Shields are grounded at each end, to create a Faraday cage around the cable. Bottles up the high electrical frequencies caused by IGBT transistor switching.


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PostPosted: Tue Nov 19, 2013 4:43 pm 
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PaulEngr wrote:
If you ground both ends of the shield, you form a transformer with the ground as the primary and the phase conductors as the secondary. This violates the concept of equipotential grounding because it creates induced voltages.


Quite the opposite. If you DON'T ground both ends of the shield, THAT violates the concept of equipotential grounding.
The "circulating" and induced currents are why you should also have a ground conductor running along with your power cables.

Here is Okonites recommendation
Grounding Shielded Cable
"For safe and effective operation, the shielding should be grounded at each end of the cable and at each splice. For short lengths or where special bonding arrangements are used, grounding at one point only may be satisfactory. "
from http://www.okonite.com/engineering/shielding.html

IEE 422 also finds multiple grounding to be acceptable, and limits the use of single point grounding to shorter runs. It is limited by the voltage rise on the sheld.

The Red book also discuses multiple grounding as well as single point, and has both as acceptable practice. [And discuses that this is a point of contention]

NESC says
[color=#000000]Grounding cable insulation shielding systems shall be interconnected with all other accessible[/color]
grounded power supply equipment in manholes, handholes, and vaults.
EXCEPTION:Where cathodic protection or shield cross-bonding is involved, interconnection may be
omitted.
[color=#1c1c1c][[/color]I would also say that you have your primary and secondary reveresed, since the primary is usually the "driving" side. But PoTAEtoe, Po TAHtow, it is all perspective]


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PostPosted: Mon Dec 02, 2013 8:21 am 
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The Okonite website http://www.okonite.com/engineering/shielding.html does talk about grounding both ends but if you read farther down is also talks about grounding at one end to reduce circulating current.


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