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Old 02-16-2015, 10:06 AM   #1
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Default Sanity check please Medium Voltage terms'

For Medium voltage terminations. 6.9kV field conductors to motor lead terminations.

Shield conductor terminates on one end only correct? To prevent a ground loop. We have the shield grounded at the Board.

I would appreciate your thoughts and opinions.

Thanks,
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Old 02-16-2015, 10:17 AM   #2
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Shield is only terminated at the source... In your case the starter...
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Old 02-16-2015, 10:19 AM   #3
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thanks, we verified that the shields for A B & C phase are grounded at the source.
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Old 02-16-2015, 11:49 AM   #4
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Typically around here the shields are landed at both ends. If you don't there could be hazardous voltages on the shield at the other end.
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Old 02-16-2015, 12:21 PM   #5
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Thanks for the reply Don, I am looking into this further now. I see what you are saying, especially if the shield is broken, from the switch gear & the end device. Will check also to ensure the shield is not broken.
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Old 02-16-2015, 12:28 PM   #6
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Quote:
Originally Posted by don_resqcapt19 View Post
Typically around here the shields are landed at both ends. If you don't there could be hazardous voltages on the shield at the other end.
If its bonded to ground at both ends then you create a loop on which current can flow and defeat the entire purpose of the shielding cable, which is to eliminate sheath currents which can break down insulation at the termination point.

Sanity check please Medium Voltage terms'-sheath-current-damage.jpg
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Old 02-16-2015, 03:32 PM   #7
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From Okonite
[quote]For safe and effective operation, the shielding should be grounded at each end of the cable and at each splice.[/quote]
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Old 02-16-2015, 03:35 PM   #8
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Originally Posted by Rollie73 View Post
If its bonded to ground at both ends then you create a loop on which current can flow and defeat the entire purpose of the shielding cable, which is to eliminate sheath currents which can break down insulation at the termination point.

Attachment 48177
Yes, currents can flow and that affects the ampacity of the cable, but the purpose of the shielding is not to eliminate sheath currents...it is to eliminate high voltage stress that breaks down the insulation, not only at the termination point but along the total length of the cable.
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Old 02-16-2015, 04:03 PM   #9
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Quote:
Originally Posted by glen1971 View Post
Shield is only terminated at the source... In your case the starter...
Explain
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Old 02-16-2015, 04:18 PM   #10
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Quote:
Originally Posted by Rollie73 View Post
If its bonded to ground at both ends then you create a loop on which current can flow and defeat the entire purpose of the shielding cable, which is to eliminate sheath currents which can break down insulation at the termination point.

Attachment 48177
Shielding is not about eliminating sheath currents, its about creating even voltage stress (potential) around the cable and eliminating corona effect.

Unless the cable was well over a dozen miles long something else destroyed it.

In MV utility systems the shields of URD and MV cable are bonded and grounded at every joint starting, ending and in between. It does create some current flow but never to the magnitude that I have seen it melt or destroy a splice.

Last edited by meadow; 02-16-2015 at 04:24 PM.
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Old 02-16-2015, 04:55 PM   #11
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Nope, sorry. SHIELDING on power cable should be grounded not only at both ends, but at all splice points etc. If you have a ground loop occurring in your SHIELD, you have a bad grounding system. The SHIELD should not be a grounding conductor. The purpose of shielding on power cables is to avoid cable damage as a result of voltage stresses and corona discharge that then causes ozone which breaks down the insulating materials.

Grounding one end of shielded cables is applicable to signal cables.
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Old 02-16-2015, 06:24 PM   #12
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The shield is a drain. I have done it both ways. Government ships want it done on both ends. This creates current on it. But, how could the government ever be wrong they must have the best engineers working for them.
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Old 02-16-2015, 07:22 PM   #13
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Quote:
Originally Posted by meadow View Post
Explain
Always been told they are grounded at one end only to eliminate circulating sheath currents, the same way single conductors are dealt with...
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Old 02-16-2015, 07:28 PM   #14
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Shield get grounded on both ends on power cable. You only ground one end of a shield when you are rejecting noise.
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Old 02-16-2015, 08:22 PM   #15
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Quote:


36-104 Shielding of thermoset insulated conductors
(see Appendix B)


(1) Except as permitted in Subrules (2), (3), and (4), shielding shall be provided over the thermoset insulation
of each permanently installed conductor with or without fibrous covering or non-metallic jacket, operating
at circuit voltages above 2000 V phase-to-phase.
(2) Shielding need not be provided for conductors having thermoset insulation where they are run
underground in raceways or directly buried in the soil and operating at circuit voltages not exceeding
3000 V phase-to-phase, provided that the insulation or the non-metallic jacket, if provided, is the ozone and
discharge-resistant type.
(3) Shielding need not be provided for conductors having thermoset insulation where the circuit voltage does
not exceed 5000 V phase-to-phase, where the conductors are installed on insulators and bound together,
in electrical equipment rooms, electrical equipment vaults, metal-enclosed switchgear assemblies, and

similar permanently dry locations where the conductor run does not exceed 15 m.


(4) Shielding need not be provided for conductors having thermoset insulations that are

(a) intended for operation at not more than 5000 V phase-to-phase;
(b) intended and installed for permanent duty; and
(c) provided in either single- or multi-conductor cable construction with
(i) a metal sheath;
(ii) metal armour of the interlocking type, the wire type, or the flat-tape type; or
(iii) totally enclosed metal raceways where installed above ground in dry locations.
(5) Subject to Rule 10-304, metal sheaths, shielding, armour, conduit, and fittings shall be bonded together

and connected to ground





Quote:

10-304 Other conductor enclosures



(1) Metal enclosures for conductors, other than those referred to in Rule 10-300, shall be bonded to ground,
except
(a) in runs of less than 7.5 m that are free from probable contact with ground, grounded metal, metal
lath, or conductive thermal insulation, and that, where within reach from grounded surfaces, are
guarded against contact by persons; and
(b) runs used for physical protective sleeving of less than 1.5 m in length, where the installation method
is such that it is improbable that they will become energized.
(2) Where single-conductor metal-sheathed or armoured cables are installed in raceways,
in order to prevent the flow of sheath currents in accordance with Rule 4-010(1)(c), the cables shall

(a) be in separate raceways or supplied with suitable continuous non-conductive jackets;

(b) have their sheaths or armour bonded together and bonded to ground at the supply end; and

(c) thereafter have their sheaths or armour isolated from each other and from ground.

We don't bond them at both ends In fact....its a violation of the CEC

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Old 02-16-2015, 08:28 PM   #16
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The power company uses the shield as the grounded conductor.
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Old 02-16-2015, 08:29 PM   #17
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Quote:
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The power company uses the shield as the grounded conductor.
The POCO can do whatever it wants Amazing what they get away with.
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Old 02-16-2015, 08:33 PM   #18
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It depends on what style of shield it is. Some are just ment for shielding others are ment to be current carrying.
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Old 02-16-2015, 08:36 PM   #19
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Quote:
Originally Posted by Rollie73 View Post







We don't bond them at both ends In fact....its a violation of the CEC

That isn't talking about shields. It is talking about the amour on armoured cable.
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Old 02-16-2015, 08:46 PM   #20
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I have never seen, and certainly never installed, shielded MV cables without the shield grounded at both ends and at any splices or taps in between. As for stray current flowing on the shield - big deal. Most cables used today have shields rated as EGC's anyway (I still pull a separate ground, though). Would you not bond an enclosure with splices in it in a 600V system for fear that your ground wire might be inadvertently carrying stray current? As someone said earlier, if you have enough current flowing on the cable shield to damage the conductor insulation, you have big problems lurking somewhere.
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