Tape Shield Bonding...

[Bigjabe]

Hi guys, I know this topic has come up a million times on here, but looking through past posts I didn't really get the answer I need.

This is regarding the bonding of metallic tape shielding of medium voltage cables, and specifically the differences b/w 1/c and 3/c cables.

I understand that in 1/c cables, there is an induced voltage on the shield... if you ground one end it builds up, if you ground both ends you have a current flow.

What I don't understand is what the difference is for 3/c cables - which from what I can tell, have their shields bonded at both ends by default. The shields are still wrapped only around individual conductors, and I don't believe the shields of the 3 conductors are in contact along the length of the cable, and thus I would expect the same induced voltages.

What am I missing here??

 

[Bigjabe]

Ok so I found some calculations that indicate the induced voltage is higher, the farther the 3 conductors are apart - in 3/c cable they are basically immediately beside each other so I guess it's negligible.

But if the conductor shield contains the electric field around the conductor, why would that distance even matter??

 

[GoldDigger]

Ok so I found some calculations that indicate the induced voltage is higher, the farther the 3 conductors are apart - in 3/c cable they are basically immediately beside each other so I guess it's negligible.

But if the conductor shield contains the electric field around the conductor, why would that distance even matter??

The distance only matters if:
A. You are allowing for one conductor being consistently closed to the shield than the others, or
B. You are looking at circumferential currents rather induced voltage.

 

[fmtjfw]

Magnetic field around single conductor is not cancelled out -- creating induced current.

Magnetic field around all three conductors is cancelled out -- not creating induced current..

 

[GoldDigger]

Magnetic field around single conductor is not cancelled out -- creating induced current.

Magnetic field around all three conductors is cancelled out -- not creating induced current..

A similar argument applies to the electric field and capacitively coupled voltage.
A big difference being that the capacitive coupling cancels out for balanced voltages while the magnetic (inductive) coupling only cancels for balanced current.

 

[Bigjabe]

Magnetic field around single conductor is not cancelled out -- creating induced current.

Magnetic field around all three conductors is cancelled out -- not creating induced current..

Yes of course, but for 3/c cables, the shields are wrapped around the individual conductors, not the overall cable, so they are still subject to the non-cancelling fields.

 

[Bigjabe]

The distance only matters if:
A. You are allowing for one conductor being consistently closed to the shield than the others, or
B. You are looking at circumferential currents rather induced voltage.

See the attached - the induced sheath voltage increases as S(space b/w conductors) increases. This also shows that for a 3c cable, the induced voltage is not zero, because S/rm will be 2 at minimum, which makes a non-zero induced voltage.

 

[GoldDigger]

Very helpful, and clearly referring to magnetic induction since it is proportional to the current. It also makes it clear indirectly that it is referring to an individual sheath on each phase.
Note that for flat configurations it is not symmetric across the three positions in the cable.

 

[fmtjfw]

Yes of course, but for 3/c cables, the shields are wrapped around the individual conductors, not the overall cable, so they are still subject to the non-cancelling fields.

Are you sure about the magnetic part for 3 conductors? The copper sheaths do not shield magnetic fields, so they could still cancel (sort of).

 

[GoldDigger]

Are you sure about the magnetic part for 3 conductors? The copper sheaths do not shield magnetic fields, so they could still cancel (sort of).

Yes, sort of!
The magnetic field of one conductor links strongly to the cross section of its own shield and much more weakly to the cross section of the other wires' shields.
The cancellation is only partial, but would be much greater if a coil or a ferromagnetic common core (via structural steel?) were involved.

 

[MD84]

I have seen where when phases are closely together (triplexed) it is not if great concern. When phases are separated cross bonding is performed.