Running 480V cable next to 4160V cable

[mull982]

For a new installation in a quarry I have to run both a 4.16kv and 480V cable out to a piece of equipment. The length of the run is about 300ft. The type of cable that I am using for both runs is 3/C Type GC for the 480V cable and 3/c SHD-GC for the 4.16kV

Both of these cable runs will be run together on the ground for the 300ft that I mentioned above. I am going to try to keep about a 1ft seperation between the cables for the entire run.

My question was weather or not I could possibly have any problems with running these cables together like this, with possible induction or some other problem. In theory I am thinking that because these cables are in a 3/c arrangement, the magnetic fields of each phase should cancel out within a cable therefore not posing any external field to the cable lying next to it, specifically when looking at an external field from the 4.16kV cable onto the 480V cable.

Does anyone have any thought or issues with this installation?

 

[jghrist]

For a new installation in a quarry I have to run both a 4.16kv and 480V cable out to a piece of equipment. The length of the run is about 300ft. The type of cable that I am using for both runs is 3/C Type GC for the 480V cable and 3/c SHD-GC for the 4.16kV

Both of these cable runs will be run together on the ground for the 300ft that I mentioned above. I am going to try to keep about a 1ft seperation between the cables for the entire run.

My question was weather or not I could possibly have any problems with running these cables together like this, with possible induction or some other problem. In theory I am thinking that because these cables are in a 3/c arrangement, the magnetic fields of each phase should cancel out within a cable therefore not posing any external field to the cable lying next to it, specifically when looking at an external field from the 4.16kV cable onto the 480V cable.

Does anyone have any thought or issues with this installation?

There are two kinds of induction, one from magnetic fields and one from electric fields. The induced current or voltage from magnetic fields depends on the current in the conductors and as you say, these more or less cancel out when the cables are in a 3/c arrangement. The magnetic field doesn't care about the circuit voltage. Induced voltages from electric fields depend on the cable voltage, but since the 4.16 kV cable is shielded, there are no electric fields outside the shield to worry about.

 

[Lxnxjxhx]

Also,

Also,

if you run cables over a conducting "ground plane", there is an additional shielding effect, even if there is no shield between the cables.

This works better the closer the cables are to the conducting metal, and is the reason why the old-time 6.3 vac vacuum tube filament supply was twisted and run flat against the ferrous metal chassis.

Supposedly there is a "virtual image" of the conductor on the other side of the metal plane.
If the conductor is two inches above the plane, the virtual image is two inches below. With this spacing of four inches the cancellation between the real and virtual conductor is not so good. It's better when the cable is flat against the metal so the real and virtual images are almost on top of one another.

I guess it's the same principle as twisting the conductors together. The closer they are, the more the fields cancel.

 

[mull982]

There are two kinds of induction, one from magnetic fields and one from electric fields. The induced current or voltage from magnetic fields depends on the current in the conductors and as you say, these more or less cancel out when the cables are in a 3/c arrangement. The magnetic field doesn't care about the circuit voltage. Induced voltages from electric fields depend on the cable voltage, but since the 4.16 kV cable is shielded, there are no electric fields outside the shield to worry about.

I was only aware of induction due to magnetic fields which I understood to be caused by a change in flux resulting from current magnitude. I was not familiar with induction caused by electric fields. Can you explain more on induction resulting from these electric fields and how they are dependent on voltage magnitude?

 

[drbond24]

To answer the OP, I agree with the essence of jghrist's post. You won't have a problem.

There are two kinds of induction, one from magnetic fields and one from electric fields.

Huh? I'm not sure I understand what you are trying to say. An electric field is the vector field around a stationary charged particle, otherwise known as an electrostatic field, right? Not applicable in this situation. A magnetic field (or in this case an electro-magnetic field) is created when charged particles are moving, as in current through a cable. Induction is the result of the intermingling of multiple electro-magnetic fields, meaning both fields must be the result of moving charged particles. Two stationary charges in close proximity would indeed effect one another, but the result would not be induction, it would just be the force between two charged particles.

Correct me if I'm wrong, it has been a fews years since college.

 

[jdsmith]

Huh? I'm not sure I understand what you are trying to say. An electric field is the vector field around a stationary charged particle, otherwise known as an electrostatic field, right? Not applicable in this situation. A magnetic field (or in this case an electro-magnetic field) is created when charged particles are moving, as in current through a cable. Induction is the result of the intermingling of multiple electro-magnetic fields, meaning both fields must be the result of moving charged particles. Two stationary charges in close proximity would indeed effect one another, but the result would not be induction, it would just be the force between two charged particles.

In general electric fields and induction aren't associated. The way it was explained to me is that the electric field is relevant in this case and it illustrates why MV cables are shielded. With a shield in place the electric field due to the cable is nonzero only in the insulation. Electric fields are zero or close to zero within a conductor, so with a known phase voltage and a grounded shield we know fairly accurately the strength of the electric field in the insulation, and hence the burden that's being placed on the insulation.

But I'm not sure what actual situations would cause an increased electric field in the insulation, and therefore I don't know what real world problems the shielding is designed to prevent.