Generation/ Low Resistance Grounding/ Distribution System 1

[GJJ]

Gentlemen,

I have a plant that is serviced by a 5KV generation and distribution system. All of the loads on this system are three phase loads. If all of the generators are low resistance grounded, do I need to run a ground wire along with the phase wires from the 5KV switchgear to each of the 5KV loads (pad mounted gear, transformers, etc.) All of the loads in question will be grounded via "local" ground rods & loops.

If I do not run a ground wire with the phase conductors, is the earth an allowable return path for returning a phase to ground fault to the generator source?

I have looked in the National Electrical Safety Handbook (NESC), IEEE Std. 142, NEC and Soares Book on Grounding to shed some light or guidance, but have come away with very little.

Any guidance on this subject would be greatly appreciated.

 

[busbar]

A somewhat related reference is ANSI/IEEE C62.92.4-1991 …Neutral Grounding in Electrical Utility Systems, Part IV—Distribution. Although §2 deals primarily in system grounding, the multigrounded system is successfully used in many areas on the North American continent.

In cases where earth return is relied upon to operate medium-voltage overcurrent devices, it is common to keep tabs on ground-electrode resistances to assure reliable operation.

Reliance upon earth return to operate low-voltage overcurrent devices is not reccommended and generally prohibited by consensus standards.

 

[dpc]

Per the NEC, you cannot use earth for an equipment grounding conductor. You must use one of the several accepted types of equipment grounding conductors and it must be run with the circuit conductors. See Article 250.118. Also see 250.186 for additional requirements for impedance grounded systems above 1000V.

If you have multiple generators, each with its own grounding resistor, the total fault current will be the sum of all the generator neutral currents.

Of course, if this is an electric utility installation, you are not required to meet NEC, but I'd strongly recommend a grounding conductor be run with each set of phase conductors.

 

[jbartos]

Suggestions to GJJ (Electrical) Jan 29, 2004 marked ///\\I have a plant that is serviced by a 5KV generation and distribution system. All of the loads on this system are three phase loads. If all of the generators are low resistance grounded, do I need to run a ground wire along with the phase wires from the 5KV switchgear to each of the 5KV loads (pad mounted gear, transformers, etc.)
///The low resistance system grounding (also known as medium resistance system grounding) is normally connected to the ground very close the generator neutral. A connection of equipment ground should be made between the low resistor and ground. This equipment ground is run to all conductive surfaces downstream to bring them all to the same equipotential level to avoid any hazardous conditions. The galvanized rigid conduit is normally considered sufficient for the ground return path. When the conduit distances become longer, e.g. hundreds of feet, it is prudent to add the grounding conductor and perform ground bonding in each pull box or junction box downstream.\\ All of the loads in question will be grounded via "local" ground rods & loops.
///This is a good idea to reinforce the return path if the rigid steal conduit installation is in effect. However, it would not be covering certain fault, which relies on the conduit return path good resistance.\\If I do not run a ground wire with the phase conductors, is the earth an allowable return path for returning a phase to ground fault to the generator source?
///Theoretically, it could perform well; however, practically, the rigid steal conduits and ground conductors may provide lower ohmic return path.\\I have looked in the National Electrical Safety Handbook (NESC), IEEE Std. 142, NEC and Soares Book on Grounding to shed some light or guidance, but have come away with very little.
///There are good coverages in:
Donald Beeman "Industrial Power Systems Handbook," First Edition, McGraw-Hill Book Company, 1955,
Chapter 6 System Grounding
Chapter 7 Equipment Grounding\\

 

[jghrist]

If the 5 kV cable is shielded, the metallic sheath may be adequate for use as an equipment grounding conductor. See NEC 250-91(b). This is the NEC-1996 rule number; my 2002 code is at the office - probably the same number.

 

[dpc]

jghrist - lots of changes in Article 250 between 1996 and 2002. I went back to 1996 version to check. Requirements for equipment grounding conductors are now in 250-118.

But unless I'm reading it incorrectly, copper sheath is only allowable as grounding conductor for mineral-insulated (MI) cable. Armor can be used for AC and MC cable.

 

[jbartos]

Suggestion to the original posting marked ///\\If I do not run a ground wire with the phase conductors, is the earth an allowable return path for returning a phase to ground fault to the generator source?
///It depends on its resistivity or impedance.\\

 

[jghrist]

dpc - After reviewing the NEC a little more, I see that you are correct. Since the post concerned 5 kV cable, and MI is 600V cable, the answer seems to be no, you can't use the shield, at least if you are subject to the NEC. Most of my work is with utilities who are not required to follow the NEC. It is common for US utilities to depend on the shield for the ground return path. The shield is sized to carry the available fault current.

 

[alehman]

A possible reason for the NEC rule - In order for the shield to be at ground potential it must be connected to ground at both ends. If it's grounded at one end only (which is permitted), the non-grounded end will have significant induced voltage.

See

http://okonite.com/engineering/shielding.html

 

[jbartos]

Comment: Visit the link in the previous posting for:
"""For short lengths or where special bonding arrangements are used, grounding at one point only may be satisfactory."""
However, if there is a grounding conductor with the run, both sides of the cable shield are supposed to be grounded.

 

[alehman]

That applies only if the grounding conductor is part of the cable assembly (e.g. inside the CLX jacket). This is to prevent arcing should the ground contact the shield.