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Difference between revisions of "Electrical grounding"

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Revision as of 09:56, 17 September 2013

Electrical grounding can be classified in either system grounding and equipment grounding.

Requirements

Requirements for system grounding are covered in detail in the Natl. Electrical Code (NEC),* Chap. 2, Article 250.[1]

System grounding

System grounding includes grounding of the power supply neutral so that the circuit protective devices will remove a faulty circuit from the system quickly and effectively.

Equipment grounding

Equipment grounding includes grounding of the noncurrent-carrying conductive part of electrical equipment and of enclosures that contain electrical equipment for personnel safety.

Purpose

Equipment grounding is a very important aspect of the electrical system. Grounding of electrical equipment has two purposes:

  • To ensure that persons in the area are not exposed to dangerous, electric-shock voltage
  • To provide current-carrying capability that can accept ground-fault current without creating a fire or explosive hazard

To protect personnel from electric shock, all enclosures that house electrical devices that might become energized because of unintentional contact with energized electrical conductors should be effectively grounded. If the enclosure is grounded adequately, stray voltage will be reduced to safe levels. If the enclosures are not grounded properly, unsafe voltages could exist, which could be fatal to the operating personnel.

The lightning arresters installed in electrical systems cannot operate satisfactorily unless they are grounded well. Under elevated static voltage or lightning strikes, lightning arresters will short-circuit the above-normal voltages to ground. If the lightning arresters are not grounded properly, elevated voltage will enter the windings of transformers, control, and/or motors, causing component failures.

Difficulties

Obtaining a satisfactory ground can present some difficulties. Wellheads normally can be considered an excellent grounding source through the well casing. Ground rods can vary from acceptable in moderately wet soils to very inadequate in dry soils. Whenever possible, use the wellhead for grounding of the secondary electrical system. If a wellhead is not available, ground rods can be used.

Design

In designing an electrical grounding system, consider the following:

  1. For personnel safety, ground to the wellhead or to properly installed ground rods all the secondary-electrical-system devices. This includes the transformer tank, disconnect switch enclosure, motor control, and motor frame.
  2. Ground to the wellhead or to properly installed ground rods all secondary lightning arresters. Use different conductors to ground the secondary enclosures and lightning arresters. The wire that grounds the lightning arresters should be a continuous, unbroken cable that is no smaller than No. 6 wire.
  3. Primary lightning arresters also should be grounded at a utility primary ground, and not to the secondary ground or the wellhead.
  4. Do not attach utility static wires or grounding of transformer connections to the wellhead. If connected to the wellhead, this can adversely affect the cathodic protection of well casing and production tubing. This part of the electrical system might include many miles of line exposure and many grounds that could influence the corrosion of the production equipment. Grounds for this part of the system should be grounding rods or ground pads located at the bottoms of the utility poles. Other satisfactory grounds are wells drilled or ground mats constructed for this purpose at the electrical substation.
  5. If possible, install ground rods at each location for each of the separate grounding wires run to the wellhead. During the servicing of the wells, the wellhead grounds may be removed. When service work is completed, reconnect these wellhead grounds.
  6. Do not connect grounds of telephone systems to the grounds of motors. Induction motors can generate harmonic voltages that can cause noise on telephones when they share common grounds.

*Natl. Electrical Code and NEC are registered trademarks of the Natl. Fire Protection Assn. Inc., Quincy, Massachusetts 02269.

References

  1. NFPA 70, Natl. Electrical Code (NEC). 2005. Quincy, Massachusetts: NFPA.

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See also

Electrical systems

Electrical distribution systems

Power factor and capacitors

Hazardous area classification for electrical systems

Alternating current motors

Induction motors

Synchronous motor

Motor specifications

NEMA motor characteristics

Alternating current motor drives

Motor enclosures

PEH:Electrical Systems