Earthing Systems Design Steps – Part Eight


In Article Earthing Systems Design steps – Part One ", I indicated the following points:





Earthing Systems Design Steps

A grounding system design process has (3) main steps:
  1. Data Collection,
  2. Data Analysis,
  3. Grounding Design Calculations.





In the above Article and Article 
Earthing Systems Design steps – Part Two ", I explained the first step: Data Collection.


Also, in Article 
Earthing Systems Design steps – Part Three ", I explained the second step: Data Analysis.


And in Article 
" Earthing Systems Design steps – Part Four "  I explained What we are going to design for grounding system in any building?




What we are going to design for grounding system in any building?

Grounding system in any building can be broken down into several subdivisions:

  1. The building exterior grounds,
  2. The electrical service grounding,
  3. The building interior bonding,
  4. Equipment grounding and bonding,
  5. Lightning protection.





In this Article, I explained the Building Exterior Grounds.


In Article 
Earthing Systems Design steps – Part Five "  I explained the electrical service grounding and indicated the five subdivisions of The Building Interior Bonding which were:





The Building Interior Bonding

Building Interior Bonding can be broken down into several subdivisions:

  1. Bonding of Equipment for Services,
  2. Bonding for Other Systems,
  3. Bonding Other Enclosures,
  4. Bonding for Over 250 Volts,
  5. Bonding of Piping Systems and Exposed Structural Steel,
  6. Bonding of Interior Metal Columns and Beams.





And I explained three subdivisions in this article.


Also in Article 
Earthing Systems Design steps – Part Six ",  I explained the subdivisions: Bonding for Over 250 Volts and Bonding of Piping Systems and Exposed Structural Steel.


In Article " Earthing Systems Design steps – Part Seven ", I explained the last subdivision of The Building Interior Bonding: Bonding of Interior Metal Columns and Beams.


Today, I will explain the forth grounding system division existing in any building: Equipment grounding and bonding.


You can preview the following Articles for more info:




Forth: Equipment grounding and bonding


1- Equipment Bonding





Equipment Bonding 

Equipment bonding within the facility includes:
  1. Equipment within the facility,
  2. Cable tray installations within the facility.










1.1 Equipment within the facility


  • Equipment within the facility must be carefully considered as to its need to be bonded to the facility ground system. Of course, all electrical equipment must have a grounding conductor as dictated by the NEC. Additional grounding is sometimes needed as shown in (Fig.1).


Fig.1


  • The frequent use of ground plates (Fig.2) provides accessible grounding points throughout the building.



Fig.2


  • When removable grounds are required near a grounded column or beam, a stud can be welded to the steel and the bonding jumper can be attached using a lug (Fig.3). Providing mechanical protection to the stud is recommended.




Fig.3









1.2 In cable tray installations

The cable tray’s bolted joints do not always provide the required low resistance. To ensure low resistance joints between segments of cable tray:

  • A separate ground conductor must then be run the length of the tray, bonded to each tray section and to adjacent steel columns.

  • Or, jumpers can be used across each joint. The cable can be welded to the tray if it is steel (Fig.4) or bolted to the tray if it is aluminum (Fig.5).



Fig.4




Fig.5









2- Equipment Grounding





Equipment Grounding

Equipment Grounding within the facility includes:
  1. Equipment Fastened In Place (Fixed) or Connected By Permanent Wiring Methods,
  2. Specific Equipment Fastened In Place (Fixed) or Connected By Permanent Wiring Methods,
  3. Equipment Connected By Cord and Plug,
  4. Nonelectrical Equipment.
  







2.1 Equipment Fastened In Place (Fixed) or Connected By Permanent Wiring Methods

  • The primary reason equipment and enclosures are grounded is to provide a low impedance path for ground-fault current. Such a path helps to ensure that the overcurrent protective device operates in an effective manner to protect people and property exposed to ground fault currents.

  • NEC Section 250-110 establishes six general conditions under which exposed noncurrent-carrying metal parts of fixed equipment supplied by or enclosing conductors or Components likely to be energized must be grounded:


The First Condition:
It requires grounding whenever such metal parts are located within a zone that extends within 8 feet (2.4 m) vertically and 5 feet (1.5 m) horizontally of ground or any grounded objects which may be contacted by persons. This establishes a reach or touch zone that ensures protection if persons could come in contact with such objects.


The Second Condition:
It requires that any exposed metal parts, if not isolated, be grounded if installed in wet or damp locations. The NEC defines a wet location as one which is subject to saturation with any liquid and other locations underground or in concrete slabs. Damp locations are those locations subject to moderate degrees of moisture such as partially protected outdoor locations and some basements.


The Third Condition:
It requires grounding of metal parts when in electrical contact with metal.


The Fourth Condition:
It covers grounding in hazardous locations. These high-risk locations are covered in Articles 500 through 517 of the NEC and installers and designers of electrical systems should review these articles prior to designing or installing electrical systems in these types of locations.


The Fifth Condition:
It requires exposed noncurrent-carrying metal parts of fixed equipment to be grounded anytime such equipment is supplied by a wiring method that provides an equipment grounding conductor (metal-clad, metal-sheathed, metal raceways or any other wiring method which has provisions for an equipment grounding conductor).


The Sixth Condition:
 It requires that where fixed equipment operates with any terminal at over 150 volts to ground, any exposed noncurrent-carrying parts of such equipment must be grounded.



Note:

These six conditions provide the general guidelines for grounding exposed metal parts. There are several exceptions to these guidelines but in general, these provisions ensure that noncurrent-carrying metal parts are grounded to protect personnel from the risk of electrical shock.







2.2 Specific Equipment Fastened In Place (Fixed) or Connected By Permanent Wiring Methods

  • The NEC contains provisions under which exposed noncurrent-carrying metal parts of specific fixed equipment shall be grounded. NEC Section 250.112 requires that these metal parts in the following equipment must be grounded:


  1. Switchboard frames and structure,
  2. Pipe organs,
  3. Motor frames,
  4. Enclosures for motor controllers,
  5. Elevators and cranes,
  6. Garages, theaters and motion picture studios,
  7. Electric signs,
  8. Motion picture projection equipment,
  9. Power limited remote-control, signaling and fire alarm circuits,
  10. Lighting fixtures,
  11. Skid-Mounted Equipment,
  12. Motor-operated water pumps,
  13. Metal well casings.


  • In general, any exposed noncurrent-carrying metal parts associated with any of the above mentioned specific equipment shall be grounded. Of course, there are some exceptions to these general provisions. Designers and installers of electrical systems who plan to work on these specific types of equipment should reference the NEC for specific application guidelines.








2.3 Equipment Connected By Cord and Plug

  • NEC Section 250.114 contains the provisions for grounding cord-and-plug connected equipment. In general, four conditions exist under which exposed noncurrent carrying metal parts of cord-and-plug connected equipment, which is likely to become energized, shall be grounded:


The First Condition:
It requires grounding in hazardous locations. These high-risk locations are covered in Articles 500 through 517 of the NEC and installers and designers of electrical systems should review these articles prior to designing or installing electrical systems in these types of locations.


The Second Condition:
It covers equipment which operates at over 150 volts to ground. As with fixed equipment, there are several exceptions for this provision, such as for motors, metal frames of electrically heated appliances and listed equipment which incorporates double insulation systems.


The Third Condition:
It applies to cord-and-plug connected equipment installed in residential occupancies. All of the following equipment, when installed in residential occupancies, must be grounded:

  1. Refrigerators, freezers, and air conditioners,
  2. Clothes-washing, clothes-drying, dish-washing machines; ranges; kitchen waste disposers; information technology equipment; sump pumps and electrical aquarium equipment,
  3. Hand-held motor-operated tools, stationary and fixed motor-operated tools, and light industrial motor operated tools,
  4. Motor-operated appliances of the following types: hedge clippers, lawn mowers, snow blowers, and wet scrubbers,
  5. Portable handlamps.



The Fourth Condition:
It applies to cord-and-plug connected equipment in other than residential occupancies. All of the following equipment, when installed in other than residential occupancies, must be grounded:

  1. Refrigerators, freezers, and air conditioners,
  2. Clothes-washing, clothes-drying, dish-washing machines; information technology equipment; sump pumps and electrical aquarium equipment,
  3. Hand-held motor-operated tools, stationary and fixed motor-operated tools, and light industrial motor-operated tools,
  4. Motor-operated appliances of the following types: hedge clippers, lawn mowers, snow blowers, and wet scrubbers,
  5. Portable hand lamps,
  6. Cord-and-plug-connected appliances used in damp or wet locations or by persons standing on the ground or on metal floors or working inside of metal tanks or boilers,
  7. Tools likely to be used in wet or conductive locations.



Fourth Condition Exceptions:
There are two exceptions from grounding in other than residential occupancies:


The first exception: permits tools and portable lamps used in wet or conductive locations to be ungrounded provided the tool or lamp is supplied through an isolating transformer with an ungrounded secondary of not over 50-volts.


The second exception: permits hand-held, motor-operated tools, stationary and fixed motor-operated tools, light industrial motor-operated tools and appliances to be operated ungrounded provided they are listed and they employ a system of double insulation which is distinctively marked on the tool or appliance.







Exception to NEC Section 250.114

An exception to Section 250.114 does permit listed tools and appliances that use a system of double insulation to be operated ungrounded.







2.4 Nonelectrical Equipment

  • NEC section 250.116 states that the metal parts of the following nonelectrical equipment shall be connected to the equipment grounding conductor:


  1. Frames and tracks of electrically operated cranes and hoists,
  2. Frames of nonelectrically driven elevator cars to which electrical conductors are attached,
  3. Hand-operated metal shifting ropes or cables of electric elevators.


  • Because metal siding on buildings is not electrical equipment, it is outside the scope of the Code [see 90.2(A)] and, therefore, is not subject to the Code provisions for grounding and bonding.

  • However, luminaires, signs, or receptacles installed on buildings with metal siding could cause the siding to become energized. Grounding of metal siding, while not required, does reduce the risk of shock to persons who may come in contact with siding that has become energized.





Now, we finish the forth grounding system division existing in any building: Equipment grounding and bonding as we have five divisions of grounding system in any building which are:





Grounding System Subdivisions

Grounding system in any building can be broken down into several subdivisions:

  1. The building exterior grounds,
  2. The electrical service grounding,
  3. The building interior bonding,
  4. Equipment grounding and bonding,
  5. Lightning protection.






In the next Article, I will explain The Methods for Performing of Grounding System Design Calculations. Please, keep following.



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