Earthing Systems Design steps – Part Five


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 and Today I will explain Other Building’s Earthing System Divisions as follows.



You can preview the following Articles for more info:




Second: Electrical Service Grounding


Article 230 of the NEC contains the requirements for installing electrical services for buildings and dwellings.




Note:

Electrical designers, however, should keep in mind that local authorities, including local electrical utilities, often have requirements which supersede or augment the NEC. Electrical designers should contact the local authorities and determine if requirements for electrical services exist which differ from the NEC.





NEC Requirements for grounding electrical services


The requirements for grounding electrical services are contained in Article 250 of the NEC as follows:
1- Section 250-24(a) requires that a grounded electrical system, which supplies a building or structure, shall have at each service a grounding electrode conductor connected to the grounding electrode system.


2- The grounding electrode conductor shall also be connected to the grounded service conductor in order to:
  • Limit the voltage to ground imposed on the system by lightning, line surges, and (unintentional) high-voltage crossovers. 
  • Stabilize the voltage to ground during normal operation, including short circuits.


3- This connection may occur at any accessible point from the load end of the service drop or service lateral to the grounded conductor (neutral) terminal block in the service disconnecting means. There are Three locations where 250.24(A)(1) permits the grounding electrode conductor to be connected to the ground service conductor as follows (see fig.1):

  • At load end of service drop,
  • In accessible meter enclosure,
  • In service disconnecting means.


Fig-1


4- Outdoor installations (like Outdoor Transformers) must have a grounding electrode, either at the transformer or elsewhere outside the building which must be connected to the grounded service conductor. Fig.2 illustrates two grounding electrode connections, one at the service equipment installed inside the building and one installed at the transformer, located outside of the building.



Fig-2





Third: The Building Interior Bonding


Keep in mind that the purpose of bonding is different from that of grounding as in below table:


Bonding
Grounding
definition
Bonding is connecting Metallic components of electrical systems to ensure electrical continuity of the components and the capacity to conduct safely any fault Current likely to be imposed.
Grounding is an intentional connection to the earth or some other conducting body that serves in place of the earth.
The purpose
The purpose of bonding is to create an equipotential plane that ensures that all metallic components are at the same potential to ground.
The purpose of grounding conductive materials, such as metal raceways and equipment enclosures, is:

  1. To limit and stabilize the voltage to ground on such enclosures because Unintentional contact with higher voltage lines or lightning strikes results in increased voltages on the electrical equipment.
  2. To provide a low impedance path for ground-fault current. The low impedance path ensures that the overcurrent device which is protecting the conductors will operate.






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.




1- Bonding of Equipment for Services




Definition:

Service equipment is any equipment necessary for the main control and means of cutoff of the supply of electricity to a building or structure.



1.1 Requirements for Bonding of Equipment for Services


The following service equipment must be effectively bonded together:
  • Service raceways, cable trays, service cable armor/sheath, cablebus framework, service equipment enclosures and any metallic raceways which contain a grounding electrode conductor.
  • All enclosures containing service conductors, including meter fittings, boxes, or the like, interposed in the service raceway or armor





Note:

Standard locknuts, sealing locknuts, and metal bushings are not acceptable as the sole means for bonding a raceway or cable to an enclosure on the line side of the service disconnecting means regardless of the type of or condition of the knockout (see fig.3).  Electrical continuity has to be ensured through the use of a supply-side bonding jumper that connects the raceway to the enclosure.





Fig-3

1.2 Bonding Methods of Equipment for Services



Five basic methods are listed, any one of which can be used to bond the service equipment together:


  • The first method: using the grounded service conductor. On the line side of the service equipment there is no separate equipment grounding conductor. The grounded conductor assumes this role on the line side of the service. The permissible means for any connection made to the grounded conductor include CADWELD exothermic welded connections, listed pressure connectors (wirenuts), listed clamps, and other listed means.


  • The second method: using threaded connections. This includes threaded couplings or bosses. It is important that these connections be made wrenchtight to ensure a low impedance connection.


  • The third method: using threadless couplings or connectors. These fittings are available for rigid metal conduit, intermediate metal conduit and EMT. Once again it is important that the connections be made up wrenchtight to ensure the low impedance ground path. Installers should note that the NEC specifically prohibits the use of standard locknuts or bushings, even if a double arrangement is used, (one on the inside and one on the outside) to achieve the bonding required by this section.


  • The fourth method: using bonding jumpers. Bonding jumpers ensure electrical continuity by providing a low impedance path across concentric or eccentric knockouts that may be part of the service equipment.


  • The fifth method: using other approved devices. This would include fittings such as bonding-type locknuts and grounding bushings (see fig.4). These fittings are designed to make good contact with the metal enclosure and help to ensure good electrical continuity.



Fig-4




2- Bonding for Other Systems
  • Other systems (communication circuits, radio and television equipment and CATV circuits) may be present in the building or structure. An accessible means must be left at the service equipment, in an external location, which can be used for connecting bonding and grounding conductors of other systems (see fig.5). Recall NEC Section 250-58 requires a common grounding electrode system to be installed and prohibits separate grounding system installations to minimize the occurrence of potential differences in equipment in which multiple systems interface.




Fig-5



  • There are three permissible methods to facilitate the interconnection of these systems:

  1. The first method: using the exposed metallic service raceways. 
  2. The second method: is to connect to the exposed GEC. 
  3. The third method: is to bond a copper or other corrosion-resistant conductor of at least a No. 6 AWG copper, to the service raceway or equipment.




3- Bonding Other Enclosures

  • In addition to the service equipment enclosures, other noncurrent-carrying enclosures are also required to be bonded by the NEC. Section 250-96 requires that metal raceways, cable trays, cable armor, cable sheaths, enclosures, frames, fittings and any other metal noncurrent-carrying parts be bonded if they are to serve as grounding conductors. This requirement applies regardless of whether a supplementary equipment grounding conductor is present. 


  • The purpose of this rule is to ensure that these metallic components cannot become energized because they are isolated from a low impedance ground path. If these components were not properly bonded and they were to become energized due to some fault condition, the overcurrent device may not operate. This would result in personnel being put at risk to serious electrical shock hazards. 





Note:

Prior to making any bonding or grounding connection, Any nonconductive paint, enamel, or similar coating shall be removed from the metal surface to which they are making a connection. Failure to do so could drastically increase the impedance of the ground path.



In the next Article, I will continue explaining Other Building Interior Bonding Divisions. Please, keep following.




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