Earthing Systems Design steps – Part Six

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: Data Collection, Data Analysis, 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: The building exterior grounds, The electrical service grounding, The building interior bonding, Equipment grounding and bonding, 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:

• Introduction to Grounding System Design – Part One

• Introduction to Grounding System Design – Part Two

• Types of Earthing System – Part One
• Types of Earthing System – Part Two
• How to Select the Best Earthing System

• Earthing System Components – Part One

• Earthing System Components – Part Two

• Earthing System Components – Part Three
• Electrical Properties of the Earthing System

In Article " Earthing Systems Design steps – Part Five ", I 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: Bonding of Equipment for Services. Bonding for Other Systems Bonding Other Enclosures. Bonding for Over 250 Volts Bonding of Piping Systems and Exposed Structural Steel. Bonding of Interior Metal Columns and Beams

And I explained three subdivisions in this article and today I will continue explaining other subdivisions of The Building Interior Bonding.



4- Bonding for Over 250 Volts


4.1 The bonding requirements for electrical circuits which operate at over 250 volts to ground:


NEC Section 250-97 requires that such circuits be bonded to ensure electrical continuity of metal raceways or cable armors or sheaths that contain any conductor other than service conductors shall be ensured by one or more of the methods specified for services in 250.92(B), except for (B)(1).



4.2 Methods of Bonding for Circuits Over 250 Volts

First case: Where oversized concentric or eccentric knockouts are present in a box or an enclosure (see fig.1) The permissible methods which can be used to achieve the required bonding are: Threaded connections, Threadless couplings and connectors, Bonding jumpers, Other approved devices. Notes: These methods are the same as those used for service equipment with the exception of the grounded conductor which is not permitted for over 250 volt applications. If the box or enclosure has been listed for use with these concentric or eccentric locknuts it will be identified or labeled as such. If a box or enclosure is encountered and such identification is not provided, one of the methods listed above must be used and the exception is not applicable.

Fig-1

Second case: Where oversized concentric or eccentric knockouts are not encountered or where they are encountered in a box or enclosure which has been tested and listed as suitable for bonding The permissible methods which can be used to achieve the required bonding are: Threadless couplings and connectors for cables with metal sheaths, Two locknuts, on rigid metal conduit or intermediate metal conduit, one inside and one outside of boxes and cabinets, Fittings with shoulders that seat firmly against the box or cabinet, such as electrical metallic tubing connectors, flexible metal conduit connectors, and cable connectors, with one locknut on the inside of boxes and cabinets, Listed fittings. Note: The second case permits the connection of a raceway or cable to an enclosure without having to use special bonding hardware under any of the three methods.

5- Bonding of Piping Systems and Exposed Structural Steel

5.1 Metal Water Piping

5.1.1 Requirements for bonding interior metal water piping systems Section 250-104 of the NEC contains requirements for bonding interior metal water piping systems, other piping systems and structural steel. Metal water piping system(s) installed in or attached to a building or structure shall be bonded to the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor where of sufficient size, or to the one or more grounding electrodes used. The use of nonmetallic water piping mains can result in the interior metal piping system of a multiple-occupancy building to be isolated from ground and from the other occupancies. Therefore, the water pipe is permitted to be bonded to the panelboard or switchboard that serves only that particular occupancy. The grounded conductor of each separately derived system shall be bonded to the nearest available point of the metal water piping system(s) in the area served by each separately derived system. This connection shall be made at the same point on the separately derived system where the grounding electrode conductor is connected.

Metal Water Piping Bonding

Notes Installers of electrical systems should note that even if for some reason the metal water piping is not used as part of the grounding electrode system it is still required to be bonded per Section 250-104. The purpose of such bonding is to ensure that the metal water piping throughout the building or structure is at the same potential to ground as the service ground. Keeping the water piping at the same potential helps to ensure that an electrical shock hazard could not exist if the metal piping were to become inadvertently energized. A separate bonding jumper to the metal water piping system shall not be required where the metal water piping system is used as the grounding electrode for the separately derived system and the water piping system is in the area served. A separate water piping bonding jumper shall not be required where the metal frame of a building or structure is used as the grounding electrode for a separately derived system and is bonded to the metal water piping in the area served by the separately derived system.

5.2 Other Metal Piping

Requirements for bonding Other Metal Piping Any interior piping systems, such as, domestic well water, or any piping which contains a liquid or a gas, and “may become energized,” shall be bonded. Once again the permissible bonding locations are to the service equipment enclosure, the grounding electrode conductor, the service grounded conductor or the one or more grounding electrodes that comprise the grounding electrode system.

General Rule for Metal Piping Bonding If the interior metal piping system contains any electrical devices, such as, solenoids or mechanized valves, the piping “may become energized” and it should be bonded.

5.3 Exposed Structural Metal

Requirements for bonding Exposed Structural Metal Any Exposed structural metal that is interconnected to form a metal building frame and is not intentionally grounded or bonded and is likely to become energized shall be bonded to the service equipment enclosure; the grounded conductor at the service; the disconnecting means for buildings or structures supplied by a feeder or branch circuit; the grounding electrode conductor, if of sufficient size; or to one or more grounding electrodes used. Where exposed structural metal that is interconnected to form the building frame exists in the area served by the separately derived system, the grounded conductor of each separately derived system shall be bonded to the at the same point on the separately derived system where the grounding electrode conductor is connected.

Bonding of Exposed Structural Metal 

Notes This requirement does not apply to isolated steel girders or beams which may be installed in a building or structure. Such beams or girders are not “interconnected to form a steel building frame” and need not be bonded. A separate bonding jumper to the building structure metal shall not be required where the building structure metal is used as the grounding electrode for the separately derived system and the water piping system is in the area served. A separate bonding jumper to the building structural metal shall not be required where the water piping of a building or structure is used as the grounding electrode for a separately derived system and is bonded to the building structural metal in the area served by the separately derived system.

In the next Article, I will continue explaining the last Building Interior Bonding Division: Bonding of Interior Metal Columns and Beam. Please, keep following.

Back To Course EE-5: Grounding System Design Calculations