What is the difference between Bonding, Grounding and Earthing?

Introduction

One of the most misunderstood and confused concept is difference between Bonding, Grounding and Earthing. Bonding is more clear word compare to Grounding and Earthing, but there is a micro difference between Grounding and Earhing.

Earthing and Grounding are actually different terms for expressing the same concept.

Ground or earth in a mains electrical wiring system is a conductor that provides a low impedance path to the earth to prevent hazardous voltages from appearing on equipment. Earthing is more commonly used in Britain, European and most of the commonwealth countries standards (IEC, IS), while Grounding is the word used in North American standards (NEC, IEEE, ANSI, UL).

We understand that Earthing and Grounding are necessary and have an idea how to do it but we don’t have crystal clear concept for that. We need to understand that there are really two separate things we are doing for same purpose that we call Grounding or Earthing. The Earthing is to reference our electrical source to earth (usually via connection to some kind of rod driven into the earth or some other metal that has direct contact with the earth).

The grounded circuits of machines need to have an effective return path from the machines to the power source in order to function properly (Here by Neutral Circuit).

In addition, non-current-carrying metallic components in a System, such as equipment cabinets, enclosures, and structural steel, need to be electrically interconnected and earthed properly so voltage potential cannot exist between them. However, troubles can arise when terms like “bonding”, “grounding”, and “earthing” are interchanged or confused in certain situations.

In TN Type Power Distribution System, in US NEC (and possibly other) usage: Equipment is earthed to pass fault Current and to trip the protective device without electrifying the device enclosure. Neutral is the current return path for phase. These Earthing conductor and Neutral conductor are connected together and earthed at the distribution panel and also at the street, but the intent is that no current flow on earthed ground, except during momentary fault conditions.


Here we may say that Earthing and grounding are nearly same by practice.
But In the TT Type Power Distribution System (in India) Neutral is only earthed (here it is actually called Grounding) at distribution source (at distribution transformer) and Four wires (Neutral and Three Phase) are distributed to consumer. While at consumer side all electrical equipment body are connected and earthed at consumer premises (here it is called Earthing).
Consumer has no any permission to mix Neutral with earth at his premises here earthing and grounding is the different by practice.

In both above case Earthing and Grounding are used for the same Purpose. Let’s try to understand this terminology one by one.

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Bonding

Bonding is simply the act of joining two electrical conductors together. These may be two wires, a wire and a pipe, or these may be two Equipments. Bonding has to be done by connecting of all the metal parts that are not supposed to be carrying current during normal operations to bringing them to the same electrical potential.
Bonding ensures that these two things which are bonded will be at the same electrical potential. That means we would not get electricity building up in one equipment or between two different equipment. No current flow can take place between two bonded bodies because they have the same potential.

Bonding itself, does not protect anything. However, if one of those boxes is earthed there can be no electrical energy build-up. If the grounded box is bonded to the other box, the other box is also at zero electrical potential.

It protects equipment and person by reducing current flow between pieces of equipment at different potentials.

The primary reason for bonding is personnel safety, so someone touching two pieces of equipment at the same time does not receive a shock by becoming the path of equalization if they happen to be at different potentials. The Second reason has to do with what happens if Phase conductor may be touched an external metal part.
The bonding helps to create a low impedance path back to the source. This will force a large current to flow, which in turn will cause the breaker to trip.
In other words, bonding is there to allow a breaker to trip and thereby to terminate a fault.

Bonding to electrical earth is used extensively to ensure that all conductors (person, surface and product) are at the same electrical potential. When all conductors are at the same potential no discharge can occur.


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Practices for grounding and bonding of cable trays

Metallic Cable Trays

Cable tray may be used as the Equipment Grounding Conductor (EGC) in any installation where qualified persons will service the installed cable tray system. There is no restriction as to where the cable tray system is installed. The metal in cable trays may be used as the EGC as per the limitations of table 392.60(A).
All metallic cable trays shall be grounded as required in Article 250.96 regardless of whether or not the cable tray is being used as an equipment grounding conductor (EGC).

The EGC is the most important conductor in an electrical system as its function is electrical safety.

 

 

Grounding and bonding of cable trays

There are three wiring options for providing an EGC in a cable tray wiring system:

1. An EGC conductor in or on the cable tray.
2. Each multi-conductor cable with its individual EGC conductor.
3. The cable tray itself is used as the EGC in qualifying facilities.

Correct bonding practices

To assure that the cable tray system is properly grounded

If an EGC cable is installed in or on a cable tray, it should be bonded to each or alternate cable tray sections via grounding clamps (this is not required by the NEC® but it is a desirable practice)
In addition to providing an electrical connection between the cable tray sections and the EGC, the grounding clamp mechanically anchors the EGC to the cable tray so that under fault current conditions the magnetic forces do not throw the EGC out of the cable tray.

A bare copper equipment grounding conductor should not be placed in an aluminum cable tray due to the potential for electrolytic corrosion of the aluminum cable tray in a moist environment.

For such installations, it is best to use an insulated conductor and to remove the insulation where bonding connections are made to the cable tray, raceways, equipment enclosures, etc. with tin or zinc plated connectors.

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18 Key Terms Defined In NEC System Grounding Requirements

System Grounding Arrangement

The topic of system grounding is extremely important, as it affects the susceptibility of the system to voltage transients, determines the types of loads the system can accommodate, and helps to determine the system protection requirements.
The system grounding arrangement is determined by the grounding of the power source. For commercial and industrial systems, the types of power sources generally fall into four broad categories:

1. Utility Service – The system grounding is usually determined by the secondary winding configuration of the upstream utility substation transformer.
2. Generator – The system grounding is determined by the stator winding configuration.
3. Transformer – The system grounding on the system fed by the transformer is determined by the transformer secondary winding configuration.
4. Static Power Converter – For devices such as rectifiers and inverters, the system grounding is determined by the grounding of the output stage of the converter.

Categories 1 to 4 fall under the NEC definition for a “separately-derived system”. The recognition of a separately-derived system is important when applying NEC requirements to system grounding. The National Electrical Code does place constraints on system grounding.

As a starting point, 18 key terms from the NEC need to be defined:

1. Ground

A conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth or to some body that serves in place of the earth.

Ground definition (photo credit: ibiblio.org)

2. Grounded

Connected to earth or to some body that serves in place of the earth.

3. Effectively Grounded

Intentionally connected to earth through a ground connection or connections of sufficiently low impedance and having sufficient current-carrying capacity to prevent the buildup of voltages that may result in undue hazards to connected equipment or to persons.

4. Grounded Conductor

A system or circuit conductor that is intentionally grounded.

Grounding simple scheme (photo credit: diy.stackexchange.com)

5. Solidly Grounded

Connected to ground without inserting any resistor or impedance device.

6. Grounding Conductor

A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes.

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9 Recommended Practices for Grounding

Basis for safety and power quality

Grounding and bonding are the basis upon which safety and power quality are built. The grounding system provides a low-impedance path for fault current and limits the voltage rise on the normally non-current-carrying metallic components of the electrical distribution system.
 
During fault conditions, low impedance results in high fault current flow, causing overcurrent protective devices to operate, clearing the fault quickly and safely. The grounding system also allows transients such as lightning to be safely diverted to earth.
Bonding is the intentional joining of normally non-current-carrying metallic components to form an electrically conductive path. This helps ensure that these metallic components are at the same potential, limiting potentially dangerous voltage differences.

 
Careful consideration should be given to installing a grounding system that exceeds the minimum NEC requirements for improved safety and power quality.

 

Recommended practices for grounding

1. Equipment Grounding Conductors
2. Isolated Grounding System
3. Branch–Circuit Grounding
4. Ground Resistance
5. Ground Rods
6. Ground Ring
7. Grounding Electrode System
8. Lightning Protection System
9. Surge Protection Devices (SPD) (formerly called TVSS)

 

1. Equipment Grounding Conductors

The IEEE Emerald Book recommends the use of equipment-grounding conductors in all circuits, not relying on a raceway system alone for equipment grounding. Use equipment grounding conductors sized equal to the phase conductors to decrease circuit impedance and improve the clearing time of overcurrent protective devices.

Equipment grounding conductor

Bond all metal enclosures, raceways, boxes, and equipment grounding conductors into one electrically continuous system. Consider the installation of an equipment grounding conductor of the wire type as a supplement to a conduit-only equipment grounding conductor for especially sensitive equipment.
The minimum size the equipment grounding conductor for safety is provided in NEC 250.122, but a full-size grounding conductor is recommended for power quality considerations.

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9 Recommended Practices for Grounding

Basis for safety and power quality

Grounding and bonding are the basis upon which safety and power quality are built. The grounding system provides a low-impedance path for fault current and limits the voltage rise on the normally non-current-carrying metallic components of the electrical distribution system.
 
During fault conditions, low impedance results in high fault current flow, causing overcurrent protective devices to operate, clearing the fault quickly and safely. The grounding system also allows transients such as lightning to be safely diverted to earth.
Bonding is the intentional joining of normally non-current-carrying metallic components to form an electrically conductive path. This helps ensure that these metallic components are at the same potential, limiting potentially dangerous voltage differences.

 
Careful consideration should be given to installing a grounding system that exceeds the minimum NEC requirements for improved safety and power quality.

 

Recommended practices for grounding

1. Equipment Grounding Conductors
2. Isolated Grounding System
3. Branch–Circuit Grounding
4. Ground Resistance
5. Ground Rods
6. Ground Ring
7. Grounding Electrode System
8. Lightning Protection System
9. Surge Protection Devices (SPD) (formerly called TVSS)

 

1. Equipment Grounding Conductors

The IEEE Emerald Book recommends the use of equipment-grounding conductors in all circuits, not relying on a raceway system alone for equipment grounding. Use equipment grounding conductors sized equal to the phase conductors to decrease circuit impedance and improve the clearing time of overcurrent protective devices.

Equipment grounding conductor

Bond all metal enclosures, raceways, boxes, and equipment grounding conductors into one electrically continuous system. Consider the installation of an equipment grounding conductor of the wire type as a supplement to a conduit-only equipment grounding conductor for especially sensitive equipment.
The minimum size the equipment grounding conductor for safety is provided in NEC 250.122, but a full-size grounding conductor is recommended for power quality considerations.

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Practices for Grounding and Bonding of Cable Trays

Grounding and bonding of cable trays (on photo: Ground wire connected to cable tray; photo credit: solarprofessional.com)

Metallic Cable Trays

Cable tray may be used as the Equipment Grounding Conductor (EGC) in any installation where qualified persons will service the installed cable tray system. There is no restriction as to where the cable tray system is installed. The metal in cable trays may be used as the EGC as per the limitations of table 392.60(A).
All metallic cable trays shall be grounded as required in Article 250.96 regardless of whether or not the cable tray is being used as an equipment grounding conductor (EGC).

The EGC is the most important conductor in an electrical system as its function is electrical safety.

Grounding and bonding of cable trays

There are three wiring options for providing an EGC in a cable tray wiring system:

1. An EGC conductor in or on the cable tray.
2. Each multi-conductor cable with its individual EGC conductor.
3. The cable tray itself is used as the EGC in qualifying facilities.

Correct Bonding Practices To Assure That The Cable Tray System Is Properly Grounded

If an EGC cable is installed in or on a cable tray, it should be bonded to each or alternate cable tray sections via grounding clamps (this is not required by the NEC® but it is a desirable practice). In addition to providing an electrical connection between the cable tray sections and the EGC, the grounding clamp mechanically anchors the EGC to the cable tray so that under fault current conditions the magnetic forces do not throw the EGC out of the cable tray.
A bare copper equipment grounding conductor should not be placed in an aluminum cable tray due to the potential for electrolytic corrosion of the aluminum cable tray in a moist environment. For such installations, it is best to use an insulated conductor and to remove the insulation where bonding connections are made to the cable tray, raceways, equipment enclosures, etc. with tin or zinc plated connectors.

NEC Table 250.122 - Minimum size equipment grounding conductors for grounding raceway and equipment

Table 2 – Minimum size equipment grounding conductors for grounding raceway and equipment

Aluminum Cable Tray Systems

Table 392.60(A) – Metal area requirements for cable trays used as equipment grounding conductors

Metal area requirements for cable trays used as equipment grounding conductors

For Sl units: 1 square inch = 645
* Total cross-sectional area of both side rails for ladder or trough cable trays or the minimum cross-sectional area of metal in channel cable trays or cable trays of one-piece construction.
** Steel cable trays shall not be used as equipment grounding conductors for circuits with ground-fault protection above 600 amperes. Aluminum cable trays shall not be used as equipment grounding conductors for circuits with ground-fault protection above 2000 amperes.

Table 392.60(A) “Metal Area Requirements for Cable Trays used as Equipment Grounding Conductors” shows the minimum cross-sectional area of cable tray side rails (total of both side rails) required for the cable tray to be used as the Equipment Grounding Conductor (EGC) for a specific Fuse Rating, Circuit Breaker Ampere Trip Rating or Circuit Breaker Ground Fault Protective Relay Trip Setting.
These are the actual trip settings for the circuit breakers and not the maximum permissible trip settings which in many cases are the same as the circuit breaker frame size.

If the maximum ampere rating of the cable tray is not sufficient for the protective device to be used, the cable tray cannot be used as the EGC and a separate EGC must be included within each cable assembly or a separate EGC has to be installed in or attached to the cable tray.

For specific areas requiring bonding for electrical continuity, refer to Figures 1-4.

Figure 1 left: Expansion splice plates; Figure 2 right: Horizontal adjustable plates

Figure 3 left: Discontinuos segments; Figure 4 right: Cable tray sections vertical adjustable splice plate

Non-metallic cable trays do not serve as a conductor. It is also recommended that wire mesh cable trays not be used as an equipment grounding conductor.
Although permitted by the NEC, it is recommended due to the unique nature of the wire mesh, fittings are manufactured in the field from straight sections by cutting away the current carrying structural wires, reducing the current-carrying capability of the system. As such, the use of wire mesh cable trays as an equipment grounding conductor is not recommended.

If the wire mesh cable tray is to be used as an equipment grounding conductor, then the installation of a ground wire is recommended.

If a wire mesh cable tray is supporting cable with a built-in equipment grounding conductor or control or signal cables, then the tray should have a low impedance path to a non-system ground to reduce noise and remove induced or stray currents. A separate grounding cable attached to the wire mesh cable tray is not usually required.