Earthing Drawing

What is Earthing?

Earthing is a very important safety feature for electrical devices, circuits, and other things. Earthing is the process of transferring electrical energy immediately and directly to the ground through a low resistance channel.

Earthing Drawing1

Electrical earthing is the technique of immediately discharging electrical energy into the earth with the use of a low resistance wire.

What is the main purpose of earthing?

Earthing keeps you from getting an electric shock. Most of the time, electrical earthing or grounding is a way to connect the metal body of an electrical machine, panel, or piece of equipment to the earth or ground.
The non-current carrying section of the equipment, or the neutral of the supply system, is connected to the ground in order to complete the earthing of the electrical system. Most of the time, galvanized iron is used for the earthing.

The earthing makes it easy for the leakage current to get to the ground. The equipment’s short circuit current goes to the earth, which has no potential. So, it protects the system and equipment from damage.

What are the general requirements for earthing?

All electrical equipment should be properly “grounded,” or connected to the ground. In general, an earthing system should include the following:

  • For people’s safety, equipment must be grounded.
  • Neutral system grounding
  • Protection from static electricity and lightning strikes.
  • Separate clean earth system for UPS, Instrumentation, distributed process control (DCS), electrical control and monitoring (ECS), etc., as per requirements.
  • To keep the voltage as stable as possible in the healthy phase (If fault occurs on any one phase)
  • To act as a return conductor in electric traction systems and communication.
  • To keep electrical installation systems from catching fire.

What equipment is connected to the earth?

Most of the time, designing an earthing system for a plant should follow the rules for electricity standards. The earthing system must have an earthing network that is connected to the right number of earth electrodes. The following items should be anchored to the ground:

  • System neutral
  • The metal parts of electrical equipment that don’t carry electricity, like motors, lighting control panels, terminal boxes, light fixtures, adaptors, etc.
  • Structures made of steel, a loading platform, etc.
  • Lighting masts and poles, cable trays and racks.
  • The process equipment includes storage tanks, pressure vessels, and everything else.
  • Shields and armor for the cables.
  • Pump handling Hydrocarbon if its base plate is not the same as the base plate of the motor.
  • Incoming supply and primary/secondary HV distribution system earthing must meet design requirements. 
  • The 415V system’s neutral must be connected to the ground.

Metal structures with a thickness of more than 4.8 mm may not need lightning protection with an aerial rod and down conductors. At the base, they must be connected to the earthing system in at least two places.

What is the meaning of earth resistance?

This is the total resistance between the earth electrode and the ground in ohms (Ohms). Earth resistance is the algebraic sum of the earth continuous conductor, earthing lead, earth electrode, and earth resistance.

What is normal ground resistance?

The ideal resistance of a ground is zero ohms. There is no one standard ground resistance threshold that is recognized by all agencies.  But the NFPA and the IEEE say that the ground resistance should be less than 5 ohms

What should be the maximum value of resistance at the earthing point? 

The following should be the resistance values of an earthing system to the general mass of earth:

  • This value can’t be more than 1 Ohm for the electrical system and equipment, generating stations, and large substations.
  • 10 Ohms for all metal parts that don’t carry electricity but are made of metal and are part of a large electrical device or any metal object.
  • Earth resistance should be at least 5 ohms for lightning protection, but it shouldn’t be more than 10 ohms.
  • For the purpose of lighting protection, a resistance value of 5 ohm should ideally be used, but in no circumstances should it be more than 10 ohm.
  • All joints in pipelines, valves, plants, storage tanks, and other equipment and facilities used for oil must be made electrically continuous by bonding or another method. The resistance between each joint must not be more than 1 Ohm. 

Copper plate clean earth shall be provided for UPS, DCS, and other electronic instruments, including electronic relays.  This must be kept galvanically separate from the system and safety earthing.

 Every 20 meters, a GI earth strip must be laid along cable trays, welded to the main support for the cable tray, and connected to the earthing grid.

All multicore cables must have their metallic sheaths, screens, and armor connected to the earth at both the equipment and switchgear ends. The sheath and shield of single-core cables must only be connected to the earth at the switchgear end.

Types of earthing systems

In the process of plant electrical installation, there are two types of earthing systems: 

  1. Protective earthing (PE)
  2. Functional earthing (FE)
Earthing Drawing 2

The following explanations help establish a distinct line between functional earthing and protective earthing:

Functional earthing:

Earthing Drawing 3

 If any part of the functional earthing connection is broken, it won’t affect any kind of protection, safety measure, or provision for electrical safety. So, its main uses are in communication, measuring, and ensuring electromagnetic compatibility (EMC).

Protective earthing:

Earthing Drawing 4

 If any part of the protective earthing is broken, it affects the safety or function of a safety measure or provision made to keep electricity safe.

Functional earthing system

Functional earthing is a type of earthing that is an important part of an electrical installation and is necessary for making sure it works without problems. Interference current can be stopped, testing adapters can be grounded, and common reference potentials can be set for electrical devices and equipment.

Earthing Drawing 5

Where is functional earthing used?

  • An electromagnetically stable electrical system is constructed using function earth (FE).
  • Function earth (FE) is important for the smooth operation of electronic devices, especially in places with a lot of electromagnetic pollution, like processing plants.
  • Function earth (FE) is a very important part of how electrical plants work.
  • Some electronic equipment needs a reference voltage that is about the same as earth potential in order to work correctly. This reference voltage is given by the functional earthing conductor.
Earthing Drawing 6
  • Metal strips, flat braids, and cables with a circular cross section can all be used as conductors for functional earthing.
  • For high-frequency equipment, it is best to use metal strips or flat braids, and the connections should be as short as possible.
  • All protective and functional earthing conductors of an installation in a building must be connected to the main earthing terminal, unless a requirement specifically states anything different.

The diagrams below show a typical functional earthing system for a control system.

Earthing Drawing7

Protective earthing system

What is a protective earthing?

It is a type of earthing that is designed to protect individuals from electrical shock, and it is effective in the event that there is a malfunction in the system.

Protective earthing is the process of installing earthing conductors in a way that makes it less likely that someone will get hurt from an electrical fault in the system.  In the case of a malfunction, non-current-carrying metal portions of the system, such as frames, fences, and enclosures, might acquire high voltage with respect to the earth. If someone touches the equipment under these conditions, they will get an electric shock.

If the metallic portions are coupled to the protective earth, the fault current will pass via the earth conductor, and safety mechanisms will detect it. The circuit will then be securely isolated

How can a protected earthing system be achieved?

The following are some possible methods for achieving protective earthing:

  • Installing a protective earthing system where parts that conduct electricity are connected to the grounded neutral of the distribution system with conductors.
  • Installing overcurrent or earth leakage current protection devices that shut off the affected part of the installation after a certain amount of time or when the voltage reaches a certain limit.
Earthing Drawing 8

What is a protective earthing conductor?

The protective earthing conductor has to be able to carry the potential fault current for a period of time that is at least equivalent to the amount of time that the associated protective device is intended to be operational.

Protective (PE) conductors create the main equipotential bonding system by connecting all exposed and extra conductive parts of an installation. When the insulation between a phase conductor and an exposed conductive part fails, these conductors send fault current to the source’s grounded neutral. PE conductors are connected to the installation’s main earthing terminal.

The earthing conductor establishes a connection between the main earthing terminal and the earth electrode.

What does PE mean on a wiring diagram?

PE means protective earth conductors, and PE conductors are required to be:

Earthing Drawing 9
  • insulated, with stripes of yellow and green coloration
  • Defendable against both physical and chemical deterioration

It should be noted that construction for busways makes this possible (busbar trunking systems)

Conductors of PE must:

As shown in the below figures, connect each exposed conductor separately to the main PE conductor, not all at once.

Give each earthing bar on a distribution board its own terminal.

Earthing Drawing 10

The diagrams below show a typical protective earthing system for a control system.

Earthing Drawing 11

Typical earthing system of control loop is shown below

Earthing Drawing 12
  • All of the grounding busbars for the equipment in the Control Room (one each for shock protection, cable armouring, and shielding) are connected to the common point (star point) of the electric system.
Earthing Drawing 14
  • Cable armouring has to be linked to grounding busbars in the control room and structural steel parts in the field.
Earthing Drawing 15
  • In the control room, only one end of the shield must be connected to the ground. The shields of cables have to be connected to a grounding busbar.
Earthing Drawing 16
  • The protective ground network must be connected to all metal structures and equipment in the control building. 
  • The protective ground network must also be connected to the cable trays that bring new wires into the control building.
Earthing Drawing 17
  • Ground busbar “cable shielding” needs to be set up with the right insulation to keep it from touching other metal structures.
Earthing Drawing 19
  • Grounding lines are made of stranded single-core wires, and the wires should be joined with crimped wire-clips.

Difference between instrument earthing and electrical earthing

The term “Reference Earth” may also be used to refer to “Instrument Earth.” Instrument Earth every individual shield (screen), and over all shields, a single or multi pair wire should be segregated from electrical earthing owing to the need of preventing interference and terminated to various bus bars.

By means of a 25-square-millimeter green-yellow wire, the instrument bus bar will be linked to the grounding dispatcher.

A green-yellow wire measuring 70 square millimeters connects the grounding dispatcher to the earth pit.

Earthing in electrical systems is also known as “dirty earthing” and “protective earthing.” Its purpose is to prevent humans, electrical equipment, and the power system itself from being shocked by electric current.

Earthing Vs Grounding

Earthing VS Grounding
Human beings are shielded from the effects of electric shocks by using this technology.By using this strategy, the whole power system is safeguarded against malfunction.
The green wire is the earth wire.The grounding wire is black in color.
Earthing is mostly used to keep people from getting electric shocks.Grounding is mostly used to unbalance the electric system when it becomes overloaded.
Earthing is between the equipment body and the underground pit. It is under the earth pit.It is between the ground and the neutral point of the equipment being used.

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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