Introduction to Earthing System

1- Introduction


Every Building, equipment, power plants, substations and/or facilities included in Electrical Utility System that use electricity require earth grounding, either directly or through a grounding system. (see fig.1)






Fig (1)






Definition:

The Earthing System: sometimes simply called ‘earthing’, is the total set of measures used to connect an electrically conductive part to earth. The earthing system is an essential part of power networks at both high- and low-voltage levels.


Functions of Earthing Systems
A good earthing system is required for:
  1. Protection of buildings and installations against lightning
  2. Safety of human and animal life by limiting touch and step voltages to safe values
  3. Electromagnetic compatibility (EMC) i.e. limitation of electromagnetic disturbances
  4. Correct operation of the electricity supply network and to ensure good power quality.


Earting Main Divisions
The earthing is broadly divided as:
  • System earthing: Connection between part of plant in an operating system like LV neutral of a Power Transformer winding and earth.
  • Equipment earthing (Safety grouding): Connecting frames of equipment (like motor body, Transformer tank, Switch gear box, Operating rods of Air break switches, etc) to earth.



Rule of Electrical Inspector in Inspecting Earthing System
As an electrical inspector, you must verify correct grounding system installation and operation by doing the following:
  1. Visually inspecting conductors and,
  2. Measuring ground resistance.



The need for Testing Earthing Systems
Test the ground system resistance to provide the only concrete proof that the preliminary design assumption is accurate and that the system is adequate and totally effective.

Measurements of ground resistance or impedance and potential gradients on the surface of the earth that are due to ground currents are necessary for the following reasons:
  • To verify the adequacy of a new ground system
  • To detect changes in an existing grounding system
  • To determine hazardous touch  and step voltages
  • To determine ground potential rise  (GPR) in order to design protection for power and communication circuits



A totally effective Earthing system
A grounding system to be totally effective, must satisfy the following conditions:
  • Provide low impedance path to ground for personnel and equipment protection and effective circuit relaying.
  • Withstand and dissipate repeated fault and surge current.
  • Provide correction allowance or correction resistance to various soil chemicals to ensure continuous performance during life of the equipment being protected.
  • Provide rugged mechanical properties for easy driving with minimum effort and rod damage.




How Earthing system works
  • The grounding system is essential to complete an electrical path to ground if there is non-designed or unanticipated above-normal potential current or voltage surges during operating conditions. Personal injury, death or equipment damage can result if the grounding system is not designed and installed properly to guide these potentially dangerous charges safely to ground.
  • The grounding systems under normal conditions carry NO current. The only time they carry current is under abnormal conditions when an electrical appliance or piece of electrical equipment is faulty and has become a potential shock or fire hazard. Under a fault condition the grounding conductor that is connected to the outer shell or sheet metal of the equipment or appliance must be able to provide a very low resistance path back to the source of the power (utility company's transformer) so that enough current will flow causing a breaker or fuse to open the circuit and automatically disconnect the hazard from the system.


  • It is NOT the purpose of grounding system to send current through the ground. Sending equipment fault currents through the earth can be a fatal misunderstanding of how a grounding system works. For the most part, the only time you intentially send current into the earth is during a lightning strike or line surge due to a nearby lightning strike.



In any discussion of grounding, the question always asked is how in low resistance ground should be? The answer is by determining the lowest possible ground resistance, the lower the ground resistance, the safer the grounding.



2.1 Determining Ground Resistance


Every Building, substation, equipment operation, and facility that uses electrical power requires a grounding system for safe and proper operation.




Importance of Ground Resistance
It is essential that the grounding system be of the proper resistance to maintain safety to personnel and equipment. And to verify this, you will perform the following:
  • Accurately measure ground resistance of a grounded system facility and or components.
  • Calculate ground resistance of a designed or completed ground system.



Definition:
Ground Resistance: is the measure of resistance between a grounded system in its entirety or in part, and the soil.

You do this by using the grid electrode at the center of the grounded system and the earth at a determined distance away from the system. Record all measurements in ohms  (Ω).



Note:
As an inspector, you must make ground resistance measurements during electrical inspections to verify safe operating limits of grounded systems and their components. The initial measurements will often be of a complete system.



Electrode Ground Resistance Components
The resistance of a ground electrode has 3 basic components:



1- The resistance of the ground electrode itself and the connections to the electrode:
The resistance of the ground electrode and it's connection is generally very low, ground rods are generally made of highly conductive/ low resistance material such as copper of copper clad.
2- The contact resistance of the surrounding earth to the electrode:
The contact resistance of the earth to the electrode: The Bureau of Standards has shown this resistance to be almost negligible providing that the ground electrode is free from paint, grease etc. and that the ground electrode is in firm contact with the earth.
3- The resistance of the surrounding body of earth around the ground electrode:
The ground electrode is surrounded by earth which is made up of concentric shells all having the same thickness.


Ground Resistance Values
The ohmic values of ground resistance objectives vary from industry to industry as follows:
  • For distribution systems above 600 V, the ground resistance from any point of grounding connection must not exceed 2 ohms for a direct metallic path.
  • For systems 600 V and below, the NEC, Article 250-84, specifies  25 ohms as the maximum value for an electrode at a consumer's premises before an additional grounding electrode is required.
  • For substation grounding having equipment operating at a nominal voltage exceeding 1,000 volts, a ground grid or loop must meet the requirements of IEEE-80. The design must meet the requirements for step, touch, and transferred potentials.
  • Telecommunications industry has often used 5 ohms or less as their value for grounding and bonding.


The goal in ground resistance values is to achieve the lowest ground resistance value possible that makes sense economically and physically.



2.2 Soil Resistivity


As an electrical inspector, you should be familiar with soil resistance in order to make calculations for ground resistance layouts, when needed.




Definition:
Soil resistivity ρ (specific earth resistance): is the resistance, measured between two opposite faces, of a one-metre cube of earth. The earth resistivity is expressed in Ωm.



Factors Affecting Soil Resistivity ρ
Soil resistivity depends on:
  1. soil composition,
  2. moisture,
  3. Temperature.

It stands to reason that soil resistivity will vary throughout the year in those areas where seasonal changes bring about a change in the moisture and temperature content of the soil. For a grounding system to be effective it should be designed to withstand the worst possible conditions.



Determination of Soil Resistivity ρ Values
One of the first requirements for a new-grounded installation site or upgrade is to measure the soil resistance. You measure from the soil itself. No outside influences from buildings, pipelines, high lines, grids, or other installed (fabricated) grounds should be near whenever possible.
In spite of the relatively simple definition of ρ given above, the determination of its value is often a complicated task for two main reasons:
  1. The ground does not have a homogenous structure, but is formed of layers of different materials
  2. The resistivity of a given type of ground varies widely (as in below Table) and is very dependent on moisture content.




The other problem in determining soil resistivity is the moisture content, (see fig.2) which can change over a wide range, depending on geographical location and weather conditions, from a low percentage for desert regions up to about 80% for swampy regions. The earth resistivity depends significantly on this parameter.

Fig (2)




Note:
Where no information is available about the value of ρ it is usually assumed ρ = 100 Ωm.

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