Electrical Grounding to most engineers, technicians & electricians is a simple subject and little attention is paid. First, the ground or grounding of a circuit is misnomer. In actuality, it’s connecting the circuit to a common point of reference; for most systems that is the earth either solidly or through impedance.
Earth or Ground : It is refereed as point of zero potential in electrical system. In normal condition it does not carry any current. It is mostly used for the protection of equipment and user.
Neutral : Neutral provides return path for AC current. In 2 phase system, it always carry current. In 3 phase system, it carries imbalance current or sometimes 3rd or 5th harmonic also.
Difference between Earth, Ground and Neutral:
There is no different between Earthing and Grounding. Grounding is the commonly used for earthing in the North American standards like IEEE, NEC, ANSI and UL etc. While European, Common wealth countries and Britain standards like IS and IEC etc uses term earthing.
In simple words, Earthing and Grounding are synonymous terms refers to same thing.
On the other hand Neutral is the return path which should ideally be at zero volts. Neutral is sometimes connected to ground especially in main panel.
- 1 I. Objective of Electrical Grounding
- 2 II. Types of Electrical Earthing
- 3 III. Types of Substation Earthing System:
- 4 IV. Electrical Grounding Testing methods
- 5 V. Corrective Measures for high Earth Resistance Values
- 6 VI. Earthing system facility
- 7 VII. Basic Terms:
I. Objective of Electrical Grounding
The object of an earthing system in a substation is to provide (under and around the substation) a surface which shall be at a uniform potential and near zero or absolute earth potential.
- It stabilizes circuit potential with respect to ground and limits the overall potential rise.
- It should protect the life and property from over-voltage.
- Electrical grounding provide low impedance path to fault current to ensure prompt and consistent operation of protective devices during the ground faults.
- It should keep the maximum voltage gradient along the surface inside and around the substation within safe limits during ground faults.
II. Types of Electrical Earthing
There are basically six earthing systems in use. These six systems are the equipment grounds, static grounds, systems grounds, maintenance grounds, electronic grounds and lightning grounds.
A system ground refers to the point in an electrical circuit that is connected to earth. This connection point is typically at the electrical neutral. The sole purpose of the system ground is to protect equipment.
An equipment ground is the physical connection to earth of non-current carrying metal parts. The purpose of such grounding technique is to ensure that all metal parts of equipment that may come in contact with the personnel are always at zero volts with respect to ground.
A static ground:
It is a connection made between a piece of equipment and the earth for the purpose of draining off static electricity charges before a flash over potential is reached. This type grounding system is utilized in dry materials handling, flammable liquid pumps and delivery equipment, plastic piping, and explosive storage facilities.
This type ground is utilized for safe work practices, and is a temporary ground.
Electronic and computer grounds:
Grounding for electronic equipment is a special case in which the equipment ground and the system ground are combined and applied in unity. Electronic equipment grounding systems must not only provide a means of stabilizing input voltage levels, but also act as the zero (0) voltage reference point
Grounding requirements for lightning protection depends upon the structure, equipment to be protected, and the level of lightning protection required of desired.
III. Types of Substation Earthing System:
A. H.V Substation Earthing
The earthing system for H.V substations comprises of an earthing mat buried horizontally at a depth of about half-a meter below the surface of ground and ground rods at suitable points. All non-current carrying parts contribute little towards lowering the ground resistance. The earth mat is connected to following in a substation:
- The neutral point of each system through its own independent earth.
- Equipment framework and other non-current carrying parts.
- The earth point of lightning arresters, capacitive voltage transformers, voltage transformers, coupling capacitors and the lightning down conductors in the substation through their permanent independent earth electrode.
- Substation fence.
Within the perimeter fence of the station, the ground mat should control all step and touch potentials to safe levels during a ground fault.
B. M.V Substation Earthing:
The earthing system of M.V substation comprises of various earth points connected via low resistance metallic conductors. The earthing system has underground vertical earth electrodes of steel pipes, horizontal earthing mesh of steel rods, all form welded at joints to form station earth.
Pipe electrode is usually in the form of 30 to 40 mm dia 2.5 m long galvanised iron pipe or solid mild-steel rod. These are used in substations and power plants.
C. Small Substation / Residential Earthing Method:
It has only one or two earthing electrodes. The earthing electrode is in the form of metal plate or a pipe or rod. It is buried in the ground with charcoal. To ensure low earth resistance, the surrounding soil and space of earthing rod is kept mist.
Plate electrode is usually in the form of 1 meter x 1 meter x 12 mm thick cast iron plate. Low voltage installation utilizes these grounding techniques.
IV. Electrical Grounding Testing methods
Earth resistance of earthing system is the resistance between the earthing electrode / mesh and the earth. It is in ohms. Two methods are used for the earth testing as:
- Earth Resistance tester
- High Current Injection
The purposes of earth testing are:
- Verify connection to earth.
- Verify impedance from point of use to earth connection.
- Inspect all connections especially to ground rod.
- Measure current in the grounding conductor. Should be less than 0.1% of phase current.
V. Corrective Measures for high Earth Resistance Values
If the earth resistance is high, than follow the following steps to rectify the problem
- Clean the surface of electrode.
- Mix the Soil in the earthing pit with charcoal, salt and soft soil mixed together.
- Pour sufficient water in the pit.
- Connect earthing rod with the equipment by low resistance copper strip.
- Use more number of electrodes
VI. Earthing system facility
Following are the points required to be connected to the earthing system in the substation.
To ensure the body, structure and other mom-current carrying parts are held at earth potential and are safe to touch. E.g. enclosures, cable sheaths, fence doors etc.
To ensure that neutral points are held at earth potential and return path is available t neutral currents E.g. star neutral of transformer, generator and motors.
To prevent electromagnetic noise in measurement, control, protection and communication signals. E.g. measuring instrument chassis, computer metal casing, control cable sheath etc.
To provide discharge path to capacitive currents, earth return currents, lightning and switching surges. E.g. surge arrester pad, earthing switch discharge pads, overhead shield earthing etc.
VII. Basic Terms:
This type of earthing diverts lightning current and 50-Periodic fault current. This ensures safe tripping during fault.
If a person standing on substation floor touches a faulted structures by raised fingers, the potential between his raised fingers and the feet is called touch potential.
If fault current flowing through the ground of the substation, a potential between two steps of a person standing on the ground is called step potential.
Typical Resistance Values of Substations for Various
Installations as Per NEC
|Installation||Type||Maximum Substation Grounding Resistance Values|
|Commercial||1. Metallic buildings
2. Wet Wells, etc.
|Less than 25 ohms|
|Industrial||1. General facilities
4. High Speed Loading facilities for chemical
Less than 1 ohm to 3 ohms
Less than 1 ohm
|Utilities||1. Generating stations
2. Large substations
3. Distribution substations
4. Small substations
1.5 – 5 ohms