The connection for non-current carrying metal parts of electrical equipment to the earth, that metal parts associated with electrical installation apparatus or a machine to the ground. so that each time the immediate discharge of current takes place to the earth without danger.
Connecting of neutral point of three faces as system to the earth is called system earthing or neutral earthing.
Importance of Earthing / Why Earthing?
- for safety of the equipment against lightning of voltage surge by providing discharge path through arrester.
- If ensures that the non-current carrying metal parts are always at ground potential even through the insulation fails, so the person doesn’t feel shock when same in contact with the equipment.
- Providing a way to discharge or de-energize the transmission line or electric machine before starting maintenance on it.
- Safety purpose for home and industrial electrical and electronics equipment.
Parts of Electrical components that must be Earth.
- Tanks, frame and enclosure of electric machine such as transformer, motor, heater etc.
- Metal frame work of switch board, Metal lighting fixture, control Bord or individual control Bord, Metal cable jointing box.
- Sheaths of under-ground cables.
- Metal conduits carrying the live wire.
Components of equipment Earthing
a. Earth wire:
That parts of earthing system which inter connects the overall metallic parts of the electrical installation equipment switches distribution box, motor, generator, transformer enclosure etc. to the earth lead is known earth as continuity conductor or Earth wire.
b. Earthing joint:
The conducting wire connected between earth continuity conductor and Earth electrode is called earthing lead or earthing joints.
c. Earthing Electrode:
The metallic electrode or plate of pipe, plate or rod buries in the earth (Underground) in known as earthing electrode.
Terms Of Earthing
i. Earth resistance
It is the resistance offered by the electrode to the flow of current into the ground earth resistance doesn’t imply the contact resistance between electrode and soil but it imply the resistance of the soil between the electrode and the point of zero potential.
Note: The maximum permissable value of Earth resistance at different parts of the power station are:
Large power station = 0.5 Ohm
Major power station = 1 Ohm
Small power station = 2 Ohm
General case = 8 Ohm
Factors Affecting the Soil or Earth Resistance
In the presence of moisture in soil salts are more dissolved in soil, that where present in the solid form and hence, increases the conductivity of soil and decrease the soil resistance.
In the temperature is less than 0 Degree Celsius, the water starts to freeze and the increases the resistivity.
Conductivity of soil is greatly increased by increasing the amount of soluble salt in the soil.
A soil resistivity at a particular location also varies with the depth generally the lower layer have greater moisture content, so have low resistance.
Reason behind 3 pin in power socket
The 3 pin in the plug is the earth pin. It connected to the earth via a low-resistance path. All the equipment extended metal surface has a earth wire connected to the metal enclosure, so that all equipment case are connected together and to the earth making shock much less likely.
When a short circuit occurs in the faulty device, excess wire to trip and open the fuse thereby removing the load from the supply and making the appliance safe.
Types of electrode use for Earthing
1. Natural earth electrode:
It refers to any metallic conductor having a sufficient and constant surface with the earth. Thus it may be affected by used of metal piping covered with conductive anti-corrosive or heat resistive loading, metal cables e.tc
2. Artificial earth electrode:
It is also known as made electrode. Of may be sub divided into different electrode selected upon types at soil encountered and available depth.
3. Driven electrodes:
It is most common grounding device used in the field as they are economically satisfying. The standard driven electrodes are copper clod rod consisting of an 8-10 feet length depth steel with 5-10 coating of copper.
If is used extensively overtime as a grounding electrode. 10 feet of water pipe is in direct contact with the earth joints must be electrically continuous at least one additional electrode must be installed with water pipe electrode.
They are typically then copper plates buried in direct contact with earth. It should be at least 2 feet of surface area exposed to the surrounding soil. It should be buried at lest 30 inches below ground level.
It consist of mesh of steel rods buried horizontally 0.5M to 1M below the substance area and a vertically electrodes are located to the mesh in further vertical riser of the steel bars or copper strips etc are connected between the grounding mesh and the point to be Earthed.
The instrument used for earth resistance measurement
The resistance of earth electrodes can be measured by different methods as below:
i. Earth spike method:
The current is passed through (X) for test and another earthing having negligible resistance such as (Y) and (z) are provided 30feet away each often
Then, the current is measured ammeter and voltage by voltmeter
Earth resistance = E/I
ii. The Fall of potential method:
This method is same except position of strike is moveable measurement of (x) and of I are made for several position of spike (Y) from the earthing electrode under spike 2. The value of Earth distance of spike (Y) from the earthing electrode. The acceptable value corresponding to flat position.
Type Of Earthing And Method
- Pipe Earthing
- The size of a pipe it depends upon the fault current and condition of the soil. The moist an soft soil is available in the earth lead, the length of the pipe is at 2-2.5 Meter, Diameter is 38-40mm.
- Contact between inner surface of the pipe and the soil is good, some holes of diameter 10-15mm are made on the pipe at equal intervals along length.
- The pipe should be placed in the permanently wet ground
- depth s depends upon the moisture of the soil in the ground, some cases it should be buried at 5.75 meter below the ground level. But in that case there will be less moisture of soil is available either.
- Pipe at the lead should be surrounded by charcoal,broken piece of coke and salt for field up the height.
- There were this layers of charcoal are arranged in the earth lead for best result, the first layer there is salt, in the second layer there is salt again, in the fourth layer again charcoal and, so go on till the filled over.
- During dry summer there were moisture of the soil is considerably decrease and increase the earth resistance so in order to have effective earth arrangement must be provided in this season so, 3 or 4 bucket of water poured into the earth lead for increase the moisture of the soil.
- solid copper earth rods used in area with high corrosion or in applications were exceptionally long life are needed.
- stainless steel, earth rods is used where may be issue caused by galvanic corrosion because of dissimilar metals buried close together.
- Earthing rod is used to protected from electric shock.
- It is essentially a protective conductor for a fault current to flow to earth by providing a path for fault current.
- Depth is require upon the condition of the moisture and soil in the wet ground minimum 8 feet is required for the good result.
- Rod is placed usually close to the electrical main service panel and is often made of copper or copper coated steel
3. Plate Earthing
- In this case galvanized iron is used as a electrode.
- Size is not less than 60cm*60cm*6.35mm according to the international standard.
- if there were copper plate is use size must be at least 60cm*60cm*3.18mm and copper wire should be earth lead to ground height is minimum 3meter required.
- Depth is required upon the condition of the moisture of the soil, t must be placed on the permanently wet ground.
- periodical testing of earthing arrangement is necessary because , if the soil surrounding the plate became dry and hard the earthing itself became a source of danger.