EARTHING OR GROUNDING SYSTEM - IEC
How Do Make Earthing/Grounding For Equipment?
We know earthing/grounding is very vital part of electrical power transmission and distribution system; but how can we make earthing connection? Yes, to make the earthing system uniform we must follow some standards. Here in this article we will focus on IEC types of earthing system.
What is Earthing or Grounding?
a general term used to describe the connection of metallic parts of an Electrical Installation or an appliance to earth or ground is called Earthing or Earthed. Notable that the terms ‘Grounding’ or ‘Grounded’ are sometimes used to mean the same as the above.
If you look at the most electrical power transmission and distribution system you will see several different three-phase voltage levels like- 400volts, 11000volts, 33000volts, 66000volts, 132000volts and so on.
Important point is that these all voltage levels of electricity coming from same source generator or transformer; but each level of voltage system is isolated from each other.
Transformers are isolating one from other and each isolated sub-station is earthed or grounded at one or multiple points that ensure the predetermined level of voltage and not allow to increase voltage difference.
The voltage difference may increase due to several causes, like as: fault occur between primary and secondary windings of transformer, lightning or surges may occur, static charges may buildup across the insulation etc. In high voltage system to reduce the induced voltage cable metallic screen or armour need to earthing.
Why Earthing or Grounding is Necessary in Electrical System?
There are three main reasons why it is necessary to earth, or to ground, the electrical equipment:
Keep the killer electricity safe, or to prevent electric shock to human operators, maintenance personnel and persons in the vicinity of electrical equipment;
Control the wild fault current, or to minimize damage to equipment when excessive current passes between the conductors and the casing or frame during an internal fault condition;
Destinies a reference, or to provide a point of zero reference potential in the power system for the conductors.
Types of Earthing/Grounding
There are basic five types (TNC, TNS, TNCS, TT & IT) of earthing in IEC standard that described below with diagram. In details referred to international standard IEC60364, part 4.
TNC Type Earthing: In TNC earthing system one additional earthing rod is used for equipment earthing where this earthing rod connected with transformer or generator earthing or neutral conductor.
TNS Type Earthing: In TNS type earthing system there not used any additional earthing rod for equipment like TNC system. Here additional earthing cable used to connect with generator or transformer earthing system.
TNCS Type Earthing: In TNCS system equipment earthed with generator or transformer system earthing or neutral wire as well as additional earthing rod near equipment.
TT Earthing System: In TT system additional earthing rod used to earth the equipment separately from generator or transformer earthing or neutral wire. System neutral wire connected with equipment but equipment is earthed by local earthing rod separately.
IT Earthing System: In IT earthing system equipment earthing and generator or transformer earthing is completely separated and there is no any inter connection. Generator or transformer are earthed by earthing resistor Rn or impedance Zn where equipment are earthed by local earth rod.
From the above five types of earthing system we get three common type TN, TT & IT, each of contains two letters where the first letter T or I and second letter N or T. The first letter denotes the source of power generator or transformer and the second letter denotes the consumer.
Source star connection of transformer or generator winding solidly earthed is denoted by first letter T where I denoted the star point and windings are isolated.
Similarly the second letter T & N for consumer, N further sub-divided S & C or NS & NC and NCS is the composite.
T denoted that consumer end is independently solid earthing from source earth; where N denoted impedance conductor is taken from source earth. C denoted neutral and earth conductors are common or same.
Earthing/Grounding Details In Definitions
Earth: the conductive mass of earth, whose electrical potential or voltage at any point is conventionally taken as zero.
Customer Earthed system: where the Customer provides a Main Earth Terminal for the Installation, which is connected to a sufficient number of local Earth Electrodes.
Distribution Company Earthed system: where the Distribution Company provides a connection to the Customer’s Main Earth Terminal, using the distribution network Earthing system, generally via the armouring or metallic sheath of the main incoming supply cable.
Earth Conductor: the protective conductors used to connect the Exposed Metallic Parts of an Electrical Installation and associated Appliances to Earth, via a Main Earth Terminal to local Earth Electrodes or the Distribution Company Earth.
Earth Electrode: a conductor or group of conductors in intimate contact with Earth, providing an electrical connection to Earth, and normally having a known and measurable value of Earth Resistance. It also be known as Earth Rod or Grounding Rod.
Earth Resistance: the resistance in Ohms of any point on an Installation to Earth, being measured using an approved testing device and approved procedure.
Main Earth Terminal: the main connection point at which the nominal value of Earth Resistance for an installation is taken, and at which Earth Conductors from the Earth Electrodes will be connected. This will normally be at or close to the Customer Connection Point.Functional Earth: an Earth or Earthing system which is provided for special functions such as reduction of radio frequency interference, noise filtering for computers etc. and which is separate from the main Earthing system of an Installation.
Earth Leakage Circuit Breaker (ELCB): a circuit breaker which is designed to open the phase and neutral conductors of a circuit upon detection of a leakage of current above a specified value through the Earth Conductor or through Extraneous Metallic Parts of an Installation.
Why Does Shock Electricity?
Electric shock occurs when actually two points of an electrical circuit exist unequal potential and come contact to a human body; and this potential difference exceeds a lower threshold value.
Very small value of current flow like 1 mA or less may slight perception of pain and for continuous flow for long time will cause of slight reaction on human body; increasing the value of current the reaction intensity will be greater.
If current limit accede 10 mA, human muscles become very difficult to control the reaction. When it come between 20mA to 50mA, it will cause difficulty of breathing.
Ventricular fibrillation may occur that leads to heart failure and death if current above 50mA to 100mA flow continuously through human body.
How Does Earthing/Grounding Protect Equipment From Damage?
Most of the electrical fault happen in the equipment like switchboard, circuit breaker, electric motor or connected any machineries’. Generally equipment body or casing become electrified and current flow through body due to electrical fault. If this fault current continue to flow then equipment or conductor insulation become damaged; the degree of damage depends on current flow duration. Electrical protective relay that detect fault current can be the solution for protect the equipment.