Safe earthing system

Earthing simply means connecting a part of an electrical appliance to the earth or soil. Good earthing practice is an important requirement for sound level electrical system protection, and though every electrical engineer would claim to know it well, it is a topic that it is less understood and often taken for granted. Without a good earthing practice, even the best protection gadgets may fail to operate, putting at risk the lives of human beings.


EARTHING Basics

When earthing was generated & engineers wanted to have common reference for their supply, the earth's conductive surface was the only convenient one available. One of the supply conductors would be conducted  to earth or ground, later to be considered as a reference - a practice called " System Earthing "


When a fault within an electrical device connects a live wire to an exposed conductive surface, then anyone coming in direct contact  with it while standing on earth would complete a circuit back to the earthed supply conductor and receive in electric shock. However, if it is exposed conductive surface would have been connected to earth, a practice called "Equipment Earthing". then it would offer a low resistance parallel path to not only prevent such a shock but also to offer a return path for the fault currents which would now be high enough to operate a protective mechanism that would clear / isolate the faulty circuit


Type of earthing

Depending on its usage & purpose, earth can be categorised into two basic types

                     1. Protective earth & equipment earthing

A protective earth (PE) is a conductor that grounds an equipment, & is used to prevent accidental electric shock. Such an earth keeps the exposed conductive surfaces of the equipment at earth potential and under normal conditions do not carry current. However, on a low impedance line to ground fault, heavy currents would flow causing a fuse to blow or a circuit breaker to trip, thereby protecting the circuit. Even if we have a high impedance line-to-ground fault, the small levels of faults currents through earth is still sufficient to trip a residual current circuit breaker (RCCB) & help protect lives. PE is also called an "Equipment Grounding Conductor".

The process of connecting a PE to the non-current-carrying metal parts ( Eg. metallic enclosure) of the electrical equipments is call "EQUIPMENT EARTHING"

                     2. functional earth & system earthing

A functional earth connection series a purpose other than shock protection, and may carry current under normal conditions.

It is typically used for measurements in measuring instruments or for noise filtering in EMI filters or quenching surges in surge suppressors. Probably the most widely used example of functional earth is the Neutral in an electrical supply system which is a current carrying conductor. The neutral conductor is connected to earth often at only one point to avoid earth currents. For this reason, a neutral is also called a grounded supply conductor.

The process of connecting a neutral conductor to earth is called "System Earthing"  


EARTHING arrangements
For describing earthing arrangements, the following notations are used ( Refer Fig # 1)
                           T     :       Direct connection of a point with earth ( terra ) 
                           I      :       isolated or no pint is connected with earth
                           N     :       direct connection to neutral 

The IEC 60364 distinguishes 3 families of earthing arrangements that are in place around the globe using the " TWO LETTER CODES "  - TN, TT & IT.

While the first letter indicates the connection between earth & the power supply equipment ( Generator or Transformer ), the second letter indicates the connection between earth and the electrical device being suppled                                                                                                         Continued .........2 

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