Electrical Safety Using Proper Earthing System

Earthing System Installation 


Effects of electric current on human beings: 

Danger to persons due to contact with live parts is caused by the flow of the current through the human body. The effects are: 

Tetanization: the muscles affected by the current flow involuntary contract and letting go of gripped conductive parts is difficult. Note: very high currents do not usually induce muscular tetanization because, when the body touches such currents, the muscular contraction is so sustained that the involuntary muscle movements generally throw the subject away from the conductive part; 


Breathing arrest: if the current flows through the muscles controlling the lungs, the involuntary contraction of these muscles alters the normal respiratory process and the subject may die due to suffocation or suffer the consequences of traumas caused by asphyxia; 


Ventricular fibrillation: the most dangerous effect is due to the superposition of the external currents with the physiological ones which, by generating uncontrolled contractions, induce alterations of the cardiac cycle. This anomaly may become an irreversible phenomenon since it persists even when the stimulus has ceased; 


Burns: they are due to the heating deriving, by Joule effect, from the current passing through the human body.


Time-current zones of the effects of alternating current on the human body | Source: ABB

Zone Designation, Refer to the photo above |  Source: ABB

This Standard gives also a related figure for direct current. By applying Ohm’s law it is possible to define the safety curve for the allowable voltages, once the human body impedance has been calculated. The electrical impedance of the human body depends on many factors. The above mentioned Standard gives different values of impedance as a function of the touch voltage and of the current path.

The earth fault modalities and the consequences caused by contact with live parts, are strictly related to the neutral conductor arrangement and to the connections of the exposed conductive parts. 

For a correct choice of the protective device, it is necessary to know which is the distribution system of the plant. IEC 60364-1 classifies the distribution systems with two letters.


Read: Earthing System According to BS 7671 and IEC 60364


In TT systems, the neutral conductor and the exposed conductive parts are connected to earth electrodes electrically independent; the fault current flows towards the power supply neutral point through earth. 


Earth Fault in TT Earthing System

In TT installations, the neutral conductor is connected to the supply star center, it is usually distributed and has the function of making the phase voltage (e.g. 230 V) available, useful for single-phase load supply. The exposed conductive parts, on the contrary, singularly or collectively, are locally connected to earth. TT systems are generally used for civil installations.


TN systems are typically used when the power supply is distributed to loads having their own electrical substation. The neutral conductor is directly earthed in the substation; the exposed conductive parts are connected to the same earthing point of the neutral conductor, and can be locally earthed. 

Three types of TN system are considered according to the arrangement of neutral and protective conductors:

  1. TN-C neutral and protective functions are combined in a single conductor (PEN conductor);
  2. TN-S neutral and protective conductors are always separated; 
  3. TN-C-S neutral and protective functions are combined in a single conductor in a part of the system (PEN) and are separated in another part (PE + N).

Earth Fault in TN System


In TN systems, the fault current flows towards the power supply neutral point through a solid metallic connection, practically without involving the earth electrode.

IT systems have no live parts directly connected to earth, but they can be earthed through a sufficiently high impedance. Exposed conductive parts shall be earthed individually, in groups or collectively to an independent earthing electrode. 

The earth fault current flows towards the power supply neutral point through the earthing electrode and the line conductor capacitance.

Earth Fault in IT System

IT distribution systems are used for particular plants, where the continuity of supply is a fundamental requirement, where the absence of the supply can cause hazards to people or considerable economical losses, or where a low value of a first earth fault is required. In these cases, an insulation monitoring device shall be provided for optical or acoustic signalling of possible earth faults, or failure of the supplied equipment.

Source:
  • Electrical Installation Handbook | Protection, Control and Electrical Device | pp. 440-444
  • Publisher: ABB
  • Download here


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