Protective Earthing Design

Electrical-Calculations
21 Jan, 2020


Grounding is one of the most important design aspects of an electric power system

Proper grounding contributes to the safety of equipment and personnel as well as to proper operation of control and other sensitivity circuits. The primary reasons for grounding equipment and neutral points of power system are: 

 To limit fundamental- frequency electric potential between all uninstalled conducting objects in local area .

  •  To limit touch and step potentials. 
  •  To limit over voltages on equipment and circuits for various operating conditions. 
  •  To limit system unbalances. 
  •  To provide for relaying and subsequent isolation of faulted equipment and circuits when phase – to – phase – to –ground faults occur. 
  •  To provide four- wire, three- phase power supply ( low – voltage )

system grounding:

In industrial and commercial power systems, the system should operate grounded. There are, however, other methods of operation that have been used historically and have operated satisfactorily for several years but do not have all of the advantages of the operation of a grounded system. Systems can be defined as:


1-   Un grounded system
 (although these are grounded through the distributed capacitance of the system )
Note : Disadvantage of this system is the arc fault as the neutral is isolated with respect to earth ( doesn't connected to the earth )


2-  Solidly grounded:

(needed in four-wire systems with line –to-neutral loads).
         Ω Used in low voltage system

3 - Impedance earthed system : and it may be through

  •   Resistance – grounded: ( either low – or high –   resistance ).

      Ω it used to limit fault current to an acceptable level
                Ω it also used to avoid excessive transient over voltage due to       
                 resonance with system shunt capacitance )
  •     Reactance – grounded: ( a special case that is not found in industrial and commercial systems)

  • Arc suppression coils
Note :  Resistance earthing is more commonly used , because it can allow the fault current to be limited and damp transient over voltage , if the correct value of resistance is chosen There are advantages and disadvantages with each of these methods of grounding (table 10.1), but for industrial and commercial power systems. The resistance – grounded system has the most advantages.


TABLE  Summary of advantages and disadvantages of Grounded and Ungrounded systems
Grounded system
Ungrounded system
Advantages
·         Fault detected and cleared as soon as possible.
·         Safety.
·         Balanced voltage.
·         Ground current provides an easy means for fault detection.
·         Resistance – grounding limits fault currents.
·         Single line – to – ground fault does not cause interruption of supply.
·         Lower short – circuit current contribution to ground faults.
Disadvantages
·         L-g fault causes interruption of load.
·         Local faults may trip the main supply breaker.
·         Higher L-G fault – current levels if not resistance – grounded.
·         May have marginally higher cost.
·         Equipment phases may be subjected to line – to – line outages.
·         Phase- to – phase fault current may not be high enough for over current relay tripping.
·         Susceptible to arcing fault burndowns and remote fault initition due to over voltages.
·         Susceptible to more multiple faults.
·         Locating faults difficult.
·         Not as safe due line – to – line voltage being impressed on phases to ground.
·         Regular and fast response maintenance needed .
·         Ground- fault levels high and may cause danger.







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