In electrical systems,all parts of installation that is not being under potential is connected to earth with conductors, to this is called ”Earthing”.
Earthing is made for minimize or elimate to undesirable step and touch potential hazards is apperad in a isolation fault.In order to,reduce the chance of injury to personel or condition of undertaking.
Design Criterions for Earthing Sytems
At first,specific earth resistance (r) is measured in the field that will build grounding.Grounding Mechanism is choosen according to (r) and must be appropriated soil types,easily the fault current mut be passed through it on the way to the ground. The most using types of grounding sytems are below explained.
Deep Driven Electrodes
Electrodes should be extensible a driven as deep as step and separeted by distance of at least twice the depth to which they are installed.
Burried Horizantal Electrod
Stripe conductor must be designed either a same direction or distance of least 60 of degrees and 40 cm depth under the surface of earth,their min.cross-section.3×20 mm.Cu,3,5×30 as galv.steel .Generally is prefered for rocky ground.
Earth Plates Electrod
It is not prefered as earth electrode,applied to vertical ,due to large surface area in contact with soil and economic.It is prefered prefabrikated mesh electrode instead of earth plates electrod.
Calculation Cross-section for Grounding Conductors
The cross-section of conductor is counted to ONDERDONK’s formula for copper when fault current is appeared during event (IEEE.80)
I: Fault current in A
A:Copper cross-section in C mils (1 Cmil= 1,974 mm2)
S:Fault current time in s
Tm:Max.Allowable temperature in oC
Ta:Ambient temperature in oC
For Copper Tm:1083oC,this equation is transformed into;
In doing tests,when max. Allowable temperature is 1083 oC for exothermic welding connection,same equalation valids.
The tempereture limit of 450 oC is a reasonable value for silver and solder welding connections ,so Onderdonk’s formula (IEEE.80),
250 oC limit is suggested for pressure or bolted connections,this formula (IEEE.80)
Copper is by far the most common metal used as ground grid conductors. Copper-clad steel is usually used for ground rods, and sometimes as ground conductors. There are four reasons why copper has been used primarily as ground grid conductor:
· Familiarity of electrical characteristics of copper when used as ground grid conductors.
· Higher conductivity (compared with steel) making it suitable for installations with high fault currents.
· Good mechanical strength, and most importantly,
· Freedom from underground corrosion. Grid integrity will not be compromised, if conductors are adequately sized and not subjected to any mechanical injury.
It is the unfamiliarity of using steel, lack of experience data, unavailability of any design standard and guide, and the fear of ground grid integrity due to corrosion, that steel is not commonly used. Also the IEEE standard provides limited information about the design procedure of the grounding system with materials other than copper.
The material selection used in the grounding system design depends primarily on the following factors:
· Fusing characteristics & current carrying capabilities,
· Conductor resistance,
· Mechanical Strength,
· Availability and
· Cost of conductor material.