IS 3043:1987 Simplified version relevant to Industries

 Based on the document IS 3043: 1987 - Code of Practice for Earthing, here are some of the key values and formulas relevant to an electrical engineer.

Formulas and Calculations

• Earth Fault Current (Iₑ):

  The standard defines the earth fault current as the current that flows through the earth connection. For a line-to-earth fault, the current is given by:

  I(f)​=Vp​/ (Ze + Zp)​​

  Where:



• I(f)​ is the earth fault current.

• Vp​ is the phase voltage.

• Zp​ is the phase impedance.

• Ze​ is the earth fault loop impedance.

• Earth Electrode Resistance (Re​):

  The document provides a simplified formula for the resistance of a single vertical rod electrode:

  
  
  

  Where:

  • Re​ is the earth resistance in ohms.

  • p is the soil resistivity in ohm-meters.

  • l is the length of the electrode in meters.

  • d is the diameter of the electrode in meters.

Specified Values

Touch Voltage and Clearance Time: The document specifies the maximum permissible touch voltage ($V_t$) that can be sustained for a certain duration before the circuit must be cleared. The formula for the maximum permissible touch voltage is: 

​ Where:

• $V_t$ is the maximum permissible touch voltage in Volts.

• $t$ is the clearing time in seconds.

• Earth Fault Current Duration:

• The standard recommends specific time durations for earth fault currents to be considered in design:

  • For substations at 66 kV and above, the duration should be taken as 1.0 second.

  • For all other voltage levels below 66 kV, the duration should be taken as 3.0 seconds.

• Corrosion Allowance:

  The document provides a table for the average corrosion rates of steel and copper in different soil conditions. It states that, on average, steel corrodes about 6 times faster than copper in soil. The standard recommends specific allowances for corrosion to ensure the long-term integrity of the earthing system.

• Residual Current Devices (RCDs):

  For portable and mobile generators, the standard recommends using RCDs with a fast operating time. The specified tripping characteristics are:

  • Operating time of 40 milliseconds or less.

  • Operating current of 250 mA.