FAQ’s – Earthing

  • Correctly designed and installed earth electrode systems are essential for the correct operation of equipment and to ensure the safety of people
  • The effects of touch and step potentials can be dangerous to people and livestock too
  • Accurately designed earthing systems ensure that the safe voltage limits (defined by the duration of the fault and the local ground covering) are not exceeded
  • Touch potential is the voltage between conductive parts when touched simultaneously; either between hands, or between hands and feet
  • An example of this would be a person touching metalwork, such as a metallic fence whilst their feet are in contact with the ground beneath, at the same time that an electrical fault or lightning strike occurs; and is discharged into the earthing system

  • Step potential is the voltage that can exist between two points (normally a person’s feet, as they step) on the earth’s surface; measured at a one metre interval, which is considered to be the stride length of a human
  • An example of this might be a person walking over, or near to an area containing an earthing system; at the same time that an electrical fault or lightning strike occurs; and is discharged into the earthing system

  • Transferred potential is the voltage rise of an earthing system caused by an external fault transferred to the earthing system by a connected means such as cable sheaths, metallic pipework etc.
  • The definition of a “cold” site is defined by the International Telecommunications Union (ITU); where the Earth Potential Rise (EPR) limit is 430V for a fault clearance time in excess of 200ms and 650V for fault clearance times equal to or less than 200ms
  • Soil resistivity measurements (Wenner method) involve the use of a four-pole test instrument, such as a Megger DET2/2
  • Four equally spaced test earth electrodes are inserted in the ground to a maximum depth of 200mm and each is connected to the test instrument with a separate lead
  • After each measurement is recorded the distance between the electrodes is increased, from a short distance of 1m, up to much larger distances dependent upon the space constraints of each site

  • Omega recommends that driven earth electrodes are tested annually and the values compared to those previously obtained
  • Significant variations in results should be investigated and any remedial works carried out
  • The overall resistance of a whole earthing system should be measured at five-yearly intervals, using the ‘Fall of Potential’ test method, and the results compared to those previously obtained
  • Omega recommends that all buried connections between copper tapes are made using oxy-fuel (Sil-Fos®) brazing techniques
  • All connections to driven earth electrodes should be made using exothermic welding techniques
  • Omega also recommends that cable to cable connections are made either by exothermic welding or compression joints

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