FAQ’s – Earthing

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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

Omega Red Group Ltd