Major accidents have taken place solely because of improper equipment earthing or leakage current. As a regular part of maintenance, earthing should be measured at regular intervals so as to ensure that the resistance of the earth is maintained at a minimum. The main purpose of this is to minimize the risks of fire and shock hazards.
Earthing plays an important role in the generation, transmission, and distribution of safe and appropriate current flow for any electrical system.
Purpose of Earthing or Grounding
When an electrical fault develops in domestic applications, any metal parts that the user is in contact with may become live electrical conductors and cause an electrical shock. Adequate earthing of that metal part, however, can provide an unfailing circuit path away from the user to an earth ground, preventing the shock.
If the circuit is protected by a ground-fault circuit breaker, the fault will trigger the tripping of the breaker and remove both the current and the shock potential.
In industrial applications, the condition of earth resistance or leakage current testing may have been neglected, perhaps for a long time, because of the time consuming methods involved, ignoring existing earthing measurement predictive plans, or the non-availability of good testing instruments. This condition can be so bad that it can cause transformer failures and erratic meter readings at high tension feeders, etc., leading to major accidents.
Other than the protection of humans and equipment, the purpose of earthing is to provide a safe path for the dissipation of fault currents, lightning strikes, static discharges, EMI and RFI signals, and interference.
Prior to selecting the location of a substation or commercial sector, it is necessary to measure the soil resistance value.
Ideally electrical engineers should maintain ground resistance at zero ohms. The ground resistance value may vary from industry to industry depending upon soil conditions, but basically for low voltage system it should be less than 1 ohm and for medium and high voltage system it should be in the range of 1 to 5 ohms.
The primary reason to minimize the ground resistance is to ensure personnel safety.
The following organizations provide guidelines for grounding:
- International Electrotechnical Commission (IEC)
- European Committee for Electrotechnical Standardization (CENELEC)
- Underwriters Laboratories (UL)
- National Fire Protection Association (NFPA)
- American National Standards Institute (ANSI)
- Mine Safety Health Administration (MSHA)
- Occupational Safety Health Administration (OSHA)
- Telecommunications Industry Standard (TIA)
Soil resistance values depend on soil composition, moisture, and temperature.
There are three variables which affect the resistance of the earth electrode:
- The soil
- The length of the earth electrode
- The diameter of the earth electrode
The depth of the electrode can lower the resistance value effectively. Normally, doubling the length of the earth electrode can reduce the resistance value by an additional 40%.
Another method to reduce the earth resistance is with the use of multiple parallel earth electrodes.
The amount of water in the ground, i.e. moisture, affects the soil resistance value. Moisture content differs seasonally, so periodically watering is necessary. Maintenance engineers should pour the water in an earthing pit on a regular basis as per the preventive maintenance plan to obtain lower resistance value. If any earthing pit found with higher resistance value then addition of salted water in the pit is recommended.
Soil Resistivity Measurement
The formula for soil resistivity can be described as:
P= 2 Π A R
Where: Ρ= the average soil resistivity to depth A in ohm cm
Π= the constant 3.1416
A= distance between the electrodes in cm
R= the measured resistance value in ohms with the test instrument.
For distance in feet formula will be:
Ρ = 191.5 AR
Where A= distance between the electrodes in feet.
Normally single electrode earthing is used for domestic applications, but for power generating substations and industries we use a grid network with multiple electrodes.
The “3-pole fall of potential” testing method can be used for a complex earth system. In this technique, the earth grid is disconnected from the earth electrode. Two auxiliary electrodes (one current electrode and a second potential electrode) are placed beside the electrode to be tested at an equal distance in a straight line. The current passed through the auxiliary current electrode is to be noted and recorded. In this way, the potential difference generated between the auxiliary potential electrode and the current electrode can be measured.
Measurement of earth resistance and leakage current without disconnecting the circuit can be done using portable instruments. The clamp-on type of earth resistance tester can measure earth resistance and leakage current. In this measurement technique, a predefined current is injected in the circuit under test and the induced magnetic current thereby produced in the circuit is noted at a several intervals.
Image: LEM Instruments
- All overhead transmission conductor lines that may be exposed to lightning should be protected by a means for diverting any electric surge to earth.
- The grounding cables of transmission towers should be tested frequently.
- Transformer earthing terminal measurements should be conducted to ensure proper contact between the soil and the earth point.
- Low earthing wire resistance is most important for motors, power distribution panels, and control panels.
- The earthing of a telecommunication control cabin or signal relay board is important to reduce stray electrodynamic stress and noise.
- Separate earthing should be provided for PLC and SCADA instruments in a control panel.
- Good earthing is required for petrochemicals pipelines and oil storage tanks.
- LEM Instruments: Ground Ristance - Principles, Testing, Techniques & Applications (PDF)
- Author’s own experince as engineer in various steel plants