Earthing Audit
Home - Earthing Audit
Home - Earthing Audit
Earthing of electrical systems is designed primarily preserve the safety of the system by ensuring the potential on exposed conductors (eg: metal body/frame, electrical conduits, cable ladders, trays etc.) within the limit consistent with level of insulation applied and to limit the rise of potential of non-current carrying conductors under earth fault conditions.
Basic objectives of equipment grounding are
To obtain this aim, proper low resistance connection to earth is necessary. However, this is hard to attain most of the time and varies according to several factors :
The main objective of soil resistivity testing and measurement is to attain a set of values which can be used to obtain an equivalent model for the electrical performance of the earth.
Those value can determine
Thus, soil resistivity calculations and measurements are crucial aspects when designing earthing installations
Resistivity of the soil
The resistivity of the soil at many sites has been found to be non-uniform. Variation of the resistivity of the soil with depth is more predominant as compared to the variation with horizontal distances. Wide variation of resistivity with depth is due to stratification of earth layers. In some sites, the resistivity variation may be gradual, where stratification is not abrupt.
To design the most economical and technically sound grounding system for EHT / HT stations, it is necessary to obtain accurate data on the soil resistivity and on its variation at the station site. Resistivity measurements at the site will reveal whether the soil is homogeneous or non-uniform.
In case the soil is found uniform, conventional methods are applicable for the computation and when the soil is found non-uniform, either a gradual variation or a two-layer model is adopted for the computation of earth resistivity.
The resistivity of the earth varies within extremely wide limits depending on its moisture content and temperature, probably between 1 and 10000 ohm meters.
The purpose of this test is to verify the adequacy of installed ground grid and to reconfirm that grid has been maintained periodically throughout its service life.
The reason for having a properly designed, installed, and maintained grounding grid is to eliminate the shock hazards and abnormal operating conditions that can arise due to fault currents.
By measuring the resistance of electrical connections, this test determines the damaged components, broken conductors, loose connections, adequate tension on bolted joints, eroded contact surfaces, contaminated or corroded/oxidized contacts, poor soldering/welding etc., which are due to surface and atmospheric conditions
The ground integrity test, which consists of testing the quality of continuity between two points on the ground grid, is generally performed in such cases where safety is a concern, particularly in older substations/ HV systems, the ground integrity test is typically performed before any other tests.
To verify that there is a low-resistance path for ground currents, all accessible ground leads need to be inspected, and those that are buried under the earth’s surface need to be tested periodically.
This test can be done by measurement with high current injection of off-grid frequency
Fault loop impedance measurement is important in a L.V electrical network to define safety and proper functioning of the electrical installation. This test is to ensure disconnection time of protective devices, determines the amount of fault current at every location in a network after installation. Efficiency of Protective Equipotential Bonding can also be tested if provided.
Fault Loop Impedance — (also called as Earth Fault Loop Impedance, loop impedance) The impedance of the earth fault current loop (phase to earth loop) starting and ending at the point of earth fault. This impedance is denoted by the symbol Zs.
The earth fault loop comprises of
Note : Earth fault loop impedance, earth loop resistance and Earth Electrode Resistance are different phenomena
Fault : The occurrence of an earth fault in an installation creates two possible hazards.
Firstly, voltages appear between exposed conductive parts and extraneous conductive parts, and if these parts are simultaneously accessible, these voltages constitute a shock hazard
Secondly, the fault current that flows in the phase and protective conductors of the circuit feeding the faulty equipment (the earth fault may, of course, occur in the fixed wiring of the circuit itself) may be of such a magnitude as to cause an excessive temperature rise in those conductors, thereby creating a fire hazard.
Earth fault protective measures as per IS3043 & IS732
IS3043 : Fault clearance time for 230 volts is 0.17 sec at DRY condition and 0.032 sec at WET condition.
IS3043 : Safety is achieved by “earthed equipotential bonding and automatic disconnection of supply”
The protective measure known as 'earthed equipotential bonding and automatic disconnection of the supply' is intended to give a high degree of protection against both hazards.
The characteristics of the protective devices and the cross-sectional area of conductors shall be so chosen that if a fault of negligible impedance occurs anywhere between a phase conductor and a protective conductor or exposed conductive part.
Automatic disconnection of the supply will occur within the minimum possible safe time.
Zs = U0 / Ia
Where
Zs = fault loop impedance
Ia = fault current ensuring the automatic operation of disconnecting device
U0 = Nominal voltage
Every circuit is provided with a means of overcurrent protection.
If the earth fault loop impedance is low enough to cause these devices to operate within the specified times (i.e. sufficient current can flow to earth under fault conditions), overcurrent devices may be relied upon to give the requisite automatic disconnection of supply.
If the earth fault loop impedance is high, it does not permit the overcurrent protective devices to give automatic disconnection of the supply under earth fault conditions.
To review the step and touch voltages at an existing station for comparison with tolerable body withstand values. To confirm design calculations of step and touch voltages in a new station. These voltages can be different from the measured values due to various approximations in the modelledsystem.
In general, a substation ground grid is designed to limit step and touch voltages on and around the yard within the tolerable limits. Depending on the assumptions made in the design, the actual step and touch voltages during the fault might differ from the designed values.
A better assurance that a substation ground grid meets its design objectives would come from actually measuring step and touch voltages by injecting a known amount of current in the ground mat from a remote ground electrode and measuring the resulting voltage gradients.
The actual magnitudes of step, touch, and transfer voltages can then be determined by scaling up for the substation fault current.
Step and touch potential
The difference in surface potential that could be experienced by a person bridging a distance of 1m with the feet without contacting any grounded object is defined as step voltage.
The potential difference between the ground potential rise of a grounding grid or system and the surface potential where the person could be standing while at the same time having a hand in contact with a grounded structure are object is defined as touch voltage.
Various types of touch voltages are as follows
Grounding measurements made with a full phase-to-ground fault current in a substation provide the most accurate step, touch, and transfer voltage data. However, this type of test is rarely performed unless some other, more demanding reason such as determination of circuit parameters, equipment performance, and protection characteristics exists for the tests.
Measurement of touch and step potential can be done by measurement with high current injection of off-grid frequency
The earthing resistance of an electrode is depending on
The first two factors are very small fractions of an ohm and can be neglected for all practical purposes and third factor is therefore important in deciding which of many protective systems to adopt.
Earth conductivity is basically electrolytic in nature and is affected by the following factors
Many of these factors vary locally and some seasonally
It should also be noted that soil temperature has some effect but is only important near and below freezing point, it is therefore recommended that earth electrodes to be installed at depths to which frost will not penetrate
IS/IEC 62305-3: When dealing with the dispersion of the lightning current (high frequency behavior) into the ground, whilst minimizing any potentially dangerous over voltages, earth-termination system is the important criteria. In general, a low earthing resistance (i.e. less than 10 Ω when measured at low frequency) is recommended (only if individual electrodes are installed for lightning)
The objectives of earth electrode & earth grid/mat are
