Unsa ang Resistance sa Earth?
Unsa ang Resistance sa Earth?
Pagtakda sa Resistance sa Earth
Ang earth electrode mao ang metal rod o plate nga gisulod sa yuta ug gikonekta sa earth terminal sa electrical system. Ito naghatag og low-resistance path alang sa fault currents ug lightning surges aron ma-dissipate sa ground. Ito usab makatabang sa pag-estabilisa sa voltage sa sistema ug pagsulay sa electromagnetic interference.
Ang mga earth electrodes mahimo mopili gikan sa materyales sama sa copper, steel, o galvanized iron, nga gipili alang sa ilang conductivity ug corrosion resistance. Ang sukal, hugis, length, ug depth sa electrode depende sa kondisyon sa yuta, current rating, ug ang partikular nga aplikasyon sa earthing system.
Mga Factor nga Moadto sa Grounding Resistance
Ang resistance sa earth mainly depende sa resistivity sa yuta tali sa electrode ug ang point of zero potential (infinite earth). Ang resistivity sa yuta na-influence gikan sa daghang factors, sama sa:
Ang electrical conductivity sa yuta, nga main ang electrolysis. Ang concentration sa tubig, asin, ug uban pang chemical components sa yuta nadetermina ang iyang conductivity. Ang moist soil nga may high salt content adunay lower resistivity kaysa dry soil nga may low salt content.
Ang chemical composition sa yuta, nga naaffect ang iyang pH value ug corrosion properties. Ang acidic o alkaline soil mahimo mog-corrode sa earth’s electrodes ug madugangan ang iyang resistance.
Ang grain size, uniformity, ug packing sa soil particles naaffect ang iyang porosity ug moisture retention capacity. Ang fine-grained soil nga may uniform distribution ug compact packing adunay lower resistivity kaysa coarse-grained soil nga may irregular distribution ug loose packing.
Ang temperature sa yuta, nga naaffect ang iyang thermal expansion ug freezing point. Ang mataas nga temperatura mahimo madugdugan ang conductivity sa yuta pinaagi sa pag-dugdug sa ion mobility. Ang mas bataas nga temperatura mahimo mapahimulos ang conductivity sa yuta pinaagi sa pag-freeze sa iyang water content.
Ang resistance sa earth usab depende sa resistance sa electrode mismo ug ang contact resistance tali sa surface sa electrode ug ang yuta. Pero, kini nga mga factor kasagaran negligible kaysa sa soil resistivity.
Measure sa Earth Resistance
Adunay daghang methods aron sukdan ang earth resistance sa existing systems. Ang uban sa common methods mao ang:
Fall of Potential Method
Kini nga method, usab gitawag og 3-point o potential drop method, nanginahanglan og duha ka test electrodes (current ug potential) ug an earth resistance tester. Ang current electrode is placed at a distance from the earth electrode, matching its depth. Ang potential electrode is placed between them, outside their resistance areas. Ang tester injects a known current through the current electrode and measures the voltage between the potential and earth electrodes. Ang earth resistance is then calculated using Ohm’s law:
Where R is the earth resistance, V is the measured voltage, and I is the injected current.
Kini nga method simple ug accurate pero nanginahanglan og pag-disconnect sa tanang connections sa earth electrode before testing.
Clamp-On Method
Kini usab gitawag og induced frequency testing o stakeless method. Wala kini nanginahanglan og test electrodes o pag-disconnect sa tanang connections sa earth electrode. Kini gamit og duha ka clamps nga gisulod sa existing earth electrode. Ang usa ka clamp induces a voltage to the electrode and another clamp measures the current flowing through it. Ang earth resistance is calculated using Ohm’s law:
Where R is the earth resistance, V is the induced voltage, and I is the measured current.
Kini nga method convenient ug fast pero nanginahanglan og parallel earth network with multiple electrodes.
Attached Rod Method
Kini nga method involves one test electrode (current electrode) and an earth resistance tester. Ang current electrode is connected to the earth electrode with a wire. Ang tester injects a known current through the wire and measures the voltage between the wire and the earth electrode. Ang earth resistance is then calculated using Ohm’s law:
Where R is the earth resistance, V is the measured voltage, and I is the injected current.
Kini nga method wala nanginahanglan og pag-disconnect sa tanang connections sa earth electrode pero nanginahanglan og good contact between the wire and the current electrode.
Star-Delta Method
Kini nga method uses three test electrodes (current electrodes) arranged in an equilateral triangle around the existing earth electrode. An earth resistance tester injects a known current through each pair of test electrodes in turn and measures the voltage between each pair of test electrodes in turn. The earth resistance is calculated using Kirchhoff’s laws:
Where R is the earth resistance, V<sub>AB</sub>, V<sub>BC</sub>, V<sub>CA</sub> are the measured voltages between each pair of test electrodes, and I is the injected current.
Kini nga method wala nanginahanglan og pag-disconnect sa tanang connections sa earth electrode pero nanginahanglan og more test electrodes than other methods.
Dead Earth Method
Kini nga method uses two test electrodes (current electrodes) connected in series with an earth resistance tester. One test electrode is inserted near the existing earth electrode, and another test electrode is inserted far away from it. The tester injects a known current through both test electrodes into the ground and measures the voltage between them. The earth resistance is calculated using Ohm’s law:
Where R is the earth resistance, V is the measured voltage, and I is the injected current.
Kini nga method wala nanginahanglan og pag-disconnect sa tanang connections sa existing earth electrode pero nanginahanglan og very long wire between both test electrodes.
Slope Method
Kini nga method uses one test electrode (potential electrode) and an earth resistance tester. Ang potential electrode is moved along a straight line away from the existing earth electrode at regular intervals. The tester injects a known current through the existing earth electrode into the ground and measures the voltage between it and the potential electrode at each interval. A graph of voltage versus distance is plotted and extrapolated to find the intercept on the voltage axis. The earth resistance is calculated using Ohm’s law:
Where R is the earth resistance, V<sub>0</sub>is the intercept on the voltage axis and Iis the injected current.
Kini nga method wala nanginahanglan og pag-disconnect sa tanang connections sa existing earth electrode pero nanginahanglan og moving the potential electrode along a straight line.
Improve sa Earth Resistance
Ang earth resistance mahimo molambo pinaagi sa pag-reduce sa soil resistivity o pag-increase sa electrode surface area. Ang uban sa common ways aron lamboon ang earth resistance mao ang:
Adding salt or other soluble substances around the electrode to increase soil conductivity by electrolysis.
Adding charcoal or other moisture-retaining substances around the electrode to keep the soil moist throughout the year.
Using multiple electrodes connected in parallel to increase the total surface area in contact with soil.
Using longer or deeper electrodes to reach lower layers of soil with lower resistivity.
Using electrodes with larger cross-sections or hollow shapes to reduce electrode resistance.
Using electrodes with special coatings or alloys to prevent corrosion and increase contact resistance.
It is recommended to measure earth resistance periodically (yearly or half-yearly) and take necessary actions if it exceeds the desired value for application.
Conclusion
Ang earth resistance mao ang important parameter para sa designing ug maintaining earthing systems. Ito depende sa various factors such as soil resistivity, electrode size, shape, depth, material, etc. Adunay various methods aron sukdan kini sa existing systems, sama sa fall of the potential method, clamp-on method, attached rod method, star-delta method, dead earth method, ug slope method.
Ang earth resistance mahimo molambo pinaagi sa pag-add sa salt, charcoal, o uban pang substances around electrodes, using multiple electrodes, using longer or deeper electrodes, using larger or hollow electrodes, o using special coatings or alloys for electrodes. Ang earth resistance kinahanglan sukdan periodically ug keep within acceptable limits for safety and performance reasons.
