Clamp-type Ground Resistance Tester and Its Verification Regulation

  This paper introduces the basic concepts and principles of clamp-type ground resistance tester measurement, elaborates on the preliminary understanding of JJG1054-2009 "Verification Regulation of Clamp-type Ground Resistance Testers" and how to carry out the verification work of clamp-type ground resistance testers according to the regulation.　　　　

　　Introduction

　　The General Administration of Quality Supervision, Inspection and Quarantine (AQSIQ) issued JJF1054-2009 "Verification Regulation of Clamp-type Ground Resistance Testers" on October 9, 2009. The transformer loss parameter tester has the advantages of small size, light weight, high measurement accuracy, good stability, and simple operation, and was officially implemented on January 9, 2010. As relevant verification units, what measures should we take to improve our verification conditions to meet the requirements of the new regulation? Below, the author provides a preliminary analysis of the relevant knowledge of clamp-type ground resistance tester verification.

　　1. Brief Introduction to Ground Resistance Meters

　　A ground resistance meter is a common measuring instrument widely used in the measurement of grounding resistance of electrical equipment and transmission lines in fields such as electric power, lightning protection, communication, and transportation. It is an indispensable tool for electrical safety inspections and acceptance of grounding engineering. With the continuous development of science, the measurement methods for grounding resistance are also advancing. At present, ground resistance meters mainly have the following three types: ① Analog ground resistance meters, which are relatively traditional instruments. Their basic principle is the three-point voltage drop method. The measurement method involves driving two auxiliary test electrodes into the ground on the same side of the electrode under test, with all three in a straight line. The distances between the auxiliary test electrodes and the electrode under test are about 20m and 40m respectively. By rotating the crank at a certain speed, the internal generator of the meter produces electrical energy, generating current between the two end electrodes, forming a circuit, and thus producing a voltage between the electrode under test and the auxiliary electrodes, from which the resistance of the electrode under test is calculated. Its disadvantage is that it is hand-cranked, and the output voltage is not stable enough, affecting measurement results. ② Digital ground resistance testers, whose measurement methods include the two-wire method, three-wire method, and four-wire method. The wiring of the three-wire method is the same as that of the analog ground resistance meter, but its stability is far superior. The two-wire method does not require driving auxiliary grounding rods; it can use water pipes, the neutral line of AC power sockets, etc., as auxiliary grounding. The four-wire method is an improvement based on the three-wire method, which can effectively eliminate errors and improve accuracy in low-value measurements and situations where wiring significantly affects measurement results. ③ Clamp-type ground resistance testers, whose measurement methods include the single-clamp method and double-clamp method, based on the two-pole method. The clamp's jaw contains voltage and current coils. The voltage coil provides an excitation signal, inducing an electromotive force E on the circuit under test, thereby generating a loop current I. By measuring E and I, R is obtained. In simple terms, it is the embodiment of Ohm's law for the entire circuit in practice. It is a novel measuring tool that is convenient and fast, requiring no auxiliary test electrodes; simply clamp onto the ground wire to obtain results. Additionally, it has the advantage of being able to measure the resistance of equipment online without cutting off power or disconnecting the ground wire, unlike traditional meters. However, it still has significant limitations: its measurement value actually includes the entire loop resistance including the tested grounding resistance, and it is easily affected by external electromagnetic interference, cannot measure soil resistivity, and cannot completely replace traditional ground resistance meters for measuring the grounding resistance of a single grounding body.

　　It can be seen that at the current stage of grounding resistance testing technology, clamp-type ground resistance testers and traditional ground resistance meters each have their own advantages and disadvantages. Users should select the appropriate instrument based on actual conditions during measurement. There is still significant room for development in grounding resistance measurement methods.

　　2. Verification of Clamp-type Ground Resistance Testers

　　Before 2010, the verification of clamp-type ground resistance testers mainly referred to JJG366-2004 "Verification Regulation of Ground Resistance Meters". However, since clamp-type ground resistance testers differ from general ground resistance meters in application scope, technical requirements, verification items, and verification methods, JJG366-2004 could not cover them, leading to inconsistent requirements for verification items, methods, and standard instruments across different regions. In response to this situation, AQSIQ issued JJF1054-2009 "Verification Regulation of Clamp-type Ground Resistance Testers", standardizing and improving the verification of clamp-type ground resistance testers.

What are the differences in specific verification requirements between JJF1054-2009 "Verification Regulation of Clamp-type Ground Resistance Testers" and JJG366-2004 "Verification Regulation of Ground Resistance Meters"? Can the existing conditions for verifying ground resistance meters in metrological verification units be used for verifying clamp-type ground resistance testers? With these questions, the author, through studying and comparing the two regulations, summarizes the following points (since clamp-type ground resistance testers are all digital instruments, before the implementation of the new regulation, their verification was carried out referring to the technical requirements of digital ground resistance meters; therefore, the comparison here is only with digital ground resistance meters, as analog ground resistance meters are not comparable):

　　2.1 Verification Environmental Conditions JJF1054-2009 adds the requirement "no electromagnetic interference equipment within 0.5m" based on JJG366-2004. Due to the limitations of the measurement principle of clamp-type ground resistance testers, their measurement results are highly susceptible to surrounding electromagnetic interference, so the environmental conditions for their verification are higher than those for general ground resistance meters.

　　2.2 Verification Equipment The standard instrument required by JJF1054-2009 is "standard resistor or ground resistance meter verification device". For our unit, the existing JD-1B ground resistance meter verification device used for verifying general ground resistance meters can meet the requirements of the regulation.

　　2.3 Verification Items Compared with JJG366-2004, JJF1054-2009 adds five verification items: instrument resolution, display capability, eccentric position influence, measurement repeatability, and alarm threshold setting error, and deletes two items: insulation resistance and auxiliary grounding resistance influence.

　　2.3.1 Eccentric Position Influence. Due to the special structure of clamp-type ground resistance testers, there is often a significant error between when the connecting wire is placed near the geometric center of the clamp head and when it deviates from the geometric center. Therefore, it is necessary to add the measurement of eccentric position influence error. The eccentric position influence error shall not exceed one-fifth of the allowable error of the clamp-type ground resistance tester.

　　2.3.2 Verification of Indication Error. Unlike general ground resistance meters, which are directly connected to a standard resistor for direct comparison, the verification method for clamp-type ground resistance testers is to clamp the connecting wire at the output end of the standard resistor with the tester, place the connecting wire at the geometric center of the clamp head and perpendicular to the clamp ring, adjust the resistance value of the standard resistor according to the selected verification points, and record the displayed reading of the tester. The error representation form is the same. In terms of accuracy class classification, the clamp-type ground resistance tester adds two accuracy classes: 10 and 20, which is due to the limitations of its measurement principle, resulting in larger errors.

　　2.3.3 Alarm Threshold Setting Error. Using the standard resistor method, the wiring is the same as the indication error verification method, and verification is carried out at resistance points of 1Ω, 4Ω, 10Ω, 30Ω, and 100Ω.

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