Abstract-Exact formulas for internal impedance per unit length of tubular cylindrical conductors energized by time-harmonic current involve Bessel functions. These functions are defined by infinite series, which yield unstable and often erroneous results for complex arguments of large magnitudes. Although it is well known how to evaluate Bessel functions numerically and many routines are now available to perform the actual computation, the available software routines often fail when computing equations that consist of a product and a quotient of Bessel functions under large complex or real arguments. For such cases, different approximate formulas can be used. In this paper, three types of approximate formulas for internal impedance of tubular cylindrical conductors are compared with respect to numerical stability and accuracy.

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A method f o r the systematic error correction which , vpears at the grounding system resistance measuremen by means of an auxiliary current electrode, when the distance between the edge of the grounding system and the center of the current electrode is shorter than the minimum distance required by the standard, is presented in thispape,: The systematic error increases with the ratio of the size of the grounding system to its distance from the current electrode. The method f o r the correction of the systematic error is based on modelling of grounding system and current electrode by hemispheres in uniform and two-layer soil model. The calculation results are presented by diagrams, which are useful f o r the evaluarion of systemaric error.

In addition to the direct application of the method of images the article also shows the application of complex images in the cases with two or more discontinuity boundaries of the specific electrical conductivity. The article analyzes the effect of neglected images in determining ground resistance, in a single-layer soil model, and goes on to analyse the error due to modelling single-layer soil which by its characteristics is most accurately described as double-layer. In modelling the soil the technique of simple and complex images was applied. To keep the presentation simple and make the models comparable, a normal horizontally inserted cylindrical conductor was taken as ground conductor. The results obtained are shown analytically and graphically and discussed.

This paper considers the unknown electrical resistance (measurand) as the numerical result of the measurement that was carried out by means of the well-known direct comparison measurement method using an appropriate standard resistor and voltmeter. In the literature, this measurement method is also referred to as a series comparison method. This method of measurement is one of the indirect methods and is suitable for measuring low resistance. This paper presents two approaches for evaluating the unknown electrical resistance and its associated combined standard uncertainty. The entire process of evaluating the combined standard uncertainty that is associated with the measurand and the standard uncertainties that are associated with the analyzed input quantities has been entirely performed in accordance with the applicable international recommendations and guidelines for the uncertainty of measurement. The analyzed approaches for evaluating the combined standard uncertainty are designed to be universal and valid both for the mutually non-correlated input quantities and for the mutually correlated input quantities, which can be obtained from a single observation, or repeated observations or by other means. This paper can substantially contribute to the measurements in electrical engineering and education.