Tolerable Touch Research Articles

A Systematic Approach to Safe and Effective Earthing System Design for High Voltage Substation

In high voltage substation, earthing system is an essential part not only for providing the protection to people who are working or walking in the vicinity of earthed facilities and equipment against the danger of electric shock, but also for maintaining the proper function of the entire electrical system. By selecting a proper conductor and electrode size based on standard criteria, the existence of large potential differences during fault or lightning conditions will able to be avoided. Standard equations for earthing are used in the design to obtain the desired parameters such as tolerable touch and step voltage, earth resistance, grid resistance, minimum size of earthing conductor and electrode, as well as resistivity of soil. Therefore, it would enable the establishment of the safe limits of potential differences under both normal and fault conditions. The main objective of this research work is to perform a necessary background information pertinent step by step approach for safe and reliable earthing system design for high voltage distribution substation in Malaysia. It including a set of applicable standard to be used as well as demonstrate an earthing design simulation developed using Microsoft Excel auto mathematical functions. The case study for high voltage substation design and simulation at two locations with different soil resistivity and properties is also performed in this research work. The results revealed that the design of high voltage substation earting system is much dependent on the property and resistivity of soil where the substation will be constructed.

Comparison of the safety criteria used for ground grid design at 161/23.9-kV indoor-type substation

This paper compares seven different standards, i.e., IEEE Std. 80, 2000 edition, IEC 60479-1, BS 7354, EA-TS 41-24, CENELEC HD 637 S1, DL/T 621 and ITU-T K.33 used for ground grid design at 161/23.9-kV indoor-type substation in the system of Taiwan Power Company. The calculated results from equations proposed by Sullivan and the Current Distribution Electromagnetic Grounding and Soil structure analysis (CDEGS) software package are also included. The results show IEC 60479-1 gives the lowest limits of tolerable touch and step voltages whereas BS 7354 gives the highest when no chippings on the ground surface are considered. The designed maximum predicted touch voltage obtained from Sullivan’s equation is higher than those obtained from the formulas of BS 7354, EA-TS 41-24, DL/T 621 and CDEGS, the mesh voltage is higher than those from IEEE Std. 80, 2000 edition and CDEGS.

인체전류를 기반으로 하는 감전의 위험성 평가방법

In order to mitigate the possible hazards from electric shock due to the touch and step voltages, the high resistivity material such as gravel is often spread on the earth's surface in substations. When the grounding electrode is installed in two-layer soil structures, the surface layer soil resistivity is different with the resistivity of the soil contacted with the grounding electrodes. The design of large-sized grounding systems is fundamentally based on assuring safety from dangerous voltages within a grounding grid area. The performance of the grounding system is evaluated by tolerable touch and step voltages. Since the floor surface conditions near equipment to be grounded are changed after a grounding system has been constructed, it may be difficult to determine the tolerable touch and step voltage criteria. In this paper, to propose an accurate and convenient method for evaluating the protective performance of grounding systems, the propriety of the method for evaluating the current flowing through the human body around on a counterpoise buried in two-layer soils is presented. As a result, it is reasonable that the grounding system performance would be evaluated by measuring and analyzing the current flowing through the human body based on dangerous voltages such as the touch or step voltages and the contact resistance between the ground surface and feet.

Safety assessment of AC earthing systems in a DC traction-supply substation

The paper assesses safety of AC earthing systems in a DC traction-supply substation (TSS) by comparing the designed mesh and step voltages to the maximum tolerable voltages. Generally, IEEE 80 criteria are based on the maximum tolerable body current to prevent fibrillation of the human heart, and this current is then translated into tolerable touch and step voltages. As a result, safety assessment of AC earthing systems in a DC TSS is referred to a procedure by which the designed mesh and step voltages are computed and compared with the tolerable touch and step voltages. Computation results of AC earthing systems for the orange line at O19 TSS in Taipei rapid-transit systems are presented as an example.