In recent times the areas of application of direct current systems are being expanded. This may lead to increased risk of electric shock. The subject of investigations described in this paper is a non-linear model of the resistance of the human body under the conditions of direct current electric shock. The aim of the study was to determine the approximate model of the human body resistance at a hand-to-hand DC flow, which may be useful for assessing the effects of the electric shock. The investigations were carried out on three subjects using the hand to hand current path. Experimental studies were conducted within a voltage range of up to 80 V, while the analytical form of the identified model enables extrapolation of the obtained results to a wider range of touch voltages. The identified models allow to evaluate the shock hazard within a voltage range of up to 1500 V. The adopted shock hazard criterion was the probability of ventricular fibrillation of 5 %. The result of the studies was a direct correlation between maximum allowable duration of shock and the touch voltage determined for the three subjects on the basis of the relationship between shock duration and the touch current specified in the IEC report, and of non-linear models of resistance identified on the basis of experimental studies. It was demonstrated that for touch voltages of up to 650 V the fibrillation phenomenon should not occur when the shock duration is shorter than 10 seconds.
Wychody produktów wzbogacania oraz ich jakooeae -a zatem równie¿ wartooeae produkcjizale¿¹ od wzbogacalnooeci wêgla surowego, stopnia z³o¿onooeci i konfiguracji uk³adu technologicznego przeróbki wêgla oraz parametrów rozdzia³u operacji przeróbczych. Porównanie efektów wzbogacania wêgla surowego w ró¿nych uk³adach technologicznych, zw³aszcza porównanie wartooeci produkcji o zadanej jakooeci, pozwala oceniae przydatnooeae poszczególnych wariantów wzbogacania (Yagun i in. 2002). Dotychczas w kilku publikacjach wykazane zosta³o (Pielot
The aim of this study is to analyze the effects of mine power network impedance on the starting time of induction motors, as well as on the operation of overcurrent protection relay. Proper selection of the time-current characteristic of overcurrent protection is crucial for the operation of the drive. A specific feature of mining power grids is their high impedance, which results from long cable lines with relatively small cross-sections. This causes relatively large voltage drops and significantly reduces the starting torque of the motor. Reduced starting torque increases the starting time and intensifies the motor overheating. This study analyzes a series of standardized time-current characteristics used in Invertim company protection devices. A simulation study of startup current and starting time was conducted for an exemplary medium-power motor with a large inertia fan at different values of power supply voltage below the rated value. Parameters of the motor equivalent circuit were calculated on the basis of manufacturer data. A new shape of the time-current characteristic has been proposed that would allow for prolonged starting at significant voltage drop in the mine network, ensuring protection from failed starting. This solution can be implemented in digital protection relays in addition to the standard characteristics.
Operational safety of electrical machines and equipment depends, inter alia, on the hazards resulting from their use and on the scope of applied protective measures. The use of insufficient protection against existing hazards leads to reduced operational safety, particularly under fault conditions. On the other hand, excessive (in relation to existing hazards) level of protection may compromise the reliability of power supply. This paper analyses the explosion hazard created by earth faults in longwall power supply systems and evaluates existing protection equipment from the viewpoint of its protective performance, particularly in the context of explosion hazards, and also assesses its effect on the reliability of power supply.
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