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Standards for Insulating links
ANSI/CPLSO-14
ANSI/UL2737 (Withdrawn)
ASTM F2973
MIL-L-24410 (Withdrawn)
Tests by Independent Organizations
Load Insulator
Miller & Hirtzer
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319 i body and skin impedance. The skin was modeled by 4 = 150 ohm, C, = 0.25 pF. The circuit in Figure 2 was used for transient analysis, but the body impedance was modeled by the circuit shown in Figure 6. RS1 Figure 6. Transient equivalent circuit of the human body STEADY-STATE CURRENT Equations The steady state current flowing through the operator when the crane touches the high voltage line was calculated by using complex arithmetic and the equivalent circuit of Fgure 2 with the appropriate body impedance model. The system impedance is shown in equation 1. The body impedance is given in equation 2 and 3. The current flowing through the crane worker's body is calculated in equation 4. i = 1,2 Zm = 750 Q where: V is the line-to-line voltage Z is the system impedance L is the source inductance C; is the insulator capacitance R[ is the isolator resistance (clean or p-lli C, is the skinelectrodecapacitance RI is the skin resistance 4 is the body resistance C,, is the equivalent shoe capacitance & is grounding resistance ?d) 1 2 3 4 Crane without protection link When a crane, not protected by an insulating link, touches the high-voltage line, the worker is exposed directly to the high voltage, which can produce a lethal shock. Fgure 7 shows that several ampere's of current flows through the worker's body. The body impedance is significantly higher than the source impedance, therefore the lattets effect on the body current is negligible. 140 120 100 80 60 40 20 0 0 20 40 60 80 100 120 VOLTAGE IN kV Figure 7. Current through the crane worker not protected by insulator link a. Body impedance is modeled by the circuit of Figure 4. b. Body impedance is modeled by R, = 700 Q The body current depends on the model used for the calculation but the current value, at all voltage levels, is in the range of lethal shock, independent of the model used. The ground resistance also affects the current because it is added directly to the body resistance. However, the decrease of current, even at a grounding resistance of several thousand ohms, is not sufficient to avoid the lethal electric shock. As an example, 144 kohm grounding resistance is necessary at 12.47 kV to reduce the current below the dangerous heart fibrillation limit of 30 mA. The use of insulating shoes reduces the current below 5 mA, but most of the system voltage will appear across the sole of the worker's shoe. The sole impedance is in the neighberhood of 15 Mohm, therefore neither the body impedance nor the grounding resistance affects the current. The 5 mA is below the let-go threshold level, but the high voltage across the sole will cause breakdown between the foot and ground. This will increase the current to several amperes and electrocute the crane worker. Crane with protection link Figure 8 shows that in clean and dry conditions the leakage current increases with insulator capacitance. It is minimally affected by the insulator resistance, and the effect of body
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