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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ANSI/UL2737 (Withdrawn)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MIL-L-24410 (Withdrawn)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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323 Discussion of IEEE paper 90 Sbl 338-4-EWN.1, "Efficiency of Insulating Link for Protection of Crane Workers", by G.G. Karady. G.GEZA, WIRC, Lenox, MA: The author should be con- gratulated for an informative paper. me application of a circuits analysis program such as biiCRXAP to a practical problem in the high voltage area is very in- teresting. ?he equivalent circui.t sham in Fig. 2 of the paper is simple yet quite useful in modeling the problem. The discussor has several catunents about the circuit. Tne insulator is, as expected, mcdeled by a parallel RC circuit. The worker's body is modeled by an impedance, which depends on voltage, and eventually reaches the asymptotic (i.e., steady state) value of 850a This variation of body impdance with voltage is represented in the paper by parallel RC circuits, but it is not quite clear which circuits produced best results. Also, a clarifying discussion of the com- ponent values of these body impdance circuits would be appreciated. For example, was the circuit of Fig. 4 or 6 used, and what is the value of RS2 ir! ~i 6? Also, for many other applications the body mpe%nce (resistance) value of 500n is used, which is sig- nificantly lmer than the value assmed by the author. A discussion of the consequences of this difference would be appreciated. Finally, the insulating rubber shoes are represented by a capacitance C,2 in the cir- cuit. The discussor's work with rubber products indi- cates that they are quite lossy. When represented by a parallel equivalent RC circuit, the capacitive reac- tance of the rubber boot is roughly equal to its equivalent loss resistance. The values of 170 pF and 15 Mfi given in the paper are confirmed by the discussor's experience. The very high value of the equivalent parallel resistance can be neglected without hesitation in the equivalent circuit of Fig. 2. The paper also presents a useful discussion of the ef- fects of extremely short duration transient currents, such as those when rapid fault clearing is ensured. By mparing the limit in Fig. 13 with the plots in Fig. 11, it is clear that short duration currents of very high peak values can be tolerated, as long as these currents exist for a very short time. It is the total available energy, and not the current amplitude alone, that is of imprtane in this case. MIQIOCAP has provisions to calculate this energy. It would be interesting to canpare MICROCAP calculations with the results of eg. (6) of the paper, which is conserva- tive, as pointed out by the author. Manuscript received August 10, 1990. GEORGE G. KARADY, Arizona State University,Tempe, AZ. In reference to Dr. Gela's discussion, I would like to point out that the proper equivalent circuit for the human body in steady state condition is shown in Figure 4. In transient conditions, the best model, according to the literature, is derived from the results of measurements presented in Figure 5. The corresponding equivalent circuit is shown in Figure 6. The capacitance and resistance values, used for the analyses, are listed in Figure 5. This is a conservative approximation. The steady state current was calculated using the model shown in Figure 4. The resistance and capacitance values were varied in a considerable range. Also the current was calculated by modeling the body impedance by a 700 or a 500 ohm resistance. This sensitivity analyses showed that the current is determined by the model parameters, if the worker is not protected by an insulator link. However, the current endangers the worker even if the most optimistic parameter values are used. If the worker is protected by an insulator link, the current depends mostly on the link impedance. The equivalent circuit used for the body representation has a minor effect. The results shown in Figure 8 were calculated by modeling the body impedance by a 700 ohm resistance. Following the discusser's suggestion, an additional analyses was performed. In this analysis the insulating rubber boot was represented by a capacitance of 170 pF and a resistance of 15 Mohm. The results show, that the resistance increases the current. For example, at 69 kV, the current is increased from 2.55 mA to 3.68 mA. However, both values are under the critical 5 mA limit. Since the presentation of the paper, the author received a report [l] from Dr Hamilton, which showed that in the laboratory, the polluted insulators' resistance can be considerable lower than listed in Table 2. In the worst case, the resistance can be as low as 8 - 20 kohm. However, in real life, the insulators are cleaned and the very heavy pollution, used in the laboratory, will not occur. Never the less, the calculation was repeated with these new values. It was found that the current increased significantly. However, this large current will dry the insulator, which increases the resistance. Unfortunately, even light pollution, used in the paper, generated life danger. Therefore, this new finding does not affect the conclusion of the paper although it increases our understanding of the effect of pollution. The energy was calculated by both MICROCAP and equation 6 for the evaluation of the effect of transient current. The latter produced higher values because it assumed that all the energy from the capacitance is transferred to the worker. Therefore, the health effect of impulse current was evaluated by using the more conservative values, calculated from equation 6. 1. J.D. Morgan, H.B. Hamilton, "Evaluation of Links for Safety Applications", 1982 Oct. (Report) Manuscript received October 1, 1990.
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