Skip to content
Search for:
Standards for Insulating links
ANSI/CPLSO-14
ANSI/UL2737 (Withdrawn)
ASTM F2973
MIL-L-24410 (Withdrawn)
Tests by Independent Organizations
Load Insulator
Miller & Hirtzer
Search for:
Download
Download Page
Download Right Page
1
2-3
4-5
6-7
8-9
391 Discussion GEORGE GELA (EPRI PDC-L, Lenox, MA): The authors should be complimented on the informative and practical paper on a topic related to safety while working near energized power lines. The main premise of the paper, namely, that probabilistic methods are needed to assess the performance of the link, is well presented. Of course, as the authors clearly state, probabilistic methods require information (such as that in Figs. 4 through 6 of the paper) which cannot be provided by standard pollution tests. The acceptable risk to the worker is a subject of debate by standardization and regulatory bodies. The IEEE/PES/T&D ESMOL (Engineering in the Safety, Maintenance and Operation of Lines) Subcommittee has been tackling this question for several decades from the worker’s viewpoint. For personnel performing live work on lines and substations, the acceptable risk appears to be significantly lower than the value of “once or twice out of every 10 cases”, as suggested by the authors. The general philosophy is that the acceptable risk should not exceed risks normally encountered from other sources. The question of protection of utility personnel and the public from electrocution by making contact with vehicles operating near energized line, has also been actively addressed by ESMOL. Results of studies and suggested guidelines for effective protection are summarized in several recent publications, see Refs. [A through D]. Three methods of protection are used: (1) equal potential zones, (2) insulation, and (3) isolation [A]; the three methods can also be used in combination, and additional temporary grounding rods can be installed for the duration of the work. Advantages and disadvantages of these protection methods are outlined in Ref. [A]. Grounding systems must, of course, withstand the short circuit currents (symmetric and filly offset) anticipated at the worksite (i.e., taking full account of the X/R ratio at the worksite), see Refs. [B, C]. The discusser proposes that similar principles and approaches can also be employed while operating cranes near energized lines. Of course, the insulating link provides additional protection for the rigger. However, when the protection methods developed through ESMOL are also adopted by the crane operators, the risk to the rigger can be reduced further, i.e., below the values quoted by the authors. A danger which cannot be eliminated by the use of the link is that of electrocution or injury due to step and touch voltages produced by short circuit current upon contact with the energized line. This current, upon entering the earth, can produce significant voltage drops near the points or entry. A person standing on the ground and touching the vehicle or another grounded object, may be subjected to a significant potential difference (touch voltage) across the body. Similarly, significant potential difference can be developed between the person’s feet while walking near the point of entry of the short circuit current (step voltage). This risk is small, since the probability is low that the correct set of conditions exists to produce sufficiently large voltages to endanger the rigger while he/she is in the most vulnerable location. However, precautionary steps, such as the above listed methods, can provide effective protection for utility and non-utility workers alike. The gain of the additional degree of protection needs to be weighed against the increased site preparation time and equipment costs when using the above methods (installation of temporary grounds, barricading, insulating gloves, footwear, etc.). It should be observed that the possible danger from step and touch voltages exists in addition to and independently of the possible hazard from leakage current through the link, which the authors address in the paper. Although the leakage current alone may not be dangerous if its magnitude is below 100 mA for a short duration, as the authors correctly point out, the possible risks resulting from the combination of the leakage current and the step/touch voltages may need to be investigated. The discusser would also like to inquire about the test procedure used by the authors for short duration 60 Hz tests. Was the AC source pre-set to the desired test voltage level and then switched on and kept for the specified period of time? In this case, how was the turn-on transient controlled? Also, at the end of the test, was the AC voltage tumed off suddenly, or was it reduced before switching off the source? How was the turn-off transient controlled? Alternatively, if the AC source was switched on at a reduced voltage, and then the voltage was raised to the desired level, how did the time interval for raising the voltage compare in duration with the test time interval of 1 sec? Similarly for the termination of the test. Answers to these questions will help understand the significance of Fig. 6, which would otherwise suggest that the duration of the energization period (1 sec versus 10 sec) is essentially immaterial. In fact, the discusser would have expected the “10 sec” curve to the to the left of the “1 sec” curve of the length of the energization period is of significance. As a minor side remark, Reference [12] should read CEIhEC Publication 507: 1991 (not 750). Also, point-by-point comparisons of Figs. 5 and 6 of the paper suggests that the “1 sec” curve in Fig. 6 actually corresponds to heavy contamination (ESDD = 0.3 mg/cm2). Does the “10 sec” curve correspond to the same contamination level? Also, the caption of Fig. 7 should refer to Link 1 to conform with the text above the Figure. [A] ESMOL TF 15.07.06.03, “Methods for Protecting Employees and Others from Electrical Hazards Adjacent to Electrical Utility Vehicles”, IEEE paper 94 SM 606-4 PWRD, awaiting publication in IEEE Transactions on Power Delivery. [B] IEEE Std 1048-1990, “IEEE Guide for Protective Grounding of Power Lines”. [C] ESMOL WG 15.07.06, “Factors in Sizing Protective Grounds”, IEEE paper 94 SM 607-2 PWRD, awaiting publication in IEEE Transactions on Power Delivery. [D] ANSI/IEEE Std 516-1987, “IEEE Guide for Maintenance Methods on Energized Power-Lines’’ (currently under revision). ’ Manuscript received February 21, 1995. U. A. GILLIES Consultant, Rhododendron, OK. Placing insulation in the direct current path to the load handle in case of an accidental contact with an energized distribution power line is a method of providing protection, however, a second source of the “hazard” not covered is the step and touch potentials developed by the current flow through the crane. The authors have estimated that the crane outriggers may have grounding resistance of 50-150 ohms in dry conditions and 10-20 ohms wet. These values may be optimistic as dry ground conditions in many areas may well exceed the 150 ohms mentioned. In any case, slnce the local ground resistance is a major impedance in the fault circult, (Table l), the ground potential rise around the crane Will be quite high and the worker must not contact the crane body and should be well away from the outriggers for maximum ”protection” from the crane contact. ”Isolating” the worker from +he TTCI~C 15, in my opinion, -1s- -ri*ic=l and directly grounding the crane can reduce the ground resistance and thus the hazard. Fig 6 suggests that the flashover probability decrease with the longer energization time. Tests with similar insulators have indicated that the probability increases with 60 Hz energization time. The authors state
Page load link
Go to Top