1、 1 Copyright 2012 IEEE. All rights reserved. Interpretation Section 23. Clearances Rule 235C2b(1)(c)i Clearance for wires, conductors, or cables carried on the same supporting structure Vertical clearance between line conductors Additional clearances - Sag-related clearances (2007 Edition, pages 134
2、-139) Rule 235C2b(1)(c)i Clearance for wires, conductors, or cables carried on the same supporting structure Vertical clearance between conductors at the support Additional clearances - Sag-related clearances (2012 Edition, pages 145-150) (26 March 2012) IR566 Reference: In 2007, NESC Rule 235C2b(1)
3、(c) was changed by adding the underlined words. This addition remains in 2012. (b) For purposes of this determination the vertical clearances required in Rule 235C2b(1)(a) and (b) apply to the following conductor temperature and loading conditions whichever produces the greater vertical clearance at
4、 the structure when: i. The upper conductor is at final sag at 120F or the maximum operating temperature for which the line is designed to operate and the lower conductor is at final sag at the same ambient conditions as the upper conductor without electrical loading, or ii. The upper conductor is a
5、t final sag at 32F with the radial thickness of ice, if any Question: This rule was clear in the 2002 NESC. Since the 2007 change, it is unsure what ambient temperature is to be used with the 120F temperature for the upper conductor. Additional clarity or definition to the intended requirements is r
6、equested. _ National Electrical Safety Code and NESC are both registered trademarks and service marks of the Institute of Electrical and Electronics Engineers, Inc. The NESC is available from the Institute of Electrical and Electronics Engineers, Inc., 445 Hoes Lane, Piscataway, NJ 08854, USA (http:
7、/standards.ieee.org/) 2 Copyright 2012 IEEE. All rights reserved. For a line that is limited by clearance to the circuit below it, the maximum operating temperature for which the line is designed to operate during the winter may be less than 120F. Was it the intent of NESC to associate the 120F temp
8、erature upper conductor temperature with a normal summer ambient temperature? If yes, that would be reasonable. It seems unreasonable to require clearance for the upper conductor sagged at 120F when the ambient temperature is -20F because the assigned operating limits may not permit operation to 120
9、F during the winter season. Thus, the confusion regarding this rule as revised in 2007. Explanation: Transmission operators have amp limits and, for a line with vertically arranged circuits, that limit would likely be the same for both winter and summer. This is accomplished by prescribing different
10、 maximum operating temperatures for winter than summer. In this manner, the required clearance is assured regardless how cold the ambient temperature may be. For example, ATC may have a transmission line with an amp limit assigned for a max operating temperature of 150F assuming an ambient of 90F. T
11、his same line with a constant operating limit would only attain a conductor temperature of about 40F when the ambient temperature was -20F. In practice, ATC and many utilities have used a more conservative approach. For the above line (with an operating limit defined to operate at 150F assuming an a
12、mbient of 90F), the utility may assume the lower conductor temperature is 60F (not 90F, which was the basis for the rating for the upper conductor). Considering this more conservative approach, is this new rule violated when the ambient temperature drops to -20F? Note a clearance violation would not
13、 occur in real operation because the capacity limit restricts the operating temperature of the upper conductor to well below120F on a cold wintery day. Utilities that have conservatively applied this rule prior to 2007 may now, depending on the interpretation, have to de-rate these vertically config
14、ured lines to ensure the upper conductor would not exceed 120F during the coldest day of the year. It is not believed that was the committees intent so long as the proper clearance is ensured throughout the year by appropriately restricting the operating capacity. Interpretation The clearances requi
15、red by Rule 235C2b(1)(c) apply as specified in either (i) or (ii), whichever produces the greater vertical clearance at the structure. In the first case (i), the upper conductor is at either 120oF or the maximum operating temperature if greater than 3 Copyright 2012 IEEE. All rights reserved. 120oF
16、and the lower conductor is at the same ambient conditions as the upper conductor and without electrical loading. In the second case (ii), the upper conductor is at 32oF with ice loading as required, if any, and the lower conductor is at the same ambient conditions as the upper conductor and without either electrical loading or ice loading. In essence, (i) is normally a summer condition and (ii) is a winter condition. An interpretation was requested of the NESC 2007 and 2012 Editions. This interpretation is applicable to both.
