Line Insulation. Section APPLICATION OF RULE

Transcription

1 Section 27 Line Insulation 270. APPLICATION OF RULE Line insulation in Sec. 27 applies to insulators for open conductor supply lines only, not communication lines or secondary duplex, triplex, or quadruplex (230C3) cables. Note 1 references Rule 243C6 (this should be Rule 243C5). Rule 243C5 states that the strength requirements in Sec. 27 apply to all grades of construction. There are not separate Grade B and Grade C strength factors for insulators. Note 2 references Rule 242E for insulation requirements of neutral conductors. Rule 242E states that supply neutrals, which are effectively grounded and not located above supply conductors of more than 750 V to ground, do not need to meet any insulation requirements. Even though effectively grounded neutrals do not need to meet any insulation requirements, it is common to see neutral conductors attached to insulators for support and termination purposes. Section 27 does not apply to secondary duplex, triplex, and quadruplex (230C3) cables, however, it is common to see the messengers of these cables attached to insulators for support and termination purposes. The Code rules related to the application of line insulation are outlined in Fig MATERIAL AND MARKING Supply circuit insulators are required to be wet-process porcelain or other materials. Examples of insulators made of porcelain and other materials are shown in Fig

2 320 OVERHEAD LINES Rule 272 Fig Application of line insulation (Rule 270). The manufacturers must mark insulators for use at or above 2.3 kv with the manufacturer name or trademark and an identification (e.g., a catalog number) that will permit determination of the electrical and mechanical properties. The Code rules for marking insulators are outlined in Fig RATIO OF FLASHOVER TO PUNCTURE VOLTAGE Rule 272 requires insulators to meet standards for a flashover to puncture voltage ratio. A list of applicable standards is provided. When a standard does not exist for the flashover to puncture voltage ratio, Rule 272 specifies a not-to-exceed value of 75 percent with an exception that permits not more than 80 percent in areas of high atmospheric contamination. The flashover to puncture voltage ratio is used by manufacturers but it is not always published in insulator catalog data. Insulator catalog data typically does include the low-frequency dry-flashover value.

3 Rule 273 LINE INSULATION 321 Fig Examples of insulators made of porcelain or other materials (Rule 271) INSULATION LEVEL Rule 273 specifies insulator dry flashover voltages in NESC Table The dry flashover tests must be done in accordance with ANSI Standard C29.1. Using dry flashover voltage ratings lower than the values in NESC Table requires a qualified engineering study. Values higher than shown in NESC Table must be used for areas with severe lightning, high atmospheric contamination (e.g., salt water fog, industrial plant pollution, etc.) or other unfavorable circumstances. The low-frequency dry flashover rating is the most common value referenced when comparing insulator specifications. Additional ratings are commonly published in insulator catalog sheets and in insulator specifications. Typical insulator ratings are listed in Fig In addition to electrical ratings, the NESC specifies mechanical ratings for insulators in Rule 277.

4 322 OVERHEAD LINES Rule 274 Fig Marking of insulators (Rule 271). Fig Insulation level requirements (Rule 273) FACTORY TESTS Insulator factory tests per ANSI Standards are required for insulators at or above 2.3 kv SPECIAL INSULATOR APPLICATIONS Special consideration must be given to constant-current circuits (e.g., series street lighting). The voltage rating of the insulator must be based on the full load rated voltage of the supply transformer, not just the operating voltage of the circuit.

5 Rule 277 LINE INSULATION 323 Single-phase lines fed from three-phase lines are required to use insulators based on the three-phase line phase-to-phase voltages. See Fig NUMBER 276 NOT USED IN THIS EDITION 277. MECHANICAL STRENGTH OF INSULATORS Rule 277 specifies the mechanical strength requirements for insulators. This is done in the form of strength factors that are applied to the insulators. Overload factors for wire tension, wire vertical (weight) loads, and wire transverse (wind) loads are not applicable. Rule 277 requires insulators to withstand the loads in Rules 250, 251, and 252. The overload factors in Rule 253 are not required and the extreme wind loads of Rule 250C are not required. Said another way, the overload factor for the loads on insulators is 1.0. The strength factor percentages provided in Rule 277 do not distinguish between Grade B and Grade C construction. After Rule 277 states that the extreme wind loads in Rule 250C are not applicable, the rule continues on requiring proper allowance for Rule 250C loads. Rule 250C only applies to structures and supported facilities more than 60 ft high. Fig Insulators of singe-phase circuits connected to three-phase circuits (Rule 275).

6 324 OVERHEAD LINES Rule 278 The strength factor percentages listed in Rule 277 are of the insulator s rated ultimate strength. The method in which rated ultimate strength is calculated for various types of insulators and various insulator materials is discussed in Note 1 and Note 2. Notes 1 and 2 reference ANSI and IEEE Standards for the test method used to determine rated ultimate strength. The rated ultimate strength of an insulator may not be very clear on an insulator catalog sheet. Catalog sheets may use terms like routine test load, maximum design load, average failing load, average breaking load, or maximum working load. The insulator manufacturer should be consulted or the ANSI and IEEE Standards should be reviewed to verify that the percentages provided in Rule 277 are being applied to the insulator s rated ultimate strength as specified in Notes 1 and 2 of Rule 277. Examples of cantilever, compression, and tension loads on insulators are shown in Fig AERIAL CABLE SYSTEMS Aerial cable systems primarily consist of tree wire conductors described in Rule 230D. The insulators supporting aerial cables must meet Rule 273. The covered conductors of aerial cable systems are considered bare conductors for electrical insulation requirements. The insulators and spacers of an aerial cable system must meet Rule 277. Rule 278B2 requires that the insulating spaces used in aerial cable systems withstand the loads specified in Sec. 25. Section 25 includes Rule 253, which contains overload factors. Rule 277 does not require overload factors as Rule 277 specifies the loads in Rules 250, 251, and 252, not Rule GUY AND SPAN INSULATORS 279A. Insulators. The Code rules for the material properties of guy insulators and the electrical and mechanical strength of guy insulators are outlined in Fig The requirements for mechanical strength of a guy insulator hinge on the word required strength of the guy. Guy insulators do not require the application of strength factors like line insulators in Rule 277. Guy insulators indirectly have overload factors applied to them via the overload factors that are applied to wire tension and wind loading. See Fig Guys may be ungrounded and insulated per Rule 279A2a or grounded per Rule 215C2. The grounding methods for guys are provided in Rule 092C2. See Fig When guy insulators are installed, they must meet the physical location criteria provided in Rule 279A2b. Complex structures with multiple guyed circuits require careful application of Rule 279A2b. The Code rules related to installation of guy insulators are outlined in Fig If a guy insulator is used exclusively for the elimination of corrosion of an anchor attached to a grounded guy on an effectively grounded system, then it is

7 Rule 279A LINE INSULATION 325 Fig Examples of cantilever, compression, and tension loads on insulators (Rule 277).

8 326 OVERHEAD LINES Rule 279A Fig Properties of guy insulators (Rule 279A1). not classified as a guy insulator. Therefore, the electrical requirements (i.e., the dry flashover and wet flashover ratings) do not apply. The guy in this case is assumed to be grounded per Rule 215C2 using the grounding methods in Rule 092C2. The corrosion protection insulator must not reduce the mechanical strength of the guy. The Code is not specific in this case whether this is the required strength of the guy as stated in Rule 279A1c or the strength of the guy wire. An example of a guy insulator used exclusively for corrosion protection is shown in Fig

9 Rule 279B LINE INSULATION 327 Fig Example of the required strength of a guy insulator (Rule 279A1). Fig Use of guy insulators (Rule 279A2a). The clearance requirements between a guy insulator and energized conductors are provided in NESC Table See Rule 235E for a discussion. 279B. Span-Wire Insulators. The requirements of span-wire insulators are similar to guy insulators. Rule 274 is referenced for insulator testing. The rated ultimate strength of a span wire insulator is based on the required strength of the span wire in which it is located.

10 Fig Installation of guy insulators (Rule 279A2b). Fig Example of a guy insulator for corrosion protection (Rule 279A3). SECTION NUMBER 28 NOT USED IN THIS EDITION SECTION NUMBER 29 NOT USED IN THIS EDITION