General Order 64

Detail Construction Requirements For Supply Lines

64    Insulators.

(a)    Insulators used on supply lines shall be porcelain, glass or other equally suitable material.

The insulators shall be so designed that their dry flash-over voltage will be less than their puncture voltage, at 60 cycles frequency.  Strain insulators shall be capable of withstanding under their mechanical loading at least as high a voltage as line insulators in general.

(b)    Insulators shall be capable of withstanding without flash-over at a frequency of 60 cycles, the voltages given in the following table:

 

Table 6 Insulator Minimum Flash-Over Voltages
Voltages of circuit	Flash-over voltage
	Dry	Wet – 1-5 inch rain per minute – 45°
750	5,000	3,500
2,300	11,000	8,000
4,000	17,000	13,000
6,600	27,000	17,000
7,500	30,000	18,000
11,000	40,000	20,000
15,000	55,000	25,000
22,000	75,000	40,000
33,000	105,000	65,000
44,000	125,000	85,000
55,000	150,000	100,000
66,000	170,000	115,000
88,000	225,000	165,000
110,000	275,000	200,000
150,000	350,000	250,000
200,000	450,000	300,000

For intermediate voltages use the value for flash-over voltages obtained by interpolation.

(c)    All insulators or insulator units, used on lines on which the voltage exceeds 17,000, shall be subjected to flash-over test at frequency of 60 cycles, or to such other tests as may be generally sanctioned by good modern practice.

(d)     Insulators used in territories where fog conditions prevail should be given more liberal factors of safety than those specified in the above table.

(e)     Insulators, supports, clamps and miscellaneous attachments shall be designed to withstand with the designated factors shall be designed to withstand with the designated factors of safety, the tension in conductors, wires or structures under the loading conditions as specified herein.