AS IEC 60720-2007 pdf Characteristics of line post insulators (IEC 60720, Ed. 1.0 (1981) MOD) 

Each line post insulator is characterized by the specified minimum bending failing load. Generally, this bending failing load is 12.5 kN; in addition, for tie-top line insulators of impulse withstand voltage levels up to and including 170 kV, itis 8 kN. The bending load is applied at the centre of the side groove in the case of a tie-top type insulator and at the point determined by dimension H in the case of a clamp-top type insulator.
2 Object The object of this standard is to prescribe specified values for electrical and mechanical characteristics and for the principal dimensions of line insulators of ceramic material (see Tables I and I). NOTE – General definitions and methods of tests are given in IEC 60383. Tosts on insulators of ceramic material or gla8s for overhead lines with日nominat voltage greater than 1000 V. AS/NZS 2917.1, Insulators Porcelain and glass for overhead power lines- Voltages greater than 1000 a.c. Part 1: Test methods- Insulator units
5.1 For tie-top type insulators only (Figures 2 and 3) Head diameter. Neck diameter. Top groove radius. Side groove radius. Distance between bottom of top groove and centre line of side groove. NOTE – By agreement between the purchaser and the manufacturer, the insulator may be manufactured without top groove. It is also possible, by agreement, to use the head of Figure 3 for types R 200, R 250 and R 325. 5.2 For clamp-top type insulators only (Figure 6) Dimensions of clamp-top bracket.
6 Fixing arrangements The fixing arrangements shall be in accordance with Figure 8. The centre hole diameters shall have Iso metric threads and may be oversize by not more than 0.25 mm (0.01 in). They shall be suitable for steel pins having standard threads after galvanizing.
7 Designation and marking Line post insulators are designated in Tables I and II by the letter R following by figures indicating the bending failing load in kilonewtons. Then follows the letter E or J indicating external or internal fixing of the metal parts. This is followed by the letter T, C or H indicating tie-top type, vertical mounting clamp-top type or horizontal mounting clamp-top type, respectively. The following figure indicates the specified lightning impulse withstand voltage in kilovolts. The letter N or L which then follows, indicates normal or longer creepage distance respectively.