BRITISH STANDARD|BS EN 60168:1995 IEC 60168:1994(2)
BRITISH STANDARD|BS EN 60168:1995 IEC 60168:1994(2)
Section 1. General
This standard applies to post insulators and post insulator units of ceramic material or glass intended for
indoor or outdoor service, and to post insulators of organic material intended for indoor service in electrical
installations or equipment operating on alternating current systems with a nominal voltage greater
than 1 000 V and a frequency not greater than 100 Hz. It may also be regarded as a provisional standard
for insulators for use on direct current systems.
The insulators covered by this standard are primarily intended for use in isolators (disconnectors) or as
bus-bar or fuse supports.
This standard covers five types of post insulators:
a) indoor post insulators of ceramic material or glass and with internal metal fittings;
b) indoor post insulators of organic material and with internal metal fittings;
c) outdoor cylindrical post insulators of ceramic material or glass and with internal metal fittings;
d) outdoor cylindrical post insulators of ceramic material or glass and with external metal fittings;
e) outdoor pedestal post insulators of ceramic material or glass.
The term “cylindrical insulators” is intended to cover insulators of the truncated conical form also.
The five types of insulators are distinguished by their electrical, mechanical and dimensional
characteristics. Figure 1, Figure 2, Figure 3, Figure 4 and Figure 5 illustrate typical examples of each type
These drawings are only general illustrations and other shapes and constructions are permitted.
This standard is intended to establish standard values of those electrical characteristics, mechanical
characteristics and dimensions which are essential for the interchangeability of post insulators and post
insulator units of the same type.
NOTE 1 General definitions and methods of test for insulators of ceramic material or glass are covered by IEC Publication 168.
General definitions and methods of test for insulators of organic material are covered by IEC Publication 660.
NOTE 2 The values of creepage distance as well as information on design aspects are provisional. These values are not directly
related to the specified electrical characteristics.
The performance of insulation under contaminated conditions has been considered, with the object of determining the significant
factors influencing performance. An application guide (IEC Publication 815) has been prepared recommending the range of suitable
designs. The guide also indicates:
— definitions of the site severity
— different methods for determining the site severity
— corresponding creepage distance
— the significance and influence of different parameters characterising the insulator profiles
— the influence of the diameter
— the influence of the angle of the installation of the insulator
— alternative measures to be considered such as greasing or washing.
3 Electrical characteristics
Each post insulator is designated for a specified lightning impulse withstand voltage based on the
standardized values given in IEC Publication 71-1. The minimum height to be chosen is determined by one
of the electrical characteristics given in the table, i.e. dry lightning impulse withstand voltage, wet power
frequency withstand voltage and wet switching impulse withstand voltage as applicable and according to
the relevant insulation coordination requirements. The operating voltage is not specified because
depending on service conditions, especially contamination, it cannot strictly be correlated with the height
of the post insulator.
The composition of the post insulator, i.e. the number, the size and the positioning of insulator units is not
specified. For a given height of a post insulator, however, the composition together with insulator profile
and size and shape of metal parts can all affect the electrical performance of the post insulator especially
the wet switching impulse withstand voltage value.
The rated switching impulse withstand voltage values are specified taking into account the
above-mentioned facts as well as the high dispersion in wet switching impulse tests. However for many post
insulator designs higher switching impulse withstand voltage values are obtainable.
The rated withstand voltage requirements given in tables are for tests carried out on single post insulators
in accordance with IEC Publication 168. With regard to this test arrangement the test conditions are
favourable and may result in higher withstand voltages than in service applications. This applies in
particular to complete disconnectors for which the actual withstand values may differ appreciably from
those given in the tables. In these cases an insulator with a greater standard height may be chosen or
special stress control fittings may be used.
4 Mechanical characteristics
Post insulators are standardized in mechanical strength classes based on values of the specified failing load
in the bending test.
These mechanical strength classes are as follows:
NOTE The dimensions of outdoor cylindrical post insulators according to d) above are only specified for strength classes 2 to 20.
For strength classes 25, 31,5 and 40 these dimensions are left for future consideration.
The specified strength classes are based on the minimum failing load in the bending test with the post
insulator mounted upright and the load applied horizontally at the top surface of the insulator. Where
insulators are to be mounted underhung, the standard values of bending strength may not be applicable
except for those insulators included in Table III. Other positions of mounting (e.g. horizontal) may also
affect the strength if the weight of the post insulator is not negligible. The appropriate strength rating for
methods of mounting other than upright shall be subject to agreement between the manufacturer and the
The bending strength at different points along the axis of a post insulator is defined in different ways
depending on the type of insulator (see Table I to Table VI and notes).
A failing load Px may also be specified and will refer to a load applied at x mm above the top face of the
insulator. The value of such loads shall be subject to agreement between the manufacturer and the
purchaser. For indoor insulators, values of P50 (x = 50 mm) are specified in the tables.
NOTE These values have been determined from the formula where h is the overall height of the insulator.
Mechanical strengths in tension or compression are not specified.
Mechanical strength in torsion is specified only for outdoor post insulators according to d) and e) above.
For special applications, additional mechanical characteristics may be required. In such cases, the values
of these mechanical characteristics shall be subject to agreement between the manufacturer and the
For insulators of organic material according to b) above, the following also applies:
For circular cross-sections, the specified failing loads apply for any load direction perpendicular to the axis
of the insulator. If this is not the case, the applicable load direction shall be the subject of agreement
between the manufacturer and the purchaser.
For non-circular cross-sections, the applicable load directions shall be specified.
Unless otherwise agreed, the specified failing loads apply at ambient temperature.
In Table II the difference in deflection between 20 % and 50 % of the failing load is given to permit the
evaluation of the flexibility of post insulators of organic material.
5 Dimensional characteristics
The following dimensional characteristics are specified:
— overall height;
— maximum nominal diameter of the insulating part;
— fixing arrangements (see clause 6);
— minimum nominal creepage distance (for outdoor post insulators only).
The composition of the post insulator is not specified, see Clause 3.
The nominal dimensions of an insulator shall not be greater than the specified maximum nor less than the
specified minimum values. The actual dimensions of insulators are subject to the appropriate
manufacturing tolerances. Tolerances of the insulator heights are given in Table I to Table IV; other
tolerances shall be in accordance with Clause 24 of IEC Publication 168, except that in checking the
creepage distance the nominal value shall be subject only to the negative tolerance.
The normal maximum values for the tolerances of parallelism, eccentricity and angular deviation, when
measured in accordance with Appendix A of IEC Publication 168, are as follows:
Parallelism for h u 1 m: 0,5 mm
for h > 1 m: 0,5 h mm
h in metres.
Parallelism measured for diameter D = 250 mm.
Eccentricity 2 (1 + h) mm
h in metres.
Angular deviation 1° either clockwise or anti-clockwise.
For post insulators of more than one unit, experience shows that if the units comply with the above
tolerances there are usually no problems of alignment when they are used in stacks of two or more units.
The above-mentioned values apply in the absence of any agreement to the contrary between the
manufacturer and the user. Other values may, however, be agreed upon according to the demands which
the application of the insulators makes on the accuracy.
NOTE Reference should be made to IEC Publication 168 regarding action to be taken when units do not comply with the above
For post insulators according to Table III, Table V and Table VI, only one class of minimum creepage
distance is specified.
For post insulators according to Table IV, there are two classes of creepage distance.
The amount by which the creepage distance of an insulator may be increased within the specified
dimensions varies according to the design and size of the insulator, and, where increased creepage distance
is required, it should be the subject of agreement between the manufacturer and the purchaser in order to
avoid designs which are unsuitable for service in polluted atmospheres.
6 Fixing arrangements