Suspension Insulators

Suspension insulators were developed to support higher voltages then standard pin types. Suspension insulators hang down from the power pole and typically can be connected together to allow for much higher voltage insulation.

This numbering system for suspension insulators was established by Raven Kotlarsky and Steve Coffman. This system covers both glass and porcelain suspensions. The numbering system uses ST (suspension type) as the prefix and covers both US and worldwide styles. The number assignment start with smallest to largest diameter, then least to most number of petticoats, then length of outer skirts and inner petticoats. In other words, tiny insulators first, big fog types last. The system provides fixed number range blocks to each major type of porcelain suspension. Cap and pin style glass suspension insulators have been given their own 1000 number block, which aids in quick identification. The ST system is divided into the following ranges by type:

ST-1000-1999 Hewlett, Hogs Liver and Related Styles
ST-2000-2999 Multipart Styles
ST-3000-3999 Classic Straight Caps
ST-4000-4999 Modern compression caps
ST-5000-5999 Multi Skirt or Multi Shed
ST-6000-6025 Solid Core Suspensions
ST-6026-6099 Half Motors
ST-6100-6499 Other Non-traditional Styles
ST-6500-6999 JD Patent Spider Discs
ST-7000-7999 Glass Cap and Pin
ST-8000-8999 Externally Ribbed Cap and Pin

Special thanks to Raven Kotlarsky for initially publishing this system and gathering the initial photographs, measurements and supporting data to document these cool insulator styles.

Early suspension insulators featured a cap and pin system that did not load the porcelain in compression and were prone to failure from heavy loads, cement expansion, or porcelain failure. This changed when A.O. Austin in a series of patents in the late 1910's developed the modern suspension insulator that loaded the porcelain in compression. These modern caps are readily identifiable by their wider, flared shape, and the occasional use of flanges to relieve the effects of flashover. The ST 4000 range includes discs that are transitional from straight tension caps to modern compression caps. As a general rule, all Ohio Brass units from about 1917 forward will fall in the ST 4000 range, and all metallic cap and pin units from other companies from about 1930 onward.

cap sizes arenot reflected in the ST numbering system. Many suspension insulators use nearly identical insulating shells, but the caps vary in size and shape based on manufacturer choice, and the load bearing capacity of the given insulator. We believe to classify each of these minor variations defeats the purpose of a handy numbering system, and that an interested collector can simply note the type of cap they have on a given insulator.

Different connection types (hook and eye, cap and pin, etc.) do not typically need a new number. Suspension insulators often use a standard insulating shell which can be fitted with a myriad of connection types based on the needs and preferences of an end user. These will simply be noted when describing the insulator. Often an otherwise identical insulator can have four or more different connection types. Giving each on a number would quickly become cumbersome.

The gallery view will show the side view of the various styles. If you click the gallery picture a detail page it will link to a detail page showing larger pictures with more views as well as multiple varieties of the same style if they have been reported. If you have a style, maker or signifnant color variation that is now shown in the gallery or detail pages, we would appreciate your assistance in providing pictures, measurements, and marking information so we can grow the data available.