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String of suspension insulators

"Suspension" and "Strain" insulators are the same thing; when used in the vertical position (with the conductor hanging below) they are called "suspension"; when used in the horizontal, to dead-end a conductor, they are called "strain".

There are two basic types of suspension insulators: unipart and multipart.

Multiparts are no longer produced, and original units are rare today. There are only two known styles: Cochrane and Byllesby, both dating from the late 1910s.

Uniparts (see left and right) were first developed in the 1910s and are the common style still in production today. Each unit has a single glass shell bonded to a steel cap (above) and a steel pin (below). The modular units connect with each other cap-to-pin, forming a chain or "string".

The mechanical strength of a string is determined by the rating of the metal parts of a single unit, but the electrical rating can be increased by simply adding more units. This system allows for much higher insulation value than any pin-type insulator: the largest one-piece pin-type power insulator ever made in the U.S. was the Corning Pyrex No. 701, which is rated at only 70,000 volts, while a string a suspension insulators could be assembled that stands off 1,000,000 volts or more.

There are three basic styles of unipart suspension insulator shells: (1) "sombrero" (so-called by collectors), which is flat, with multiple ridges or "skirts" on the under-side which increase the leakage distance; (2) fog or "bell", in which the disk is curved into a bell or nearly hemispheric shape, the indent being to keep the ridgeless underside from being wetted as easily; and (3) aero(dynamic) style, a wide flat disc (also with no ridges) which presents little cross-section and so is least affected by high winds.

String of suspension insulators in Cupertino, CA

String of suspension insulators in Santa Clara, CA
Multipart
Byllesby — shells: 5 — width: 6¾" — length: 22" — weight: 15#
Purple Byllesby multipart suspension insulator
Photo: Gary Romprey

Aqua Byllesby multipart suspension insulator by by Hemingray

This insulator was invented around 1917 by Henry M. Byllesby (formerly of Edison, Westinghouse, and T.H.E. Co.), who formed his own engineering company (H. M. Byllesby & Company) in Chicago. "The insulator consists of a micarta core on which are assembled glass disks separated by asbestos washers and held in place by a simple interlocking washer and cotter key. After the parts are in place the unit is heated uniformly to a temperature of 100 deg. Fahr. and [tar] compound poured into the core through the top casting" (Electrical World, April 21, 1917) It's reported that 15,000 of these units were made, although they are very rare today, with only 2 purple strings and perhaps 3 original aqua strings known. The purple unit pictured sold on eBay in June, 2010 for $20,000. The aqua shells were aparently produced by Hemingray (shell fragments were found at the company dump), but the maker of the purple shells is unknown, possibly the same glasshouse that made the Cochrane bells. Two sizes were made, one with 5 disks (pictured), and other with 11. "The eleven-disk insulators will be used exclusively on the 110,000-volt line which is being built from Wisconsin to Minneapolis to serve the Twin Cities. The line west of Stillwater is being requipped with them. Five disk insulators are being placed on a number of 60,000-volt lines in various parts of Minnesota. The Montana Power Company was one of the first companies to try out the glass disk feature." (ibid) The five disc units were also used in California.

Cochrane — shells: 6 — width: 6" — length: 25" — weight: 16#
Purple Cochrane Bells multipart suspension insulator

Patented by Harry Hamilton Cochrane of Butte, Montana in 1916 (No. 1,194,957), these "Cochrane Bells" were used only in Montana, mostly on the 65KV, 13½-mile Cochrane line which ran from the Bertha substation near Corbin to the Porphry Dike mine. The shells are stacked on a parafin-soaked oak rod with cork washers between; cast iron caps screwed to either end of the threaded rod clamped the assembly together. The shells come in purple shades from near-clear to a rich full purple; pictured is the typical medium purple. A much rarer stationary version with ten larger bells also exists.



Unipart
A Proposed Numbering System
I propose an S-numbering system for glass suspension insulators. The system is keyed primary off the glass shell, with a suffix to indicate variations in the metal cap and pin.

To make the system more mnemonic and useful, the numbering would be S-DDNN where DD is the diameter in inches of the shell, rounded down to the nearest inch. NN is an arbitrary sub-code to distinguish the individual styles near the same size. Within a size, numbers are assigned in petticoat order. That is, the fewer skirts, the lower the number. "Bell" or "aero" type glass, with just a single outer shell, would come first. Complicated "sombrero" types with numerous inner skirts would come last.

Since the smallest size insulators is about 5", that leaves the lower range of numbers available for multiparts and other uses. I would put multiparts in the 00NN range, and leave 01, 02, 03 and 04 for other uses (radio strains?).


Unipart Suspension Table
S-#### Maker Style# Width Shape Skirts Hardware Weight Color(s)
S-5## Sediver 5" Disc 1 Clevis 2 lb. Aqua, Off Clear
S-5## Sediver 5¼" Disc 1 Clevis 3 lb. Olive Tint
S-6## Pyrex 65221 6" Disc 3 Clevis 4½ lb. Clear, Clear w/Carnival
S-6## Pyrex 65241 6" Disc 3 Hook 4½ lb. Clear?
S-6## Pyrex 65251 6" Disc 3 Clevis+ 4¼ lb. Clear?
S-6## EIV 6¾" Disc 3 Ball 4 lb. Aqua
S-7## Pilkington 7" Disc 3 Clevis 4 lb. Ice Green
S-7## Pilkington 7" Disc 2 Clevis 4 lb. Light Yellow Olive
S-9## Pyrex 9" Disc 4 Hook 6 lb. Clear w/Carnival
S-9## Pyrex 9" Disc 4 Clevis 5 lb. Clear
S-10## EIV 10" Disc 4 Clevis 8 lb. Aqua
S-10## EIV 10" Disc ? Ball 8 lb. Aqua
S-10## EIV 10" Disc 3+ Ball 13 lb. Aqua
S-10## Sediver 10" Bell 1 Ball 9 lb. Aqua
S-10## Sediver 10" Bell 1 Ball 8 lb. Aqua
S-10## Sediver 10 1/8" Bell 1 Clevis 10 lb. Aqua
S-10## Pyrex 65234 10 Disc 4 Ball 8¼ lb. Clear
S-10## Pyrex 65244 10 Disc 4 Clevis 8¼ lb. Clear
S-10## Pyrex 65216 10 Disc 5 Ball 12 lb. Clear
S-10## Pyrex 65226 10 Disc 5 Clevis 12 lb. Clear
S-10## Pyrex 65214 10 Disc 4 Ball 8½ lb. Clear w/Carnival
S-10## Pyrex 65224 10 Disc 4 Clevis 8½ lb. Clear w/Carnival
S-10## Russian 10 Disc 3 Ball 7 lb. Light Blue-Aqua
S-11## EIV 11¼" Disc 4 Ball 12 lb. Aqua
S-12## EIV 12½" Disc 5 Ball 21 lb. Aqua