Signal insulators saw extensive use on both telephone and low voltage electric distribution. There are probably more varieties of colors and styles in the signal catagory than any other. This is also one of the more challenging areas for U number identification!
Almost every company making porcelain insulators made some signal types. Due to their frequent use for electric street lighting and low voltage distribution, colors were often used to indicate their functions. Blue or yellow was often used to indicate a "hot" line mixed in with telephone lines, or to differentiate between circuits. Green was extensively used in the east to mark series street light circuits.
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