PLM was founded in 1951 by Frances Calvert, a cable engineer previously employed by General Electric.  G.E. had developed a line of interlocked armor cable, but there were no fittings available for terminations.  Calvert manufactured the first fittings in his basement and, later that year, sold the business to Jon Pescke, Quigg Lohr, and Carroll Miller, whose initials (PLM) formed the company name.

Starting with fittings for armored cables, PLM soon developed splices, terminations, and other accessories for joining cables.  In 1957, PLM developed the first coupler designed with a ground check interrupting circuit for safe disconnect, and the first coupler capable of handling 500 amperes at 7500 volts.

In 1963, PLM and Adalet (high quality manufacturer of hazardous locations enclosures and accessories) were acquired by the Scott Fetzer Company (subsidiary of Berkshire Hathaway).  The operations of Adalet and PLM merged in 1977.