United states patent number 4,021,092 was assigned to today's Perma-Link® plugs and connectors which are classified among the most rugged and safest wiring devices available. They incorporate many of the same features as Ericson® Perma-Tite® 2 and Perma-Grip™ devices and utilize A 2-screw cable clamp. Perma-Link® plugs and connectors are designed to provide many years of trouble free service in the most demanding industrial and construction environments. Ericson® manufacturing company engineers and produces a comprehensive range of plugs and connectors for demanding industrial and construction environments. From the inVention of the first dead front, back-wired electrical plug in 1929 and patent in 1936 to present day innovations that are designed to meet or exceed UL and IEC watertight standards.