The "U" style connector was invented by 3M® in the 1970's for communications applications. It eliminated the need to strip and twist the POTS (Plain Old Telephone) wires by using insulation displacement "U" shaped crimping contacts. Insert the wires, crimp the circle shut and you have a secure connection. Over time, a variety of wire sizes and tap types were created and color codes used to tell the wire sizes. The UG is used to tap into an existing line without causing any kind of interruption. Good for sizes 19-26AWG solid telecom or alarm wire.
UR type telcom splices.
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