Due to the Vietnam War, Copper was pulled from buildings and aluminum wire was used instead. Aluminum wire is a soft metal with high resistance and because of this the wire will expand when any kind of a electrical load (POWER) is applied. Where ever the wire is cut and installed under screws for switches and plugs when power is used on the circuit it will cause the wire to expand and the screw will slowly over time unscrew and leave a gap between wire and screw and start to spark in wall and cause fire.

Pig tailing with the proper Wire Connector and attach copper wire to aluminum wire, then install copper wire under screw will stop the fire danger. Aluminum wire is corrosive every place the insulation from the wire is removed and the bare wire is exposed. It is at this point that the wires should be pigtailed on the screws to the switches and plugs.