Touch a receptacle that has been running a space heater for two hours and it will feel slightly warm. Touch one that has nothing plugged into it and finds itself warm anyway, and something is wrong behind the wall plate.

Heat in an electrical connection almost always means resistance. Current passing through a tight, clean connection produces very little of it. Current passing through a loose screw, a corroded terminal or a spring clip that has lost its tension produces a great deal, and that heat concentrates at exactly the point that is already failing.

The most frequent cause in houses built after the late 1960s is the push-in terminal on the back of the device. Those terminals hold the conductor with a small metal spring rather than clamping it under a screw. They are quick for the installer and they work for years, until repeated heating and cooling relaxes the spring. Once the contact patch shrinks, resistance climbs, more heat is produced, and the process feeds itself.

Other causes include an over-loaded circuit sharing that receptacle, an undersized extension cord plugged into it, a plug whose blades have spread and no longer grip the contacts, or aluminium conductors terminated on a device never listed for them.

The practical test is simple. If a receptacle is uncomfortable to hold a fingertip against, if the plastic has yellowed or browned near the slots, if there is a faint smell of hot plastic, or if the plug arcs slightly when removed, that circuit should stop being used until someone opens the box.

What makes this worth taking seriously is that a failing connection rarely announces itself with a tripped breaker. The breaker is watching for too much current, and a high-resistance joint does not draw too much current. It simply gets hotter, quietly, behind a wall plate, until the surrounding material reaches a temperature it was never meant to see.

Where a device has to be replaced, using the screw terminals rather than the push-in holes on the back is the single cheapest improvement available. Wrapping the conductor clockwise around the screw so tightening pulls it inward, and seating it flat under the plate, gives a contact area many times larger than a spring clip and removes the failure mode entirely.

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