Why does an inspector look at receptacles for evidence of scorching around the slots?

Scorching around a receptacle's slots is heat damage, and heat at a receptacle comes from resistance where there should be almost none. Two mechanisms produce it. A loose or worn connection — at the terminal screw, at a back-stab connection, or between worn contacts and a plug — creates resistance, and current through resistance generates heat. Or the circuit has been loaded beyond what the device and conductors can carry, so everything heats together. Both are progressive: heat degrades the connection further, which raises resistance, which produces more heat. Discolouration is therefore evidence that the process has been running for some time, and it is one of the few electrical defects with visible external evidence. The inspector reports it as a fire risk and recommends the device and its connections be evaluated and replaced.

Scorching is heat damage produced by resistance at the connection or by overloading. Both are progressive, and visible discolouration indicates the process has been under way for some time.

Resistive connections at receptacle terminals generate heat proportional to current, progressively degrading the connection. Back-stab connections, worn contacts and overloading are common causes, and scorching or melting at the device face is the visible evidence.

Brown Means It Has Been Hot, for a while. Heat at a receptacle is resistance, and resistance only gets worse.

Note whether the receptacle also holds plugs loosely. Worn contacts and scorching together confirm the mechanism and raise the urgency.

• •Treating discolouration as cosmetic or as paint
• •Using scorching as an indicator of device age
• •Confusing heating evidence with grounding faults