Smoke or burn marks on an outlet are often caused by heat resulting from an overloaded circuit, however, there are several other things that should be checked as well. Smoke or burn marks should be noted as a red flag that there may be a fire safety concern.
When an electrical current leaps across a gap, this intermittent connection causes a spark, or arcing which translates to heat.
The source of the heat causing the outlet face to be burned or show smoke can be due to a number of causes, including the outlet having the hot (energized) wire touching the neutral or ground, or it just being close enough for arcing to occur. Loose wire connections that are not secured down tightly can also be at fault.
Corrosion or Heavy Dust Buildup
Sometimes corrosion has built up over time and is creating the problem. Occasionally, dust or dirt can also be the culprit. These can potentially result in heat build up, or arching. While these can cause arcing, they are rarely the cause.
Outlets that are old and have had the a lot of use over the years get worn out in the area where the metal pieces of the plug come in contact with the metal pieces in the receptacle. When it reaches a point where the area of contact is much smaller then when the outlet was new, then the resistance builds up which creates heat. When the heat gets excessive it may cause a burning smell and/or burn marks, or smoke marks on the outlet or the cover of the outlet.
Where the wiring connects to the outlet, the screws or the wiring may have worked loose, therefore there is not a full connection and resistance and heat build up. This also results in smoke or burn marks to the plastic.
An overloaded outlet or circuit may also cause smoke on the face of the outlet, as well as a smoke or burnt odor. Overloading from a power surge can create the same condition.
Outlets on overloaded circuits, or with too many appliances connected and operating at the same time, may likewise have smoke or burn marks on them.
Replacing outlets that are burnt should be done. Any outlet that has smoke or burn marks on it, should be checked for damage, for wear and tear, and for proper wiring connections. Replacing the outlet is the best idea.
Caution: When plugging in a lamp or other appliances be sure that there is a good connection. Sometimes when we plug in a lamp the plug may fall out or drop down. This is an indication that there is not full and proper contact/electrical contact. When there is insufficient contact of the metal parts, then heat can build up.
Damage or Improper Wiring
If an outlet becomes damaged from use, being hit by a hard object or an electrical cord being jerked out of the outlet, then arcing may occur. Note that this is seldom the reason for burn marks or smoke on an outlet.
Improper wiring, can also generate the smoke. If an arc fault has occurred, the excessive heat from the arc may cause the plastic to overheat and therefore it burns slightly.
Who to consult? An electrician.
Electricians can generally advise buyers on burn't outlets, do replacement of outlets and trouble shoot circuits if necessary. Also, qualified handymen can replace damaged or defective outlets.