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Overheating Connections
Connection points are the most likely place for overheating to occur on a circuit. The most likely cause of the overheating will be a loose connection or the presence of resistive oxides at the point of connection. Metals at an overheating connection will be more severely oxidized than similar metals with equivalent exposure to the fire. For example, an overheated connection on a duplex receptacle will be more severely damaged than the other connections on that receptacle. The conductor and terminal parts may have pitted surfaces or may have sustained a loss of mass where poor contact has been made. This loss of mass can appear as missing metal or tapering of the conductor. These effects are more likely to survive the fire when copper conductors are connected to steel terminals. Where brass or aluminum are involved at the connection, the metals are more likely to be melted than pitted.

Currents in excess of rated ampacity produce effects in proportion to the degree and duration of overcurrent. Overcurrents that are large enough and persist long enough to cause damage or create a danger of fire are called overloads. Under any circumstance, suspected overloads require that the circuit protection be examined. The most likely place for an overload to occur is on an extension cord. Overloads are unlikely to occur on wiring circuits with proper overcurrent protection.
Overloads cause internal heating of the conductor. This heating occurs along the entire length of the overloaded portion of the circuit and may cause sleeving. Sleeving is the softening and sagging of thermoplastic conductor insulation due to heating of the conductor. If the overload is severe, the conductor may become hot enough to ignite fuels in contact with it as the insulation melts off. Severe overloads may melt the conductor. If the conductor melts in two, the circuit is opened and heating immediately stops. The other places where melting had started may become frozen as offsets. The finding of distinct offsets is an indication of a large overload. Evidence of overcurrent melting of conductors is not proof of ignition by that means.

Common Electrical Problems