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Energetic weather events such as flooding means that indoor branch circuits that were never intended to get wet may experience varying degrees of moisture. After the waters recede and ambient heat dries the connection, corrosion can set in. A slight amount of impedance at the connection interface coupled with electrical current flow always results in I squared R heat rise, which sets the stage for more corrosion.

Whether the full effects surface hours, days or years later the end result will be somewhere between an annoying callback for the electrician and catastrophic loss of property or life.

Arc fault circuit interrupters address this hazard by recognizing the unique sputtery signature of a series connection with too much impedance or a parallel fault with too little impedance, but only after these defects have caused an outage. Other impedance issues can exist unseen by AFCI and GFCI equipment — an example is a poor connection within an equipment grounding conductor.

Moreover, corrosion is only one of several factors in this equation. Other problems include terminations not fully torqued or overtorqued to failure, wirenuts that disengage when wiring is squeezed back into an undersized enclosure, raceway serving as equipment grounding conductor that separates due to structural problems of the building or concrete cracking and separating. In other words, the list is immense.

Unwanted impedance in circuitry can result in either of two harmful effects, depending upon where it occurs. If a splice or termination within a phase (hot) conductor or a neutral develops a slight resistance and if the circuit is energized and a load is connected, an intense hot spot will occur. The same thing will happen if one of these conductors becomes partially severed, for example by an errant nail driven through the wall. Sometimes a hot spot will burn itself out and open up the circuit. But if there is flammable material nearby, fire often results.

Another very serious problem can result from a high impedance anywhere along an equipment grounding conductor. Since no current flows through this conductor during normal operation, this type of fault can remain dormant for years until a ground fault develops in a location such that the impedance lies between the ground fault and the service equipment where the equipment grounding conductor is connected to the neutral through the main bonding jumper.

If the impedance is great enough, the fuse or circuit breaker will fail to open and the supposedly grounded metal case of a power tool or appliance will remain energized until a person touches and completes the path to ground. Moreover, the grounding conductor's hot spot can ignite adjacent flammable material, so that there is the risk of both shock and fire.