Hidden Heat: The Unseen Hazard of a High Resistance Connection


Expertise Includes:

    • Electrical Equipment Failures
    • Electrocution & Electric Shock Accidents
    • Fires & Explosions
    • High Energy Laser Design
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    • Lightning Strike Analysis
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    • Scope of Damage - Estimate of Repair

A typical residence can have upwards of 10,000 feet of electrical conductors installed, most of which are buried in the walls, attics and crawlspaces.  A commercial building can have 100,000 to upwards of 1 million feet of electrical conductors.  At each device such as a switch or a receptacle are at least three, and typically six or more connections of these conductors within a junction box.  The connections can be in the form of twisted connectors, screw terminals, push in terminals and crimped connectors.

Every light fixture will also have a number of connections, with a minimum of three.  Now consider the number of receptacles, switches and fixtures within a building and realize that there will be a thousand to tens of thousands of wire connections within a building.

FIGURE 1 – A view of a typical junction box with both screw terminal and twist (wire nut) connections.

Each of these connection points has the potential to have a defect.  A loose equipment grounding conductor could prevent a Ground Fault Circuit Interrupter (GFCI) from functioning correctly.  A loose connection in either the hot or neutral wires can have a different type of effect.  A loosely connected wire may allow current to flow while at the same time heating up from the poor or limited amount of contact.  This heating will cause oxidation to gradually form at the interface of the connected wires.  This oxidation further degrades the connection, adding more resistance to the flow of electricity and causing a voltage drop in the circuit at the point of the bad connection.  As this localized resistance builds, the amount of heat generated at the poor connection increases.  The term for this condition is a high resistance connection.

Electrical demands on the affected circuit past the defective connection can cause the high resistance connection to get hot, even to the point of glowing red with a temperature over 700 degrees Fahrenheit.  The elevated temperatures cause the oxidation of the connection to happen more quickly.  When the electrical demand is removed from the circuit past the defective connection, say from turning off an appliance, the connection cools off.  This condition may exist for a long time, with the amount of deterioration to the connection increasing with each use.

Eventually, the amount of thermal energy radiated from the high resistance connection causes the materials surrounding the junction box to be heated.  If the demand on the circuit remains for a long enough period of time, the temperatures seen by these materials, such as a wooden wall stud, reach their ignition temperature.  The result is a fire in the wall cavity and the surrounding area.

Figure 2: View of the back of a duplex receptacle damaged by a high resistance connection that was caught in the early stages of a fire

High resistance connections are typically caused by a defect in workmanship, with the failure to tighten connections the most common.  The failure to align multiple conductors mated with a twist on connector such as a wire nut is another common cause.  In a circuit breaker, damage to the contact barbs that slide over the buss bar can occur from an improper alignment during insertion into the panel.

Figure 3: View of a damaged wall area from the receptacle shown in Figure 2

Figure 4: View of a damaged circuit breaker from a loose contact to the buss bar in a panelboard

While the likelihood of finding one of these conditions seems rare, I have personally experienced two at houses that I or my relatives have owned over the past 20 years.  The process of finding such a high resistance connection can be difficult.  A load has to be placed on the circuit at the farthest device from the circuit breaker and then each device (receptacle or switch for example) has to be examined for a rise in temperature.  This can be accomplished with thermal imaging, an infrared thermometer or simply with the back of your hand around the perimeter of the receptacle or switch.  Any devices that are warm need to be examined by a qualified individual.

If you are in need of a forensic electrical engineer, contact an expert at WARREN.

Tom Kelly has a Bachelor of Science in Electrical Engineering and a Master of Science in Electrical Engineering from Florida Atlantic University, Boca Raton, Florida, along with a Master of Business Administration with emphasis in strategic leadership from Winthrop University, Rock Hill, South Carolina. Tom’s 25-year career in electrical engineering includes forensic engineering investigations involving industrial electrical accidents, electrical equipment failure analysis, control system failures, robotics and automation components, and scope of damage assessments.  He has conducted investigations for fires, arc flash incidents, electrocution and electric shock accidents and lightning strike evaluations.

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