Reducing the Risk of Aluminum Wiring Hazards, Part 1
By Todd Keys, Structural Integrity Associates
[What are the problems associated with aluminum wiring? How significant are the risks? In part one of this article, the author discusses the properties and problems related to aluminum wiring.]
During the mid-1960s and early 1970s, aluminum wiring was used extensively in residential electrical distribution. However, problems including the risk of electrical fires soon led to changes in the way aluminum conductors were manufactured. Aluminum wiring is still used in many industrial facilities, and even with these changes, evidence suggests there are still inherent problems with the use of aluminum in electrical applications.
Who Is At Risk?
There is an extensive list of factors that can affect the amount of risk associated with a facility wired with aluminum. The only true way to determine what risk a facility presents is to have an inspector trained in aluminum wiring hazards conduct an extensive inspection of the electrical system. Some repairs that facilities may attempt could actually increase their exposure to electrical failure.
This article discusses some of the properties of aluminum conductors and the manufacturing changes that have been made in an attempt to alleviate these problems. The risk factors that exist with both the older type of aluminum conductors and the newer conductors are also discussed, along with the regulatory agencies governing when and how changes to the manufacturing processes take place.
Fire Probes Lead to Changes
In 1973, shortly after its establishment, the U.S. Consumer Product Safety Commission (CPSC) began investigating injuries and deaths resulting from electrically ignited house fires. Its research showed that "homes wired with aluminum wire manufactured before 1972 were 55 times more likely to have one or more connections reach Fire Hazard Conditions1 than are homes wired with copper."
[Note: the research conducted by the Franklin Research Institute defined "Fire Hazard Conditions" to occur when receptacle cover plate mounting screws reached 149 C (300 F), or sparks were emitted from the receptacle, or materials around the receptacle were charred.]
By this time, performance problems with aluminum conductors already had led to most manufacturers changing their conductor alloys. The CPSC research, as well as various other studies, also led to code changes concerning the use of aluminum conductors.
Prior to 1972, aluminum conductors were made of many different types of alloys. The aluminum typically being used at that time had very large coefficients of thermal expansion. This meant that devices made from this substance would expand and contract a great deal over small temperature increments. The aluminum also had a high frequency of bending and creep failures.
The aluminum industry found that alloys using specific additives helped alleviate some of these mechanical problems. Alloys were identified that were stronger, more ductile and capable of numerous bending cycles without experiencing failure. The National Electric Code (NEC) was eventually amended to require that "Solid aluminum conductors No. 8, 10, and 12 shall be made of an AA-8000 series electrical grade aluminum alloy conductor material. Stranded aluminum conductors No. 8 through 1000 kcmil shall be made of an AA-8000 series electrical grade aluminum alloy conductor material."
Aluminum alloys are categorized in series having multiples of 1000. Aluminum alloys from the 1000 series are greater than or equal to 99 percent pure aluminum. Each series greater than the 1000 series is identified as having a specific primary alloying element, along with other additives.
The 2000 series alloys have copper as the major alloying element, and the 3000 series alloys have manganese as the major alloying element. Other elements with series designations are silicon, magnesium, zinc and a combination of silicon and magnesium. The 8000 series means that the major alloying element is something other than these common elements listed above.
The 8000 series aluminum conductor alloys appear to have corrected many problems that originally occurred with aluminum wire, such as creep failure and excessive thermal expansion. However, one of the major problems with aluminum conductors remains uncorrected: the extremely fast rate that aluminum develops an oxide layer, and the high electrical resistance of this oxide layer.
Oxidation on aluminum forms a barrier on the surface that is impervious to additional oxygen and thus prevents further oxidation. It is this property that makes aluminum such a desirable material in outdoor settings. Unfortunately, the electrical resistance of this oxide layer creates problems in electrical distribution applications. The electrical resistance of aluminum oxide (or alumina) is so high that in some high temperature testing environments, alumina is actually used as an electrical insulator.
The problems with oxidation can be made worse by a microscopic metallurgical phenomenon know as fretting corrosion. This is a very complicated issue resulting from compounds that form with the interaction of aluminum and various other metals. These compounds create erosion and corrosion problems from the micromotions experienced during vibration and expansion/contraction cycles. Fretting corrosion can create a build-up of oxidation between components within a connection, resulting in high electrical resistance.
A typical failure with aluminum wiring occurs over an extended period of time. When a termination or splice is made with aluminum conductors, there is usually enough disturbance of the oxidation layer and a tight enough connection to permit adequate current flow. As the oxide layer reforms there is an increase in the electrical resistance of that connection. The increase in resistance corresponds to an increase in temperature that causes thermal expansion.
The thermal expansion loosens the connection and adds to the fretting corrosion. The whole thing is a vicious cycle that eventually elevates the temperature to the point where the insulation protecting the conductors is damaged. This typically results in a fault to the grounded enclosure or receptacle box, contact with another conductor and possibly sparks and/or flames.
Failures can occur at places other than terminations or splices. An example of this would be in a stranded conductor being run from one termination to another. Because of the oxidation layer coating each strand in the conductor, there may be poor conduction from strand to strand. At one termination, only a few of the strands may be making good contact with the termination, while at the other termination different strands are making good contact with that termination.
There must be some point in the conductor run where a crossover of the current occurs. This typically occurs at a bend where friction between the strands has disrupted the oxide layers. Failures of this sort are often characterized by damaged insulation at the crossover point. There may also be insulation damage in a spiral shape running along the length of the conductor. This is due to the overheating of the few conductor strands that were actually carrying the current.
How Prevalent is Aluminum Wiring?
What is the likelihood of running into a facility wired with aluminum? We know when specific types of wiring were in use, but it is difficult to speculate on the extent of their use. Residences built between 1965 and 1973 may have been wired with the "old technology" aluminum wire. Those built after 1973 also may have been wired with the "new technology" aluminum wire instead of copper.
It is unlikely, however, that todays newly constructed residential buildings are being wired with aluminum. Although the use of No. 10 and 12 conductors is permitted by the NEC, many of the local jurisdictions only allow aluminum conductors greater than No. 6 to be used in new construction.
Many industrial facilities still use large aluminum conductors and bus in their main distribution. Although these conductors are aluminum and pose all of the undesirable properties mentioned above, their application makes them far less susceptible to failure than aluminum branch circuit wiring. As will be described in the following sections, it is the differences in application that really impact the susceptibility to failure. This is why levels of risk associated with aluminum wiring can only be made through informed inspection.
[EDITOR'S NOTE: Part two of this article asks a number of questions to help assess risk and concludes with suggestions and sources to help you make informed decisions.]
Todd Keys is an engineer with Structural Integrity Associates, Inc., a subsidiary of HSB Group, Inc. Before transferring to Structural Integrity, he worked in the Electrical Loss Control division of The Hartford Steam Boiler Inspection and Insurance Company, where he conducted failure analysis and loss prevention evaluations including infrared thermography. Todd joined HSB in 1996 after earning a bachelors degree in Mechanical Engineering from California Polytechnic State University at San Luis Obispo.
©1997 Hartford Steam Boiler Inspection and Insurance Co.