Aluminum wiring was used in the construction of some homes in the 1960's and 1970's. In many installations it was originally installed and terminated on devices that were compatible with the aluminum wiring. (There are indications that in the early years of the the use of aluminum branch wire this may not have been the case- http://www.propertyevaluation.net/Aluminum%20Wiring.html ). Over time this original equipment has been changed out in home upgrades with receptacles and switches that are not compatible with aluminum wiring.
This brings us to the present, where many devices that are not compatible with the aluminum wiring in the home exist. When the aluminum wire is terminated on a device that is not designed for aluminum there is a real possibility of the wire overheating at the terminal due to oxidation of the aluminum, and the aluminum loosening its contact through heating-cooling cycles. This resistance and arcing creates heat, and potential fire risk.
Although statistics on fires caused by aluminum wiring is not available in Canada, electrical equipment was found to by the Council of Canadian Fire Marshals and Fire Commissioners to have caused 2% of the house fires in the study year of 2007. (http://www.ccfmfc.ca/pdfs/report_e_07.pdf)
How can you tell if you have aluminum wiring? Often you can't tell immediately because the wire is behind the walls or the lettering has worn off of the cables. The aluminum wire is easy to identify by taking a cover plate off of a switch or a plug and shining a flash light into the box to see if the wire attached to the switch or plug is copper colored or aluminum (a dull silver) colored. In plugs that I have replaced there have been instances where the insulation was melted back and had turned hard from the heat, although it wasn't evident from outward appearances. If you do have heat emanating from a plug or from your electrical panel there is a problem that needs to be addressed quickly.
There are two common solutions to the hazards created by aluminum branch wiring. One is to replace the incompatible devices – such as switches and receptacles- with new compatible devices. Another solution is to use short pieces of copper wire to connect aluminum-incompatible devices to the aluminum wiring that is in the house. These short pieces of wire are refereed to as pigtails and require special wire connectors (Marrettes) and an anti-oxidizing paste to install correctly. Due to the high cost of aluminum compatible switches and receptacles the second method is used in favor of the first.
There is a lot of information available on the internet about aluminum wiring including the references that I have made already. I would also suggest Wikipedia, and the Ontario Electric Safety Authority for further reading.
At Jaycox Industries we have experience identifying aluminum wiring and completing the work to make it safe. Feel free to call to speak to an electrician about this topic or to stop by your home for a free consultation.