When exposed to oxygen, a hard and resistant oxide coating forms within a short period of time on the surface of aluminum. The coating protects the subjacent material from further corrosion. This effect makes aluminum a highly durable material. However, the protective oxide coating on the material surface is not desirable when it comes to electrical engineering. It degrades the conductivity of the aluminum and makes contacting difficult. If an oxidized conductor is connected without any pre-treatment (to remove the coating), the contact resistance will be increased between the aluminum conductor and the connector component. This can result in temperature increases and, under worst-case conditions, cable fires. To prevent such problems, the oxide coating must be broken or removed physically. This can be done by brushing the bare aluminum conductor ends before contacts are made and also during the crimping process: Connector components for aluminum conductors are equipped with special contact grease from the factory, usually a grainy, abrasive material such as corundum. Combined with high pressure, the corundum particles cause an abrasive effect that breaks the non-conductive oxide coating on the aluminum, improving contact properties and electrical connections. The grease also prevents moisture and oxygen from entering and causing new corrosion of the contact points. Better quality cable lugs are generally equipped with plastic plugs, which prevent the contact grease from drying out or leaking during storage.
For fine-wire conductor designs, we recommend that IEC 61238-1 Cl. A-tested C8 crimps be used due to the larger oxidizing surface of the conductor. C8 crimp’s contours penetrate very deeply into a stranded bundle, equally tear up the individual strands, and thus allow for optimum contacts on all strands, even in a bundled conductor. The use of C8 crimps (that have been developed as part of the POWERLINE Aluminum series) allows for the best possible electrical values (low contact resistance) and mechanical extraction forces to be reached.
When it comes to specifying electrical connection components, the corrosive reactions of aluminum in the presence of other metals – mainly copper – must also be taken into consideration. When aluminum comes into contact with more noble metals (those with higher electropotential) such as copper, iron, or brass, an electrochemical reaction may arise by means of contact element formation. This reaction is activated by conductive liquids — in the field mostly by condensed water (condensation). In this process, the potential differences produced by the electrochemical voltage series play a crucial role. The copper electrode (anode), electrolyte (water), and the aluminum electrode (cathode) create a contact element. Any voltage across these elements is short-circuited by the contact between the copper and aluminum. The resulting current creates a decomposition process in the aluminum, which is visible as a radiant oxidation point revealing the contamination of tiny copper particles. However, the copper does not decompose. But the decomposition process negatively affects the electrical connection over the long term, with increasing contact resistances that lead to temperature increases and even to fires. Therefore, we recommend using an aluminum/ copper (Al/Cu) cable lug for connecting aluminum to copper peripherals. Bimetal connectors such as Al/Cu cable lugs, press connectors and connecting bolt pins are manufactured using a friction welding process. They are encapsulated to prevent liquids from penetrating the connection and causing any unwanted creepage. The use of Al/Cu connectors and connections is the most sensible way of combating the effects of oxidation on aluminum. Another means of protecting against moisture is installing a secondary insulation on the contact area. Depending on the field of application, mechanical load, and environmental conditions, a cold-, a roll-, or a hot-shrink tube can be used. The best protection results are achieved by shrink tubes with inside adhesive. At the same time, the electrical contacts should be thoroughly inspected during regularly scheduled maintenance.
Finally, the creepage behavior of aluminum is something that must be considered. Aluminum is a softer metal than copper and tends to expand or stretch over time, especially when subjected to higher pressure and temperature. Classic crimp connections suffering from creepage lose strength and would no longer be reliable to ensure proper connectivity. HELUKABEL’s C8 crimp exhibits a filling degree of 95 percent, which cannot be obtained with conventional crimp connections. The described expansion/ stretch process is compensated with outstanding extraction values. At the same time, we recommend performing regular maintenance and inspection on all clamping points in accordance with their load levels.
Aluminum wire was used to replace copper in the 1970’s. One of the schools we worked with had regular fires which would consume the entire unit ventilator. This was traced to the aluminum wiring and all of the fires started at the junctions to the equipment. The building was constructed and inspected by applicable state and national wiring codes. This article is helpful in understanding why this happened. If it is built correctly, it still must be inspected on a regular basis. We used thermal infrared scanners which were bulky and used nitrogen gas. Is it correct to assume from a maintenance viewpoint that both copper and aluminum wiring must be thermally inspected but there appears to be more of a risk from aluminum? We noticed that the expansion of aluminum wiring requires retightening of all connectors on a regular basis. We generally did not do this regularly with copper wiring.
250 - Spade Lug Terminal
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