Slide 20 Aluminum Wire Connector failure mechanism details

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This is Aluminum Wiring Repair Procedure: Here Color photos and descriptive captions from CPSC Meeting 9/28/95. In this document aluminum wire twist-on connector failures and repair procedures are described, including aluminum wire repair methods which work and methods which do not work and are unsafe.

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Details of Aluminum Wiring Connector Failures: the failure mechanism, resistance, current paths, connector deterioration, inadequate standard

Equivalent circuits, general case and newly made

Color photos of aluminum wire repair procedures, and photos of failed connectors are included. This document series describes hazards with existing aluminum wiring repair products, explains the aluminum wiring failure mechanism, and reviews recommended retrofit procedures including use of readily-available materials.

This information was presented to the US Consumer Product Safety Commission by Dr. J. Aronstein, 9/28/95. The minutes of that meeting were obtained under the Freedom of Information Act and posted by Daniel Friedman January 1996.

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Slide 20 20. Here are the current paths in this type of connection. Each of the paths from wire to wire has some associated resistance. The direct wire to wire path has only contact resistance, while paths through the spring sections have contact resistance as well as bulk resistance of the steel wire.

Slide 21 21. Here is the most general equivalent circuit for the twist-on connector


Slide 22 22. Because the spring scrapes and digs into the wires, the spring-to-wire contact resistance in the newly-made connections is extremely low. The equivalent circuit is then simplified to this form.


Slide 23 23. Combining the bulk resistance of the sections of the spring that are in parallel in the current path further simplifies the equivalent circuit for newly-made connections.



Current Flow in an Overheating Twist-On Connector

Slide 27 overheating aluminum wire twist on connector failure photo

This article continues below with an explanation of common twist on connectors and copper pigtailing as an attempt at aluminum wiring repair.

The key observation is that with this type of aluminum to copper wire splice, most of the current flows through the spring in the connector. In the design of twist-on wire connectors the purpose of the spring is to maintain tension on the spliced wires, not to conduct electricity itself.


Slide 24
24. Measurements on newly made aluminum-wired twist-on splices show that most of the current flows through sections of the steel spring.

More than 60% typically for an aluminum connection, but less than 10% for an all-copper connection. There is a basic difference in behavior with aluminum wire.

Connection Deterioration in a Twist On Connector Used for Aluminum Wiring Repair

This article continues with the explanation of why twist-on connectors overheat and lead to failures when used for copper to aluminum pigtailing as an attempt at aluminum wiring repair. The observation explained here is that with this type of aluminum to copper wire splice, resistance increases between the copper and aluminum wires, leading to overheating of the connection and the twist-on the connector.

Slide 25
25. As the aluminum-wired twist-on splices age, the wire-to-wire contact increases in resistance and may finally open altogether. The many wire-to-spring contacts also deteriorate (increase resistance) and some may also open completely, as shown in this equivalent circuit.

Slide 26
26. The deterioration process is observed experimentally as a slow but progressive increase in resistance, as the various current paths within the splice degrade. There are less parallel paths active through the spring, and the result can be a red-hot spring when current flows.

Slide 27
27. That is exactly what has happened here. All of the current is passing through only a few segments of the spring and those parts of the spring become red hot.

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The CPSC Recommendation against the use of twist-on connectors for aluminum wire is soundly based. There is no reason to believe that the Ideal #65 connector, recently UL listed for aluminum-to-copper combinations, overcomes the fundamental deficiencies of this type of connection for applications with aluminum wire.
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The Ideal #65 connector has a zinc-plated steel spring. A combination known to be bad in contact with aluminum. Containing a free-burning oxide inhibitor grease, and having a free-burning thermoplastic shell, the connector can ignite readily if failures of the types shown here occur.

The only justification given for marketing this connector as suitable for aluminum wiring is that it has passed the test requirements of UL486C.

That the tests of UL486C are insufficient can be understood by considering the following table.[Table below.]

Table of UL-486C Electrical Connector Standards: comparing actual field-use with test conditions
Why the UL-486-C UL High-Current Cycle test is inadequate

Slide 28 28. This table compares factors of actual use with the UL486C test conditions. It is evident that the UL listing is based principally on a high current cycling test, which does not really stress the connection in ways that it is stressed in an actual residential application.

Slide 29 29. Additionally, the UL486C standard does not use the older wire for its tests, does not require testing of the combinations most likely to be used, and tests a statis

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