DOE/EH-0127 --- Issue No. 90-01 --- 01/89

Copper and Aluminum Cable
Connector Problems

BACKGROUND

When preparing to remove old fire alarm notifier circuits to upgrade a fire alarm notification system, an electrician removed the cover to a junction box only to discover electrical cable with burned insulation and exposed conductors. Each of two 500 kcmil aluminum conductors had both their insulation boot and conductor insulation melted away from their in-line compression splice. These two exposed phase conductors were within 1/2 inch of each other. The third phase was intact, with no visible damage to the boot or the insulation.

Investigation revealed that the two damaged cables had been joined with compression connectors designed for splicing copper conductors. They were not listed for use with aluminum cable.

Problems with Aluminum Connectors

The Loss Prevention Guides of the Factory Mutual Engineering Co. describes the problems with aluminum connectors and offers cost effective preventive measures.

"...aluminum has the undesirable characteristic of forming a high resistance film of aluminum oxide on the outside of the conductor when it is exposed to the air. When the oxide is scraped or wire-brushed off the conductor, it reforms quickly. Unless the connection is properly prepared, a high resistance contact is the result and heating is likely to occur."

Aluminum expands 38% more than copper. When terminated with connectors designed for copper and exposed to load currents, some of the aluminum will be extruded from the connector. Subsequently, when de-energized, the aluminum shrinks and oxides form in the voids. The next load cycle experiences increased resistance in the connection which causes more temperature rise, more extrusion of the aluminum and higher resistance in the connection. The sequence repeats itself until the connection fails.

"Creep or cold flow is another factor that has caused heating at the connections and terminations of aluminum conductors."

"Because aluminum is basically a softer metal than copper, it creeps at lower pressure."

Aluminum connectors are "...designed with larger contact surfaces to minimize the pressure (per unit area) exerted on the conductor..."

"Aluminum has a higher creep rate than copper. Consequently, connectors for aluminum must have more surface area to reduce the pressure on conductors in a joint."

Corrective Actions:

A complete line of pressure connectors have been designed for making connections and splices in aluminum conductors of all sizes. However, per DOE Order 6430.1A, the General Design Criteria, only conductors sized #4 AWG and larger are permitted to be aluminum. The terminals of power distribution and utilization equipment must have their terminals identified with the Underwriters Laboratories, Inc. (UL) designation "AL/CU." This designation indicates that they are listed by UL as suitable for attaching either copper or aluminum conductors.

Another factor to consider when joining copper and aluminum where moisture is present is electrolytic action. A miniature electric cell is created which eats away the aluminum due to galvanic corrosion. The use of tin-plated aluminum lugs prevents corrosion.

Although this Safety Note focuses on aluminum conductors classified by UL as Building Wire and Cable, aluminum conductors may also be found in electric motors, transformers and other power distribution and utilization equipment. In a more general scope of concern, increased current flow in electrical equipment may be due to overloading and insulation breakdown as well as high resistance connections. Thermal imaging is a fast and efficient method to detect these problems in their early stages.

Recommendations

As a foundation, the application of aluminum conductors should be controlled by means of both engineering specifications and approved design, construction, and maintenance criteria. Then,

• Check all mixed aluminum and copper connections for loosening and overheating at least every six months.

• Clean and tighten all electrical connections during each maintenance period in those facilities or circumstances where it is impractical to inspect. An infrared thermal imaging survey should be made, at least annually, to detect indications of overheating so that appropriate corrective action can be taken.

• Use, in general, the following criteria to determine the condition of connections in fuses, circuit breakers, knife switches, disconnect and power switches, cables, bus bars, power panels, lighting and receptacle panels, and motor controllers:

1. Note 10-15 degrees C (18-27 degrees F) rises above ambient temperature and plan to reinvestigate within 6 to 12 months.

2. When temperature rises are between 15 to 20 degrees C (27 and 36 degrees F) schedule a shutdown for an investigation and possible repair at a convenient time.

3. When the temperature rise is 20 degrees C (36 degrees F) or above, shut down and investigate immediately.

For additional information and suggestions on this and other related topics, review the following documents:

Factory Mutual Engineering Corp:

Loss Prevention Data Sheet 5-25 (15-25); "Infrared Thermal Imaging Systems"

Loss Prevention Data Sheet 5-26 (14-16); "Aluminum Electrical Conductors"

Loss Prevention Data Sheet 5-31 (14-5); "Cables/Bus Bars"

Institute of Electrical and Electronic Engineers:

IEEE Standard 141; "Recommended Practices for Electrical Power Distribution for Industrial Plants"

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