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27.2 Case Study: Aluminum Wire Failures
in Residential Wiring
Background: In the early 1970s, aluminum wire was used
extensively as a substitute for more expensive copper wire in
residential and commercial wiring, specifically for 110 V
electrical outlets that used steel screw compressive clamping
of the wire against a brass or steel plate. The aluminum wire–
steel screw junctions were observed to fail catastrophically
through a process of overheating, leading in extreme cases to
initiation of structural fires (Meese and Beausoliel 1977 ;
Rabinow 1978 ).. Figure 27.6 shows an example of the dam-
age to the wire-screw junction and the surrounding plastic
housing and wire insulation caused during an overheating
event observed in a laboratory test. This failure was a puz-
zling occurrence, since aluminum is an excellent electrical
conductor and was long used successfully in high voltage
electrical transmission lines. Moreover, the vast majority of
Al wire–screw connections provided proper service without
overheating. However, those connections that did fail in ser-
vice often produced such catastrophic effects that the critical
evidence of the initiation of the failure was destroyed.
Capturing an event like that shown in. Fig. 27.6 required
intensive laboratory studies in which thousands of junction
boxes were tested and continuously monitored with thermal
sensors until a failure initiated, which was then automatically
interrupted to prevent complete destruction of the evidence.
This problem illustrates the “macro to micro” sampling
problem. The failure mechanism was eventually discovered
by SEM/EDS characterization to have a microscopic point of
origin, but this microscopic failure origin with micrometer
dimensions was hidden within a complex macroscopic struc-
ture with centimeter dimensions. Solving the problem
required a careful sample preparation strategy to locate the
unknown feature(s) of interest. The metallographer mounted
the entire Al-wire/steel screw/brass plate assembly in epoxy,
as shown in. Fig. 27.7, and sequentially ground and polishedCr Fe Co Ni WBright
phase0.1444
±0.00020.0135
±0.00010.5374
±0.00070.2279
±0.00050.0768
±0.0036Cr Fe Co Ni WInter
phase0.1037
±0.00020.0161
±0.00010.4817
±0.00060.3468
±0.00060.0517
±0.0025Cr Fe Co Ni WDark
phase0.3158
±0.00030.0142
±0.00010.5191
±0.00070.1336
±0.00040.0173
±0.0009BSE 5 μm. Fig. 27.5 SEM-BSE image and DTSA-II analyses of selected grains in the fine-scale region
Al wire 1 cmSteel screwBrass
plateThermal damage to
wire insulationThermal damage to
plastic case. Fig. 27.6 Residential electrical outlet wired with aluminum. The
laboratory test was interrupted after the thermal event initiated and
was automatically detected, but significant thermal damage to the
plastic casing and wire insulation still occurred
. Fig. 27.7 Metallographic mount (2.5-cm diameter) showing the
cross section of the steel screw, aluminum wire, and brass plate
Chapter 27 · X-Ray Microanalysis Case Studies