Older Homes

David A. Dini, Thomas Z. Fabian, J. Thomas Chapin
Underwriters Laboratories Inc.
333 Pfingsten Road, Northbrook, IL 60062

Copyright © 2006 Underwriters Laboratories Inc.

Abstract
This study describes a unique opportunity to measure certain electrical,
mechanical, combustibility, and chemical composition characteristics of wire and
cable conductor insulation collected from older homes from across the United
States. Detailed information about the age, building location, type of conductor,
and use conditions within the home enabled inferences to be made about the
effects of age and usage on various residential wiring infrastructures.
Comparisons between older thermoset rubber and more recent thermoplastic
materials are also made.

Background
The Fire Protection Research Foundation (FPRF) of Quincy, Massachusetts, in
cooperation with several manufacturers of electrical equipment, insurance
companies, testing laboratories, and the U.S. Consumer Product Safety
Commission, is sponsoring a multi-year Residential Electrical System Aging
Research Project. The goal of this project is to improve residential electrical fire
safety by more thoroughly understanding the effects that aging may have on the
safety of electrical system components. One aspect of this project is to
characterize the condition of various age groups of residential electrical
components by surveying, recovering, and analyzing representative samples of
actual installed wiring systems, wiring devices, and similar distribution and
utilization equipment.

With the help of qualified electrical volunteers from across the U.S., older homes
ready for demolition are being identified for this study, and when permission is
granted to access these buildings, the volunteers are called upon to photo-
document and harvest various aspects of the buildings electrical system for
further study in the laboratory. The homes generally range in age from 40 to 90
years old. Some history about the house, such as age, location, renovations, etc.
is also obtained when available. The buildings wire and cable system is
generally subjected to a thorough visual examination and dielectric testing as
part of the basic study.

Between 2004 and 2006, electrical components were harvested from 11 different
houses. In addition, wire and cable samples from 15 additional houses were
given to the study. This paper describes an additional analytical study that was
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conducted on these old wire samples to determine some of the physical
properties of the wire insulation after years of service in the house.

Samples and Nomenclature
Each sample was identified by a unique designation using the format XX-Y-Z.
XX-Y represents one or two letters followed by a number to identify the specific
house or other source from which the sample was obtained. The suffix -Z is a
sequential number to represent samples of different wiring systems from the
same house or source, or similar samples taken from different locations within
the house.

The wire samples were generally from a nonmetallic or armored cable wiring
system, or individual conductors that were part of a knob-and-tube system, or
installed in metal conduit or tubing. The age of the wire was often known from
the age of the building or other information provided by those involved in the
harvesting or obtaining of the wire. Other estimates of age were made using the
following criteria. Thermoset rubber insulated conductors were generally of a
vintage before 1960. Thermoplastic insulated conductors were generally of a
vintage after 1950. Nonmetallic cable with a cloth braid jacket was generally of a
vintage before 1970. Nonmetallic cable with a thermoplastic jacket was generally
of a vintage after 1960. Nonmetallic cable with a grounding conductor was
generally of a vintage after 1962. Armored cable with an aluminum bonding wire
was generally of a vintage after 1959.

Samples were obtained from St. Paul Travelers Insurance that were taken from
investigations on various older houses that had been involved in insurance
claims; President Woodrow Wilsons house in Princeton, New Jersey; and
residential structures in the Birmingham, Alabama; Chicago, Illinois; and
Milwaukee, Wisconsin areas. The estimated age of the samples ranged from 15
to 85 years old. Details on the samples including wire size and conductor
insulation information can be found in Appendix A.

Testing of Wire Insulation
The following tests and analysis were conducted on the recovered wire samples:

1. Dielectric Test
2. Bend / Dielectric Test
3. Ultimate Tensile Strength and Elongation Measurement
4. Limiting Oxygen Index Measurement
5. Oxygen Bomb Calorimeter Test
6. FT-IR Measurements
7. Thermogravimetric Analysis

1. Dielectric Test - Dielectric tests were conducted on 2 ft of unaltered samples to
determine the effect of aging on the wire insulation and its ability to carry power
and temporary transient voltage surges without experiencing a dielectric
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breakdown. The middle one-foot of the wire was wrapped tightly in aluminum foil.
The dielectric test voltage was applied between the conductor and the aluminum
foil. The voltage (ac) was first increased from zero to 5 kV at a rate of 500 Volts
per minute, and then held constant for one minute. If no breakdown occurred,
the voltage was then increased from zero to 20 kV at a rate of 500 Volts per
minute. If dielectric breakdown occurred at less than 20 kV, the potential at
dielectric breakdown was noted.

2. Bend / Dielectric Test - Bend tests were conducted to analyze the brittleness
of the insulation as may have occurred because of the natural aging process and
exposure to the ambient environment. Two feet of unaltered wire was used. For
this test, the wire was bent around a mandrel of a diameter as indicated in table
1. Each specimen was tightly wound for six complete turns onto the mandrel.
The winding was done at a rate of about 3 seconds per turn, and successive
turns were in contact with one another. Following this the wire was examined
under a microscope to check for crazing lines and stress whitening, which are
precursors to cracking and embrittlement. The two-foot sample was then
immersed in steel shot and subjected to an ac dielectric test between the
conductor and the shot. The voltage was first increased from zero to 1.5 kV at a
rate of 150 Volts per second, and then held constant for one minute. If no
breakdown occurred, the voltage was then increased from zero to 20 kV at a rate
of 500 Volts per second. If dielectric breakdown occurred at less than 20 kV, the
potential at dielectric breakdown was noted.

Table 1
Conductor Size
Dia. of Mandrel
(AWG)
(Inches)
14

0.313
12

0.375
10

0.563
8

0.688
6

1.250



Bend Test Sample

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3. Ultimate Tensile Strength and Elongation - Ultimate tensile strength (UTS) and
elongation tests using a mechanical tensile strength tester were conducted to
compare the mechanical properties of the various samples and the effect of
aging on the brittleness of the sample. Test samples consisted of the wire
insulation with the conductor removed. The elongation rate was 20 in/min, and
the jaws began at a distance of 4 in. The percent elongation was measured by
detecting the change in jaw distance from the initial 4 in. value to the final
distance recorded when the sample fractured
1
. The UTS was calculated by
recording the maximum load in lbf and dividing that value by the area of the
insulation. The area was calculated using the following equation,

A = (pi / 4) x (D
2
d
2
),

where A is the cross-sectional area of the specimen in square inches, D is the
outer diameter of the insulation in inches, and d is the diameter of the conductor
in inches.


Tensile Strength Tester

4. Limiting Oxygen Index - Limiting oxygen index (LOI) tests were conducted to
determine the minimum oxygen concentration necessary to support combustion
in accordance with ASTM D2863 [1]. Ambient oxygen concentration was varied
by precisely controlled combination with nitrogen. LOI tests were conducted on
unaltered wire by using a methane torch to ignite the wire. If the flame produced
a candle-like burn that did not appear as though it would extinguish, the
concentration of oxygen was reduced by 1%. If the flame extinguished itself, the
concentration of oxygen was increased by 1%. The critical value for the oxygen
index concentration was determined to be the oxygen concentration range where
the flame extinguished at the lower concentration and produced a candle-like
burn at the higher concentration. The conductor was left in the insulation to
maintain sample rigidity.

1
Standards for wire insulation often specify that gauge marks be placed on the insulation spaced
two inches apart, and that elongation be measured as the increase in distance between the
gauge marks. For the tests described here, elongation was determined by detecting the change
in jaw distance from the initial 4-inch value. Review of the plotted elongation data showed that
slippage of the sample in the jaws was not o