Testing of Hostile Electrical Connections in a Residential
by Roger L. Owens, P.E. (NAFE 412F)
A fire occurred in a residence that was being utilized as
a day care center. All parties involved in the initial investigations
into the fire cause agreed that the fire originated in the
laundry room in the vicinity of the electric clothes dryer.
The plaintiff’s expert opined that the dryer was defectively
designed and hence caused the fire. This writer was retained
by the manufacturer of the dryer and asked to determine
the most probable failure mode and to test an exemplar dryer
in order to validate those observations. The ignition point
appeared to be at a high resistance electrical connection
at the dryer heater box involving both copper and steel.
(See Figure 1)
Initial examination of the clothes dryer at the residence
revealed that the dryer heater circuitry connection had
been modified. One of the male prongs had been removed,
the insulating material stripped back and the #10 AWG stranded
copper wire was partially inserted and held in place by
a common steel wood screw. (See Figure 2)
conductor strands and the wood screw appeared heavily
heat stressed and appeared to be a viable ignition source.
A few inches away black electrical tape was deformed
by heat but not fully combusted. (See Figure 3) This
connection was duplicated on an exemplar dryer test
stand and under operating conditions ignited lightweight
in any amount flowing through any material produces
heat. The earliest quantification of this effect was
by Joule in 1845, the familiar expression Power = I2R
(current squared multiplied by resistance). This formula
is a satisfactory model for normal cylindrical conductors,
but is only qualitatively true for electrical arcs,
vacuum tubes and transistors, and conduction through
electrolytics and ionized gases. In this case heating
occurs at a connection (where a 10 gauge stranded copper
conductor was connected to the Ni-Chrome heater inside
of a clothes dryer).
Connections occur in a bewildering variety of materials and
mechanical geometries, but of course, the objective is to
have a connection that will have a low resistance (high conductance)
and an adequate current-carrying capacity for a particular
circuit. Connections may be soldered, crimped, screwed, push-in,
punch-down, and so forth, and may include materials such as
copper and copper alloys, brass, bronze, aluminum and aluminum
alloys, steel, etc., and any of these materials may be plated
with tin, silver, solder, brass, platinum, cadmium, and others.
The failure modes of a connection are either an open circuit
or an increased resistance at the connection.
The reason for a particular failure due to increased resistance
will vary from one example to the next, but could be caused
by chemical change due to oxidation of one or more materials
at the connection, metallurgical changes, such as grain or
hardness growth, or mechanical failure, such as cold flow
or fatigue. Mechanical stress is also a factor. External environmental
agents, such as heat, humidity, and caustic chemicals could
also be exacerbating factors. I2R is appropriate for calculating
the heat generated in a failing connection, but due to the
uncertainties of the thermodynamics and heat transfer calculations,
the resulting temperature of the failing connection can vary
over a wide range.
The clothes dryer involved in this case was of the electric
type, which means that an electrical motor-driven induction
fan pulled heated air through a rotating clothes drum. A
240-volt, 5200-watt electrical heater heated the air. The
connection of the copper appliance conductors to the heater
was by means of a specially designed pin and socket arrangements.
The pin was crimped to the copper appliance conductor and
the socket was crimped to the Ni-Chrome wire of the electrical
heater. The pin inserted into the socket with an interference
fit. The two single-phase connections were housed in a porcelain
holder. As designed, the connection is adequate for the
designed power level.
There was no pin in the improper connection found in the dryer
recovered from the fire scene. Instead, the wire had been
pushed into the connection socket and a wood screw was used
to hold it in place.
An exemplar clothes dryer was obtained from a reseller.
The applicable parts were removed and bench-mounted for
easy access. The connection made to the heater was duplicated
in the manner that the post-fire investigation had documented.
(See Figure 4)
exemplar bench-mounted dryer assembly was run at operating
voltage, allowing enough time for the unit to reach equilibrium
or failure, whichever came first. During each cycle, the temperature
of the modified connection was monitored.
any of several techniques artificially aged this connection.
These techniques included passing current through the
connection using a variance and step-down transformer
and by heating the connection with a hand-held heat
gun. Exposing the components of the connection to various
corrosive chemicals also artificially aged the connection.
The bench-mounted dryer components were then “cycled,”
which means the
The temperature was monitored using a hand-held IR thermometer
and by a type K thermocouple. When the temperature recorded
at the connection started escalating a cotton handkerchief
was placed over the screw and the system was re-energized.
Discussion of results
The exemplar connection exhibited an increase in internal
resistance when it had been artificially aged using internally
generated heat and externally applied heat. The connection
temperature was monitored to reach 185 to 220°F within
a few cycles with an eventual maximum temperature recorded
of 1038°F. (See Figure 5)
As the power through the connection cycled on and off, the
connection appeared to age and the temperature recorded
at the connection increased. This phenomenon of aging has
been observed in prior testing of connections performed
and discussed in an article titled Forensic Engineering
Analysis of Copper/Brass Contacts that was published in
the NAFE Journal in December 2001.
aging the connection with a drop of household bleach
dramatically increased the temperature at the connection.
This procedure was particularly effective if the application
occurred when the connection was hot. A cotton handkerchief
was draped over the connection in an attempt to duplicate
a fallen piece of fabric or clothing. The system was
then energized as before and the handkerchief caught
on fire. (See Figure 6)
The test project demonstrated that fabric ignition temperatures
could be achieved by the utilization of a common steel wood
screw as a connector for the dryer heater circuit. The aging
process at the connection that caused the high resistance
was primarily a function of electrical and thermal cycling
with the introduction of a caustic mist that would be found
in a typical laundry room. The project also demonstrated
that the most probable cause of the fire was a modification
by the owner at the electrical connection to the dryer heater.
The author wishes to acknowledge that Joe Stainton, BSEE,
P.E. was of tremendous assistance in setting up and monitoring
the experimental project.