Industrial Crimp Quality Handbook
Doc No: TM-640160065
Release Date: 00-00-02
UNCONTROLLED COPY
Page 25 of 27
Revision: C
Revision Date: 12-23-09
Single
Strand Wire
Terminal
Wire Barrel
Excessive
Crimp Depth
Wire is deformed
Or Compressed Beyond
Normal Yield Point
Causing Potential
Failure Mode or Future
Break or Fracture Point
Excessive
Bell Mouth
Dielectric Testing
(The term “dielectric” refers to an insulator.)
Some terminals are covered with insulation so that
electrical contact can be made only where it is
desirable. The crimp is made through (on) this
insulation, which is compressed and extruded due to
the pressure of the crimping dies. Obviously, not all
insulation materials can withstand this treatment and
even with the strongest materials, the crimp must be
correctly designed so as not to rupture the insulation.
Dielectric tests are made on insulated terminals after
crimping to determine that the crimp process is not
rupturing the insulation or thinning it out so that it will
not withstand impressed voltages. The test is made
by impressing a voltage between the wire to which the
terminal is crimped and conductive materials
contacting the terminal insulation.
The voltage is gradually increased until the
requirements are reached or until a breakdown occurs
which means the insulation breaks. Depending on the
use and the specifying agency, dielectric withstanding
requirements normally range from 1500 to 8000 volts,
resulting in a 300 to 600 volt rating of the terminal.
8.3 Final Tensile Value
The type of die affects the final tensile value in
several ways. See Crimping Dies, Section 6.5.
If the die (as in the indentor type) does not indent far
enough, a void may be created in the compression
joint permitting the individual strands to shift, thus
loosening the connection. Further, the air space
(void) acts as an electrical insulator.
If the die presses too tightly the individual strands can
be squeezed and elongated. This can cause a weak
point in the conductor, cause the wire to break at a
lower than allowable tensile strength, and/or create a
heat rise across the joint because of lower cross-
section and increased resistance.
Another way to cause tensile failure is not
compressing the barrel enough to hold the conductor
securely. Molex crimp tools are designed to eliminate
these problems.
8.4 Electrical Resistance
The electrical resistance across the crimp is compared
to the resistance of an equal length of wire, and
expressed as relative resistance for a particular wire
size.
Relative resistance of the crimp to the wire is given by
the formula below:
Relative resistance
W
C
R
R
Where:
R
C
= Resistance over crimp
R
W
= Resistance of wire
Relative resistance values of less than 1.0 denote a
crimped joint with less resistance than the wire;
values more than 1.0 indicate greater resistance than
the wire. Usually it is easier to measure the voltage
drop across the crimped joint. Many specifications
state the requirements in terms of voltage drop at a
specified current. Voltage drop is the more
commonly used term in the industry. If the resistance
value is desired, it may be calculated from Ohm’s
Law:
