ASTM B539-02(2013)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: B539 − 02 (Reapproved 2013)
Standard Test Methods for
Measuring Resistance of Electrical Connections (Static
Contacts)
This standard is issued under the fixed designation B539; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope that the nominal contact area offered zero resistance. When
measuring contact resistance one attempts to include as little
1.1 These test methods cover equipment and techniques for
bulk resistance as possible in the measurement, by placing
measuringtheresistanceofstaticelectricalconnectionssuchas
measuring probes as close to the contact interface as practical.
wire terminations or splices, friction connectors, soldered
joints, and wrapped-wire connections.
3.1.3 connection resistance, n—the resistance from the ter-
mination point on one end of a device containing static
1.2 Measurements under two distinct levels of electrical
contacts, through the contacts to the termination point on the
loading are described. These levels are: (1) dry circuit, (2) and
otherendofthedevice.Theterminationpointisthelocationon
rated current. One or both of these levels of loading may be
a terminal of a device where a wire or printed circuit path
required in specific cases.
electrically connects to the terminal. This resistance is the
1.3 This standard does not purport to address all of the
value of resistance displayed by the device in a circuit
safety concerns, if any, associated with its use. It is the
application.
responsibility of the user of this standard to become familiar
3.1.3.1 Discussion—The term contact resistance is often
with all hazards including those identified in the appropriate
used in commercial literature to indicate the connection resis-
Material Safety Data Sheet (MSDS) for this product/material
tance displayed by the device in a standard application. In the
as provided by the manufacturer, to establish appropriate
more rigorous usage of contact resistance, the connection
safety and health practices, and determine the applicability of
resistance is the sum of the contact resistance plus the bulk
regulatory limitations prior to use.
resistance of leads within the device that go to the static
contacts from the point that the leads are connected to the
2. Referenced Documents
external circuitry. Measurement of contact resistance indepen-
2.1 ASTM Standards:
dent of all bulk resistance is very difficult for most commercial
B542 Terminology Relating to Electrical Contacts and Their
devices.
Use
E122 Practice for Calculating Sample Size to Estimate,With 3.1.4 dry circuit, n—a circuit in which the open-circuit
Specified Precision, the Average for a Characteristic of a voltage is less than or equal to 20 mV. Current is usually low
Lot or Process in a dry circuit, but a low-current circuit is not necessarily a
dry circuit. When the applied voltage (open-circuit voltage)is
3. Terminology
too low to cause any physical changes in the contact junction,
such as break-down of thin insulating films or softening of
3.1 Definitions:
contact asperities, the circuit is said to be a dry circuit.
3.1.1 See Terminology B542 for definitions of contact
resistance, film resistance, and constriction resistance.
3.1.5 open-circuit voltage, n—the steady-state voltage
3.1.2 bulk resistance, n—the resistance a contact assembly
which would appear across the contacts if they were opened.
would have if it were solid metal of an identical geometry so
3.1.6 static contacts, n—electric junctions designed for
infrequent separation and connection, and intended to perform
These test methods are under the jurisdiction of ASTM Committee B02 on
their function only when contacting members are stationary
Nonferrous Metals and Alloys and are the direct responsibility of Subcommittee
relative to each other. This definition includes crimped,
B02.11 on Electrical Contact Test Methods.
welded, brazed, riveted, or soldered joints; friction connections
Current edition approved Aug. 1, 2013. Published August 2013. Originally
such as pin and socket connectors or taper pins, twisted-wire
approved in 1970. Last previous edition approved in 2008 as B539 – 02 (2008).
DOI: 10.1520/B0539-02R13.
splices; and connections made with screws, or bolts and nuts
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
between electrical wiring and components. The definition
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
excludes relay contacts, slip rings and commutators, and
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. switches and circuit breakers.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B539 − 02 (2013)
3.2 Descriptions of Terms for Levels of Electrical Loading: are electrically parallel. In practical cases the clean metal-to-
3.2.1 dry circuit, n—This method provides for measurement metal contact spots will carry most of the current and the total
of contact resistance under very low levels of electrical contact resistance is primarily dependent on the size and
excitation,withappliedvoltagesandcurrentsselectedtobetoo number of metallic contact spots present (see Note 1). In
low to cause breakdown of thin oxide films or other contami- addition, acceptably low values of contact resistance are often
nates in the contact interface or to cause formation of metallic obtained with true areas of contact being significantly less than
bridges across the interface where none may otherwise exist. the apparent contact area. This is the result of having a large
Dry circuit testing is intended to determine whether the test numberofsmallcontactspotsspreadoutoverarelativelylarge
contact will function properly in circuits of arbitrarily low apparent contact area.
levels of electrical excitation. Dry circuit testing procedures
NOTE 1—The term metallic contact as used here is intended to include
should be used when the possibility of films or contaminants in
the so called quasi-metallic contact spots as well. The latter case was
the contact interface exists or when the test sample is ulti-
discussed in Electric Contacts by Holm.
matelyintendedforuseinalow-levelcircuit.Thistestingmust
5.2 The practical evaluation and comparison of electrical
precede other tests on the same samples at high levels of
connections depend in large part on their contact resistance
electrical loading.
characteristics. On the one hand, the absolute value of contact
3.2.2 rated current, n—The rated current for a static contact
resistance is greatly dependent on the amount of metallic
device is determined or specified by the vendor or user of the
contact established and indicates initially how efficient the
device. The rated current may be large enough to cause
system is in producing areas of metallic contact. On the other
significant heating of the test samples. When rated current
hand, a comparison of the initial resistance to the resistance
measurements of contact resistance are required, using either
after aging indicates how stable the system is in maintaining
ac or dc test currents, the procedures outlined for temperature
the initial contact area. Both of these characteristics should be
stabilization in 9.5.3 must be followed.
considered when evaluating contact systems. The criteria
employed in evaluating contact resistance and stability are not
4. Summary of Test Methods
a part of these test methods as they depend on specific
4.1 The test methods described herein are characterized as
applications and therefore, will not be quantitatively stated.
four-terminal resistance measuring techniques, wherein a mea-
However, an estimate of contact resistance resulting from
sured and controlled test current is introduced into the sample
good metallic contact can be made for a given physical
using two“ terminals” or connecting points, and two other
situation and used as a comparison to actual measurements to
points are selected on the sample across which a voltage drop
determine how effective the system is in establishing stable
is measured. This voltage drop, divided by the test current, is
metallic contact. Resistances measured by these methods
the effective overall resistance of the sample included between
before,duringandaftersimulatedlifetestsareusedasameans
thevoltageprobes.Thevoltage-measuringpointsarechosenso
of determining the stability of contacts within a device.
as to measure as closely as possible the voltage drop due only
6. Interferences
to the contact resistance of the sample and to eliminate from
the measurement as much as possible the resistance of the 6.1 Measurement of Low Resistance:
metal pieces comprising the contact and the resistance of the
6.1.1 Contact resistances are normally very small, ranging
wiresandconnectionsusedtointroducethetestcurrentintothe from microohms to a few milliohms in cases of practical
sample.
interest. The measurement of resistance in this range requires
special techniques to eliminate effects of thermal potentials,
4.2 Two different levels of test current are specified. The
external interference, and resistance of connections and wires
choice of which level to use is governed by the application and
leading to the test sample.
requirements of the electrical connection being tested. Elec-
6.1.2 The resistance-measuring procedures in these test
tronic signal-circuit connections may require low-level (dry-
methods are four-terminal techniques. Test current in the
circuit) testing, whereas power-handling wire connectors
sample is measured and controlled, and made independent of
should be tested at rated current.
the sample resistance. Voltage-measuring probes are attached
4.3 Either ac or dc test currents may be used, with appro-
tothesamplesoastoeliminatetheeffectsofconnectionsofthe
priate instrumentation.
sample into the test circuit. If the purpose of the measurement
is to determine the contact resistance, the voltage measuring
5. Significance and Use
probesareattachedascloseasfeasibletothestaticcontacts,so
5.1 As stated in Terminology B542, contact resistance is
as to include as little of the bulk resistance of the sample as
comprised of a constriction resistance and a film resistance.
possible in the measurement of the contact resistance.
When present, the latter of these is usually much greater in
6.1.3 Two wire measurements of resistance are not suitable
value and dominates the contact resistance. For a given contact
because connections to the sample will contribute part of the
spot, when the film resistance is zero or negligible the contact
measured resistance, and these may be large, unknown, and
resistance for that spot is nearly the same as the constriction
variable.
resistance and therefore, as a practical matter, has a minimum
value which represents a clean metal-to-metal contact spot.As
Calculationsandformulaeforcontactresistanceofvarioustypesofcontactsare
real contact surfaces exhibit varying degrees of roughness, real
covered very thoroughly in Holm’s Electric Contacts, 4th Edition, Springer-Verlag,
contactsarenecessarilycomposedofmanycontactspotswhich New York.
B539 − 02 (2013)
6.1.4 Because the resistance being measured is often in the 7.1.1 Power Supply—A supply capable of providing the
microohm or milliohm range, and it is determined by measur- requiredcurrentand,inthecaseofthedrycircuitmeasurement
ing the potential across the static contacts, the value of the capable of limiting the current to 100 milliamps and the open
potential is often in the microvolt or millivolt range. As a circuit voltage to 20mV. The supply may be dc or ac, but the
result, thermal potentials may be significant in relation to the voltage measuring device must match the type of current from
potential being measured and appropriate measures are re- the supply.
quired to cancel or eliminate their effects.
7.1.2 Voltmeters and ammeters built into power supplies
6.1.5 In the dry circuit method, high potential may change a
may or may not meet the requirements of these methods with
resistance by breaking down a film. Appropriate caution is
respect to accuracy or precision. External metering should be
required to obtain valid dry circuit resistance measurements
used when necessary.
including limiting the open circuit voltage of the measuring
7.1.3 Both output terminals of the dc supply must be
apparatus that is connected to the device under test.
isolated from the power line, the case of the supply, and the
6.2 ac Versus dc Measurements: building ground. This prevents “ground loops” or undesired
connections through ground, between the power supply and
6.2.1 Either method described herein can be used with ac or
dc test currents, with appropriate changes in instrumentation to other measuring instruments (such as an electronic voltmeter)
correspond with the power supply. The methods are described attached to the sample.
as using dc test currents, and the following comments apply
7.1.4 The maximum current required in the power supply
when ac is used.
can be estimated from Table 1, which gives rated currents for
6.2.2 ac measurements should be expressed as RMS unless
various wire-size terminations according to Military specifica-
otherwise defined in the test report. Take appropriate measures
tions.
to isolate the measurements from stray signals, especially sixty
7.1.5 The output current of the power supply should be
hertz power line noise. Commercial resistance measuring
variable and readily and accurately adjustable.
instruments that use ac test currents generally are suitable
7.2 Current Measuring Device—a meter, ac or dc, to match
providing that they meet other requirements of the standard.
the power supply current type or suitable alternate measuring
circuitry is needed. The accuracy of the device must be 1 % or
7. Apparatus and Test Circuits
better of the measured current. Some power supplies include
7.1 Fig.1showsthebasicarrangementofafourwirecircuit
measurement capability of sufficient accuracy. An external
formeasuring.Intheillustration,themeasuredresistanceisthe
arrangement of a calibrated fixed resistor wired in series with
resistancebetweenthepointswherethevoltmeterisattachedto
the test current and a voltmeter across the resistor is suitable
thetestspecimen,thatis,betweenthepointsofV1andV2.The
provided that it achieves the accuracy required.
measured resistance includes the contact resistance at the
contactbetweenthetworoundedpointsandthebulkresistance 7.3 Rever
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