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December, 2015
Improving Cable Quality and Reliability — Resistance Measurement to Within 1mW
P
recision low-value resistance measurements become necessary when testing cables intended to carry significant current. This is
By Christopher E. Strangio, Founder and President, CAMI Research We measure the resistance of interest, RW,
of prime importance in high-reliability applica- tions used by the military, medical field, aero- space, and other areas where a life may depend on the accuracy of a system. For example, cables that transmit power, as you might find in AC power dis- tribution or radio transmitters, low-resistance con- nections that have faulted to high-resistance cre- ate real danger of fire or explosion. On the other hand, while cables used in medical applications do not carry significant current, questionable connec- tions to hair-thin wires between a monitor and high-precision sensors could potentially create life- critical measurement errors or result in circuit misoperation.
Small Faults, Big Problems Many unfortunate accidents have happened
due to high-resistance connections in cables and wire harnesses which, in some cases, resulted in loss of life. This underscores the importance of accu- rately measuring resistance when these compo- nents are manufactured, or by periodic inspection, as in aircraft, where wiring assemblies are continu- ously in motion. 4-wire Kelvin measurement helps locate bad solder joints, faulty crimps, recessed pins, pin contact contamination, improper wire gauge, and stress-exuded wire — errors that may go unde- tected when using other methods.
4-Wire Measurement Ohm’s law defines resistance “R” as the ratio
of voltage “V” across a component, to the current “I” passing through it: R = V/I. To measure resist- ance, we apply a test current to a wire and detect the voltage drop developed. From this, we can cal- culate the resistance.
CAMI HVX high voltage test system. In some situations, the resistance of interest
approaches the resistance value of the lead wires used to measure it, resulting in an inaccurate reading. This problem can be solved by moving the voltage measurement points out to the end points of the mating pins, thus bypassing any voltage drop that may occur in the lead wires. The Ohmmeter then appears to have four
wires coming from it. This approach is referred to as “4-wire measurement” or “4-wire Kelvin meas- urement,” in honor of the 19th century English physicist, Lord Kelvin, who originally developed it.
Extremely Low Current The current flowing through the voltage-
between the conductor’s two mating pins. However, the entire circuit includes the resistance of the lead wires. The voltage drop used in the cal- culation then includes all three of these resistanc- es. In many situations, the lead wire resistance is much lower than the resistance of the conductor or component being measured and therefore can be disregarded.
measuring circuit of a 4-wire system is extremely low, typically on the order of fractions of a µA (six or more orders of magnitude less than the source current), so virtually no voltage drop occurs across these lead wires, and its effect on the resistance measurement is negligible. If there is no current flowing through a wire,
there is no voltage drop across it regardless of its length. This means that lead wires may now be quite long, sometimes exceeding 10-ft. (3m), with- out having an effect on the measurement. Long lead wires are necessary when testing large, multi- branch wire harness assemblies, which is not uncommon. The principal advantage of 4-wire measure-
ment is that it eliminates any effect of fixture resistance (the lead wires) to obtain a precise resistance value of the unit under test (UUT). Because 4-wire measurements typically employ test currents well above those needed for 2-wire testing, a secondary advantage comes through the use of a high-current stress test for wiring by driv- ing a current of 1A or more through each conduc- tor. This allows for the ability to set a dwell time from 100ms to many minutes. Observing a slowly increasing resistance during a long dwell period resulting from thermal heating may reveal prob- lems not detected within a shorter measurement interval. Essential for enabling testing flexibility, soft-
ware that drives a 4-wire measurement system should offer user-selectable options. These options can be critical. A system should permit individual conductors within a UUT to be independently dis- abled from a 4-wire test to avoid damage to fuses or other components, and also should allow users to independently set different test currents and dwell times for different conductors.
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