To get a good idea of the changes that have been happening to cable testers, think of them as circuit analyzers. This much more inclusive classification not only covers cable harnesses, but also power cords, multiconductor cables, printed circuit boards, back planes, connectors, transformers, and motors.
Testing a wider range of product types and performing more types of tests are complementary activities. For example, many cable testers include hipot capabilities and the closely related insulation resistance measurement. In addition, as cable-harness assemblies incorporate increasing numbers of passive and active components, cable testers have provided transistor, diode, resistor, and capacitor test functionality and switch-position recognition.
Flexibility is the key word, although manufacturers achieve it in different ways. “Flexibility, ease of setup, and price are the most important factors driving the cable-test industry today,” according to Greg Von Rehder, product manager at B+K Precision. “Many customers work with cables and connectors custom to their application, and they also want to reduce test time.”
The B+K Model 205 Cable Tester comes with a universal adapter card that accepts up to 22 different connector patterns and styles. You can’t actually use the Model 205 out of the box, but you can quickly add your own connectors to the adapter card to meet your cable test needs.
Cablescan offers flexibility through improvements to the system software that controls the company’s cable testers. Ken Rockwell, general manager, said, “In the past, customers were satisfied to start a test from the control program’s command line. Today, they want to have control of all aspects of the test from entering serial numbers to changing output file locations and receiving test results directly in the calling program.
“We have written our software as COM modules that customers use in their programs to gain full control over the tester,” he continued. “Test results will be sent directly back to the customers’ programs and can be analyzed in real time. And, we are continuing this trend by developing new Microsoft .NET assemblies that provide the same kind of complete control. The anticipated wide acceptance of .NET standards will make tester interfacing, integration, and customization easier and more common.”
The situation was summed up by CableTest System’s President Raymond Simmons: “Because each customer has unique and specific test requirements, the provision of cable-test instrumentation hardly can be described as a cookie-cutter business. We manufacturers must provide a means of meeting a wide range of testing requirements while avoiding major or costly retrofits to our standard products.
“CableTest’s systems are designed with an open architecture that allows easy integration of new technologies and/or features,” he explained. “The product line supports a wide range of voltage and current sources, two- or four-wire measurements, centralized or distributed testing, and point counts from a few to tens of thousands.”
The primary capabilities of a cable tester are continuity and component test.
Continuity seems very basic, but it must be defined to meet the customer’s application. As a minimum, this means setting an upper resistance limit above which the circuit is declared to be open and a lower one corresponding to a short.
“The resistance/continuity issue often is confused,” said Brent Stringham, director of sales and marketing at Cirris Systems. “First, you need to decide whether a connection is intended or unintended. An intended connection not connected is an open. Likewise, a connected but unintended connection is a short. So, the minimum continuity resistance corresponds to the value that all intended connections must be less than, and the maximum continuity resistance corresponds to the value that all unintended connections must be greater than.”
But, what about measuring a harness containing resistance wire or discrete resistors? The overall continuity resistance range must include any values expected to be encountered. The actual upper and lower resistance values could be set directly by the user, or a nominal resistance value and a tolerance percentage could be entered. Exceptions to this arrangement are Cablescan’s Series 90 Testers that have separate continuity and resistance ranges.
What voltage should be applied during continuity testing? Is the intention to mimic the normal operating conditions of the harness or to perform a safety hipot test? Answering this question is important, because a high-voltage capability generally costs more, and fewer channels are provided on each expansion board.
For example, the Cablescan Series 90 Low-Voltage Tester has a minimum 128-point configuration that can be expanded in 128-point increments to a total of 131,072 points. The high-voltage version, Series 90HV, costs $1,000 more for the basic 64-point configuration and expands in 64-point increments up to 47,104 total points.
High-voltage testing adds a safety dimension and implies the necessity of adequate operator training. But, you may not need a high-voltage capability on more than a few points out of hundreds. One solution may be to use a separate hipot tester with a scanner.
As an example, the Guardian Model 1030S Hipot Tester from QuadTech provides an AC or DC 5-kV test stimulus, measures insulation resistance up to 50 GW, and has a built-in eight-channel scanner. This type of tester can verify one conductor against all others in the group of eight for defective insulation and shorted conductors.
If you only require low-voltage testing, typical test voltages range from 200-mV to 12-V DC, depending on the manufacturer, tester model, and range setting. Some testers provide only a fixed voltage, some offer switched ranges, and a few are continuously variable.
The Eclypse International ESP Hand-Held Fault Location Meter is an example of a low-voltage tester that uses an AC excitation signal rather than DC. In operation, the frequency of the AC signal is varied until a minimum amplitude close to zero is measured at the cable connection. An integral microcontroller executes a formula that combines the velocity factor of the cable and the oscillator frequency to determine the distance to the cable discontinuity. This technique was developed under license from NASA and is claimed to have accuracy similar to a conventional time-domain reflectometer, but at a 90% lower cost.
The general location of a cable discontinuity also can be determined by more conventional testers. According to Mr. Stringham of Cirris Systems, “You need a tester that can measure capacitance and resistance accurately to determine where a mis-wire has occurred.
“Once the tester has detected an open, capacitance is measured from the two end points of the open circuit to all other wires in the cable,” he explained. “Because most opens occur at a connector, determining the faulty end is straightforward. Similarly, resistance measurements between the end points of a shorted pair of circuits can determine the faulty end.”
Resistance testing is the most common component test capability provided by cable testers. For most testers, measuring resistance is fundamental to determining continuity, although the measurement range may be limited. For a more accurate measurement, especially of low-value resistances, Kelvin or four-wire connections are required.
Conventional two-wire resistance testing cannot distinguish between the circuit under test and the tester’s own cabling that connects to the circuit. For example, there could be several feet of wiring between the voltage source and the start of the circuit being tested and another several feet between the end of the circuit and the ammeter measuring the current through the circuit. The total resistance limiting the current includes the connecting wires.
In contrast, a four-wire measurement provides the excitation current via a separate pair of conductors (A). The voltage measurement is made by a high-impedance instrument connected across the circuit under test by its own pair of wires (B). The voltage drop along the connection wires (A) caused by the excitation current doesn’t affect the voltage measurement because almost no current flows in the voltage-measurement wires (B).
A circuit that carries heavy currents, for example an engine-starting circuit, must be tested using a four-wire connection. Because the starting circuit has been designed to carry hundreds of amps, its resistance will be many times smaller than the resistance of the tester’s internal wiring. Without a four-wire connection, you only would be able to measure the tester’s resistance, not that of the starting circuit.
Because some wiring harnesses also contain shielded wires and twisted-pair cables, for example, simple resistance measurements may not give a complete description of the actual circuit as it has been connected. Capacitance measurements can be used to confirm the continuity of an unterminated shield and that twisted-pair conductors haven’t been miswired.
Capacitance measurements help determine the location of an open circuit. Of course, a cable/harness tester with this capability also can be used to measure capacitors.
As indicated in the comparison chart that accompanies this article, a wide range of cable/harness tester capabilities is available. The problem is not one of finding the mix of features you may need, but rather matching what is available to your budget. One way to get exactly the performance required for a particular job is to buy a tester custom designed for your application.
An interactive test procedure prompts the operator both visually and audibly to respond to a series of preprogrammed commands. Each component must be tested, or the program will not proceed to the next step.
Another example of a special tester capability is the guided assembly feature in the Cirris Systems easy-wire CH+ model that indicates where connections are to be made on a wire-by-wire basis during cable construction. As the wiring harness is built, each wire is tested, ensuring that the completed cable will be error free.
Cable/harness testers have developed in many dimensions to address complexity, ease of use, embedded components, multiple types of signal media, and cost. If you don’t see a suitable product in the comparison chart, contact the manufacturers directly. They have many more models in addition to those listed, so it’s likely that you soon will find a solution to your problem.
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