Cable Testing Basics.

Do Quality Connections Really Matter?

To the crew of the Apollo 13 they certainly mattered. The Apollo 13 "disaster" was caused by a spark emitted from wiring that had melted due to excessive temperatures. This in turn caused the number-2 oxygen tank in the service module to explode. The explosion blew away an entire panel on the service module. As a result of the damage, the Apollo 13 crew members were forced to use the lunar module as a lifeboat back to Earth. Like the crew, we use machines and electrical equipment that contain miles upon miles of wiring inside them. Can you tell the quality of the wiring and its connectivity from the outside of a device? When you pick up your iPad or cell phone, do you know how well they are wired? Every time you mow your lawn, drive your car, or turn on the lights, are you confident that everything is wired as it should be?

You hope that they are wired correctly, but if they are not, or if the wiring is damaged, you will usually find out at the worst possible time--such as when you're driving through the 126°F desert and the A/C unit of your car burns up because the manufacturer installed the wrong gauge wire.

There are methods to help you assemble wires: the wiring schematic, a bill of materials, and labels. However, once assembled, how can you prove the assembly is sound and wired correctly?

First, you use your eyes. By looking you can measure workmanship quality in terms of the mechanical aspects of an assembly. For example, you can see if you have good crimps, satisfactory splice junctions, and proper isolation of shields and conductors. Conversely, what your eyes cannot tell you is how the electricity flows when it is filled with electrons. We cannot see electrons, just as we cannot see how well those electrons move. The quantity and quality of electrons within the mechanical wires, harnesses, or backplane assemblies are responsible for the functionality of a device. The data, signals or voltage produced by these electrons are needed to fire a missile, launch an aircraft, and operate life-saving medical equipment.

Considering the fact that we cannot rely on our eyes to measure electron flow in our assemblies, we have to use test equipment.

Photo top: This view of the severely damaged Apollo 13 service module was photographed from the lunar module after it was jettisoned. The command module, still docked with the Lunar Module, is in the foreground.

Photo Left: The crew of the Apollo 13 mission step aboard the U.S.S. Iwo Jima (prime recovery ship for the mission) following splashdown and recovery operations in the South Pacific. Exiting the helicopter, which made the pick-up some four miles from the Iwo Jima, are (from left) astronauts Fred W. Haise, Jr., lunar module pilot; James A. Lovell Jr., commander; and John L. Swigert Jr., command module pilot. The Apollo 13 spacecraft splashed down at 12:07:44 pm CST on April 17, 1970.

Some folks still do it the old-fashioned way: make the assembly, plug it into the product and see if the product works. This is known as functional testing. Is there a cost in doing this?

Functional Testing Case Study
A company that makes surveillance equipment for military and homeland security contracts a supplier to build a wire assembly for a $100,000.00 gyroscopic camera. The camera has lasers, night vision, HD Video, anti-shake, and 360-degree viewing. Mounted to a UAV or helicopter, it can find a fugitive hiding in a ditch or a lost child on a mountain top at night, chilled to the bone.

When the harness was plugged into the array of camera sensors and equipment and powered on (functional testing), smoke billowed out and it became apparent that the assembly was bad. As it turns out, the laser in the camera received the wrong voltage because a connection had been miswired to a higher-voltage supply line. The cost of this single electrical wiring error was--you bet--$100,000.00. Although functional testing is an important part of the proces, it should NOT be the first part of the process, as we have just seen.

Some Helpful Test Terms
Before we continue our discussion of testing, it will be helpful to understand that a continuity test checks for these electrical conditions:

1. Good Connection: The current flows where it is supposed to.
2. High Resistance Connection: Current flows where it is supposed to, but it is the wrong amount of current.
3. Open: Current is not flowing to and from the points it is supposed to.
4. Short: Current is flowing to the wrong place.
5. Miswire: This is an open and a short
combined. The wire is missing from the desired location, and is instead going to an
undesired location.
6. Low Voltage Insulation Resistance Leakage: This is a type of short.

Most of us are familiar with handheld electrical circuit testers because they are used by electricians everywhere. An elctrical circuit tester is a handy, cost-effective tool that allows you to see if electrons are flowing. Some of these are handmade with a 9V battery and a speaker that buzzes or beeps (beep box) so you can hear whether or not there are any electrons present. Some have a light (light probe) that illuminates when a connection is detected. A handheld tester cannot tell you how many electrons there are, it simply lets you know if they are flowing--which is still a step up from the total lack of testing in the above example.

It would take one or two people several hours to probe this harness using a handheld electrical circuit tester, and they would have to probe very carefully over every single circuit. They would be able to learn if the electron current flows to the places designated in the schematic, thus providing a decent continuity test (a continuity test is the testing of an electric circuit to see if current flows). If electron flow is inhibited by broken conductors, damaged components, or excessive resistance, the light on the handheld device won't turn on or the speaker won't beep. This indicates that the circuit has an “open.” This method typically misses shorts and most miswires.

This method of testing is the way it has been done in the past and is sometimes still done today. The cost is low and the man power is high, but at least it is better than the “Poke and Hope” method of a functional test.

Beeping 2.0

The galvanometer was a revolutionary instrument originally devised in 1820. It was improved to become a field-capable device in the 1920s by a British post office engineer who wanted an improved multifunctional tool to troubleshoot phone lines. This handheld device is known today as a digital multimeter. It not only detects electrons, but also measures "electrical" quality, which is based on one of three factors:

• Voltage (measured in volts)
• Resistance (measured in ohms)
• Current (measured in amps)

This tool has the same two leads (red and black) as the beep box or light probe we discussed earlier. It has a beep mode and one or two people can measure the flow of electrons through the wires of the device-under-test.

This device is still used heavily today and is the de facto cable tester.

Semi-Automated Electrical Tester

It's time to calculate how many measurements or "probes" the workers have to make. This sort of calculation is called a permutation. To calculate the number of permutations required in our scenario you would use the equation (N) x (N - 1) / 2, meaning you would probe the first point to points 2 through 100, the second point to points 3 through 100, and so on. If you had 100 points to measure the equation would be (100) x (100 - 1) / 2 = 4,950 different possible measurements that you would need to make. At 30 seconds per test this would take nearly 41 hours! This is a very common practice for testing low-volume harnesses. While it is time-consuming it can be, and is, done regularly. However, for a very large and complex harness you could be talking several weeks' worth of probing, and we have not even begun to talk about checking for insulation integrity yet!

 

Summary


• A Continuity Test checks for electron flow.
• Hand Beeping requires time. It's prone to human error due to fatigue and losing one's place on the schematic.
• If you want to check for shorts, you have to make a lot of redundant measurements.

Hmm . . . this method of manual testing is probably impractical for today's high production needs and quality standards. So how can we improve testing time, reduce human error, eliminate ergonomic issues, as well as record all the values of the circuits we are testing? Enter the Automated Cable Tester . . .

Imagine a new kind of tester--a digital multimeter that has hundreds, if not thousands, of test leads coming out of it. The tester has a switch inside that allows each lead to alternate between + and -.

It isn't too practical to have hundreds of people holding those leads, so let's turn those leads into mating connections (test points). Should you need to, this tester also allows you to add more mating connections in order to increase the number of measurements you can make.

Now, instead of having to manually probe your circuits two points at a time, wouldn't it be nice to take your schematic and load the diagram data into the tester? This way, you can take those measurements without even needing to be there. What's more, there won't be any operator fatigue, and you will get more consistent results. In addition, your tester can check those 10,000 points (as mentioned in above) in seconds because it is able to measure more than two points at once. Since you're measuring the voltage, resistance, and/or amps, go ahead and have your tester record these measurements and print a report. Too much to ask? Nope! Such is the power of automated testing!

So why use handheld digital multimeters at all? Well, as you build an assembly you might want to make sure the wires are able to conduct electricity. Or, you may need a multimeter to troubleshoot a circuit that an automated tester deemed as bad. Additionally, some assemblies simply do not justify the cost of manufacturing the mating connectors that plug into Cirris' automated tester. If you're assembling something simple like a refrigerator or a dishwasher, placing the wires in the correct place may be all that matters. In situations like these where measurements and data are not required, a multimeter can still produce good results.