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.
Each day, throughout the day, you use and operate equipment that utilizes electrical connections. From the wiring in your cell phone to the wiring in your car; from the cables in your microwave to the harnesses in aircrafts, you depend on these connections more than you realize. 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.
Electrical connections are responsible for bringing power and “life” to your devices. We are surrounded by technology, so it is not surprising that most of the tasks we perform require good electrical connections. Indeed, anything that “turns on” depends on electrical connections to function. We can use human anatomy as a comparison: We, too, depend on connections (veins and arteries) to carry blood around our bodies to keep us alive and give us the power to operate.
We typically take electrical connections for granted until something goes wrong. And if something does go awry, the consequences could be enormous, such as:
- Lost data
- Costly repair or replacement of devices or vehicles
- Death (e.g. imagine if the wiring in your aircraft is faulty)
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 the assembly is filled with electrons. We cannot see electrons, just as we cannot see how well those electrons move. The quantity of electrons within the mechanical wires, harnesses, or backplane assemblies are responsible for the functionality of a device. The data or signals 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. If you manufacture cables and harnesses or equipment that utilizes electrical connections, then it is your responsibility to make sure these connections are working as they should.
What Are You Testing For?
Here are some potential cable/harness errors you should to test for:
- Opens (current is not flowing to and from the points it is meant to)
- Shorts (current is flowing to the wrong place)
- Miswires (combination of an open and a short)
Problems like these will cause your connections to fail and may negatively affect the integrity of your device or bring harm to the people directly involved with the equipment.
Your testing needs may go beyond that. You might need to test assemblies that contain passive components such as:
- Twisted pairs
If the device you are testing has any components, there needs to be a way to account for these components when performing tests. For instance, if your test method is designed to use a low resistance to check a connection between two points, a highly resistant component will mimic an open. Your test method needs to be able to make an allowance for this component. Your test method should also check to make sure that the components are correct (e.g. appropriate resistance, diode in proper orientation, etc.).
High Voltage Testing
Low voltage testing reveals opens and shorts that will always cause problems in your cables. However, high voltage (hipot) testing can detect errors that may not cause problems immediately, but may cause problems in the future. If you are testing systems that cannot tolerate any possibility of failure, such as those that are critical for safety, then hipot testing is necessary.
Hipot testing can be likened to water pressure in a hose. If there is a hole in the hosepipe, you are more likely to see water leaking out of it (and thus find its location) if the water pressure is strong. The same goes for the pressure or “strength” of current flow in a cable: the greater the voltage, the more likely the current will “arc” and thus alert us of insulation damage, near shorts, etc.
Some problems that hipot testing can detect (that low resistance testing may not find) include:
- Damaged insulation
- Stray wires
- Contaminants such as solder flux
Continue to Where to Start - Part 2 to learn about different test methods.