- SPECIAL REPORTS
When high-voltage cable assemblies and their connectors fail, it is generally catastrophic. This is because a dielectric breakdown has occurred.
Dielectric breakdowns happen when the voltage stress is significant enough to cause an arc through the dielectric between the conductor and ground. The failure is catastrophic because the current flow through the dielectric leaves voids filled with carbon. The dielectric will no longer be able to withstand the required voltage, and in some cases, voltage can’t be raised.
There are five main reasons why high-voltage cables and connectors fail: excessive bending, elongation, excessive torque, interface failures and corona failures.
Excessive bending of the cable puts undesired stress on the entire cable, from the conductor to the shield. This stress can cause microvoids in the insulator to become elongated, become larger or even tear. This can lead to an eventual corona failure or dielectric breakdown.
Linear elongation and relaxation of the cable can cause the conductors to “birdcage” or kink. The conductor and the insulator are not one homogenous mass; they can move independently from one another. Thus, the insulation can stretch while the conductor gets pulled tighter. When the cable relaxes, the insulation can act like a Chinese finger trap and grab the conductor.
A birdcage happens when an outer layer of the conductor stranding separates from other outer strands and the inner stranding. This can cause the air between the strands to ionize, which will lead to a corona failure.
A kink in the conductor can lead to a damaged or broken conductor. If the conductor breaks, it can puncture the insulation, severely reducing the dielectric withstanding voltage. Eventually, the dielectric will no longer be able to withstand the applied voltage, causing a dielectric breakdown. A kink in the conductor can cause excessive stress on the insulator. A kink in the electrical cable will almost always lead to corona failure.
When excessive torque is applied to a high-voltage cable, the cable tends to fail directly behind the connector, where the smaller leads exit the larger connector diameter. The excessive torque applied causes the cable leads or the potting material to split and tear. The tear will get worse until a dielectric breakdown occurs.
An interface failure occurs when the mating connector is not fully mated. This creates a creep path for the voltage to ground. When voltage is applied, it will bridge the gap between the conductors and ground, causing the cable to short out. In most cases, this will leave a groove in the insert through which the current flowed. The groove will fill with carbon, and the cable will no longer be able to withstand the applied voltage.
A corona is the ionization of air within a void in a dielectric or interface inside of a connector. This ionization produces ozone, among other gases, and ozone is detrimental to dielectric materials. When ionization occurs, it leaves a path for current to flow. Over time, the dielectric will continue to break down until the voltage stress is significant enough to cause dielectric breakdown.
When performing a failure analysis of high-voltage cable assemblies or connectors, engineers should be follow the same process every time. High-voltage cables can fail for the five reasons I just mentioned. Some fail because of bad workmanship or material defects. Either way, a reputable company should have a process for identifying and correcting a failure in their high-voltage cables or connectors.
Before beginning a failure analysis of a cable assembly or connector, speak with the end-user of the product and find out as much as you can. These are the most important questions to ask:
Where was the cable assembly installed? Was it outdoors, indoors, underground, underwater or at altitude?
What were the operating voltage and current?
If the same product has failed repeatedly, look to your raw materials suppliers. Did they change anything in their product?
Documentation is critical. There is no such thing as too much information. Gather information such as test reports, work orders, raw material lists and inspection reports. If you have a tightly controlled manufacturing process and keep good records, this should be easy. In some instances, however, it can be difficult.
The next step is observation. Do more than just look at the failed assembly! Feel the product. Is anything dented, split, cut or swollen? Look for anything out of the ordinary. Give it a smell. Do you notice any odors coming from the product? Your nose may be able to detect solvents or chemicals on the product that should not be there. As you cut apart the product, carefully examine each piece.
Always keep thorough records of anything you find. Detailed records will help you come to a conclusion at the end of the analysis. It will also help you explain to your customer what went wrong. You’ll want to hang onto those records, too. They’ll come in handy for future analyses, helping you spot trends and identify recurring problems.
A picture is worth a thousand words, right? So don’t forget to complement your findings with photographs. Any observation you write down should be photographed. Take pictures of every step of the process. Don’t be afraid of taking too many pictures. With digital imaging, there’s no excuse not to. Just delete the ones you don’t need!
Anything you remove from the product should be saved in a plastic bag and marked appropriately. If you need to prove your conclusion to someone, having the bad material will help your case. It could also provide answers on how to avoid this failure again.
When you’ve finished taking the cable and connector apart, analyze your records, photographs and materials. Put all your findings together and bring in another set of eyes to review everything to make sure you did not miss anything. This step can be very long and may take a couple of weeks to identify what caused the failure.
Once you have analyzed everything, you need to come up with a conclusion as to why the cable failed. Be careful not to speculate about a root cause—use your evidence to identify the actual reason it failed. If you don’t have a reason for why it failed, then don’t be afraid to say the failure is unknown!
Finally, it’s always a good idea to write an executive summary of the failure analysis that can be distributed to the customer and others in your company. Not everyone needs or wants all your technical data. Having the executive summary will also help you quickly remember past analyses that may help you in the future.