Can High Voltage Damage Cables?
Several years ago, AMP needed to test ribbon cables with wires of 0.050” centers and rated for 300VDC. They decided to use 1000-volt hipot tester from Cirris. The 1000-volt hipot test was useful in finding problems in damaged insulation on wires that passed between adjacent tines in the IDC connector. Surprisingly, anything less than 1000VDC could miss errors.
Even though 1000VDC was effective on the ribbon cables, there were concerns that the high voltage would damage or degrade the ribbon cables. Is this a valid concern when it comes to high voltage? How can you know the right test voltage to use when testing with high voltage?
What is the goal of testing with high voltage?
High voltage testing finds “near shorts” that low voltage testing would miss. The test applies a very high voltage between conductors and checks for good isolation. For more information on high voltage testing, see the Cirris article, “Testing Cables with High Voltage.”
Myth: Maximum Working Voltage Limits
Many users are concerned that exceeding the rated working voltage of the wire or connector during hipot testing will weaken or damage the insulation. Cirris testers have safety precautions in place that prevent the cable from getting damaged in the event of a high voltage failure.
For example, think about what happens when an AC extension cord gets a short. Sparks fly, resulting in charred remains of insulation, melted metal embedded into the insulation where the short occurred, and potential for more shorts due to reduced insulation. If this is what happens when you have a short at 120VAC limited by a circuit breaker of 15 amps or more, what might 1000VDC do to your cable assembly?
If you have a thorough understanding of electricity, you will recognize several things wrong with this comparison between a 120VAC short and high voltage cable testing.
- The energy that most testers apply to a cable-under-test is insufficient (most Cirris testers limit this to 0.03 Joules). Cirris testers carefully control both the current and the time that the current is applied. If you were to touch wires together on a Cirris hipot tester during a high voltage test, no damage would occur because of the limited amount of current and quick shut down time. In contrast, an AC line short that triggers a circuit breaker takes maybe 200 milliseconds at perhaps up to 100 amps. To show this mathematically would be 0.2 seconds X 120 Volts X 100 amps = 2400 Joules, or 80,000 times more energy than a failure during high voltage testing on a Cirris tester.
- In the AC extension cord example, the wires had to touch for the failure to occur. If the direct short (breakdown) had not happened, no damage would have resulted. Remember Paschen’s law: an arc will not start in the air until around 327 VDC or 231 VAC (the peak voltage at 120 VAC is 1.41 X 120VAC RMS = 170 VDC peak, which is not enough to cause an arc). HV testing will reveal potential shorts before they become dangerous.
A620 Working Voltage Limits
IPC/WHMA A-620 and all military specifications (as far as we know) do not use working voltage as a limit to test voltage. A-620 uses 1500 VDC or 1000 VAC to test Class 3 assemblies (High-Performance Electronic Products . . . where performance on demand is critical).
High Voltage is also required in A-620 Class 2 assemblies (Dedicated Service Electronic Products . . . where high performance and extended life is required), with pin spacings less than 2mm (0.079″) due to the higher risk of shorting due to close pin spacing.
UL Labs Requirements
Components connected to the AC line in appliance and technology products typically have working voltage specifications of 120 to 300 VAC. However, the UL specifications* often require 100% testing at thousands of volts—usually, 1000VAC + 2 times the voltage of operation. When in use, 1240 VAC is regularly applied to products with 120 VAC rated working components and 300 VAC wiring.
*982, 1010 and 1082 for appliances, and 60950 for Technology Equipment (ITE)
If you have ever seen the process of manufacturing insulated wire, you may have noticed a tester checking for defects in the insulation. This testing usually consists of a small bead chain (like those old key chains) or perhaps a brush raised to a very high voltage, through which the wire is passed. For 300V rated wire, a “spark tester” applies several thousand volts. To date, no evidence suggests that this testing has degraded the insulation. As a result, this practice is standard and required by most UL and MIL wire specifications.
There is no need to worry about potential damage from re-testing a cable or harness assembly at 1500VDC when these “spark tests” have previously been performed on every wire of their assembly. As an example, MIL-W-16878E TYPE EE (TFE hookup wire) rated at 1000 volts must be spark gap tested at 5,000 volts.
Case Study: No Insulation Damage
In 2003, Sandia National Labs tested aircraft wiring for degradation, not just at 1500 volts but all the way to breakdown requiring thousands of volts. You can read the report here.
The lab did not identify damage until breakdown occurred. The first detectable damage observed did not happen until ten times the energy of a Cirris tester.
Dielectric Breakdowns and IR Testing
There is no evidence that damage occurs to a cable subjected to higher voltages if a breakdown does not happen. However, if a DWV failure (breakdown) does occur, the energy is limited so that the risk of damage is low. Moreover, if damage did occur, the IR test is designed to detect the carbon trail damage. For this reason, A-620 suggests performing IR testing if performing a DWV test, and it is not allowed to precede the DWV test.
When Not to Hipot
Specifications like A-620 give useful guidance on test voltages. To be more rigorous, you can test assemblies as though they were in higher classes. But connectors do keep getting smaller. At some point, the connector itself cannot hold up at a specified test voltage.
- High voltage testing is effective in catching “near shorts” that low voltage testing may miss. These “near shorts” can turn into real shorts in the field (latent defects).
- It is NOT appropriate to use Rated Working Voltage of wires/connectors as a limit for test voltage.
- We lack evidence that high voltage testing damages or degrades the insulation of the wires or connectors in a harness assembly unless breakdown occurs. Even breakdown with high voltage at low energy levels is not expected to create damage.
- IR testing should be done concurrently with or after DWV testing to ensure that any possible damage from a dielectric failure is detected.