Top 5 Capacitance Errors in Cable Testing
Have you struggled with capacitance errors? The stored charge in a cable or harness can create problems during testing. Capacitance failures may occur due to any number of circumstances. Here are five reasons that might explain what caused your error and how to solve it.
1. Not allowing for the cable shield capacitance
If you’re getting a single overcurrent failure during the high voltage testing of your cable, it may be caused by a highly capacitive shield around your cable.
To find out if a highly capacitive shield is causing the overcurrent failure, run the test and confirm that the shield is the net that is failing. If so, check the “High Capacitance Shield Allowed” box in the High Voltage Test Parameters window of the Easy-Wire software.
Note: this works if only ONE net is failing due to capacitance.
2. Testing long or coiled cables
Long cables and cables that are coiled have more capacitance and may create overcurrent failures. The reason for this is because Cirris testers have limits to the amount of charge that can be applied.
For long cables, try lowering the voltage in your test program. Unwind any cables that are coiled and stretch them out. You can also measure the amount of capacitance from the shield to all other wires tied together (this is what happens during the high voltage test) to find out the total capacitance in the cable you’re trying to test. Compare this data with the “maximum capacitance per net” specs found associated with each tester type at www.cirris.com. For example, the Easy-Touch® Pro specs online show:
Max Capacitance Per Net: 150 nF @ 300 VDC, 90 nF @ 500 VDC, 45 nF @ 1000 VDC, 30 nF @ 1500 VDC, 9.5 nF @ 1000 VAC
3. Floating metal
“Floating” metal is metal in your cable or harness assembly that is not grounded/assigned to a test point (e.g., a metal connector shell in your cable assembly). Floating metal—coupled with your circuit under test—will pick up a charge and may suddenly discharge during testing. Your Cirris tester will detect this as a dielectric breakdown. This error may only be intermittent and could occur at a single point in the cable assembly, making it difficult to locate.
Make sure all floating metal is attached to a test point so that it can be discharged.
4. Capacitors not identified during a Learn
After Learning a cable and reviewing the test instructions, you may discover that the test program did not Learn every capacitor in your cable. The reason for this may be because your capacitors are not within the learn range of your tester.
Verify your test instructions after learning your cable. If any capacitors are missing, add them to your test instruction list manually.
To discover the capacitor learn range for your tester, check the specs found associated with each tester type at www.cirris.com. For example, the Easy Touch® Pro component specs online show:
Capacitors: 5 nF to 100 μF ±10% ± 0.02 nF
To add a capacitor to your test instruction list:
- Go to the “Define Instructions” tab of the Test Program Editor in Easy-Wire and select the CAPACITOR test instruction in the drop-down list.
- Click “Add Instruction.”
- Manually add your capacitor instruction in the Capacitance Instruction window.
Note: The capacitors must be within the test range of your tester, or the tester will not recognize them as capacitors. Capacitors that fall outside of the test range should be entered as links so that the high voltage test will not fail.
5. Incorrect polarity of electrolytic capacitors
Cirris cable testers cannot check for the polarity of electrolytic capacitors. So, if you didn’t pay attention to the positive and negative side of your capacitor and consequently put it in the wrong way, you will send out a bad cable.
In future, use your schematic and a visual inspection to ensure electrolytic capacitors are situated accurately in the cable you are testing.
Note: The polarity of an electrolytic capacitor is usually indicated on the capacitor itself. Also, the positive lead of the capacitor is typically longer than the negative lead (however, if the capacitor is already installed, the leads are likely to have been clipped).