How to Hipot Highly Capacitive Cables
A customer recently asked Cirris about a “charge error” that occurred during a test. This error has to do with capacitance in cables. If you have ever run into a charge error or overcurrent error, Cirris can provide you with valuable solutions that will help you eliminate the error.
Whenever you have two wires in a cable that aren’t connected to one another, they form a capacitor. This is similar to a battery in a sense that when you apply voltage between the two wires, they can store charge.
When voltage is applied between the two conductors separated by a barrier, capacitance describes the amount of charge held by the conductors. The barrier insulates the conductors and allows them to hold opposite charges.
Hipot testing is a process of applying high voltage to a net (or network of wires) in a cable, holding the voltage for a specified amount of time, then reducing the voltage back to zero before moving to the next net. As the voltage to the net is ramped up, charge accumulates on the net-under-test in an amount proportional to the net’s capacitance. The mathematical equation for this is:
Q = C x V
For this equation, “Q” is charge, “C” is capacitance, and “V” is voltage. There is an electrical safety standard that mandates a maximum safe amount of charge for any electrical assembly a human could come into contact with. Cirris testers carefully monitor the amount of charge delivered to an assembly during voltage ramp up and will shut down with a charge error if this safe amount of charge is exceeded. If you are experiencing a charge error, then from the above equation, we can see that the way to reduce the charge is to either reduce the capacitance or reduce the voltage.
Because capacitance is related to the physical design of your cable (which you probably can’t change), you can try lowering the voltage. However, if your cable specs require a certain voltage that cannot be changed, contact Cirris to talk about possible options.
Current is the rate of charge flow on to or off of the net-under-test. The equation that relates charge to current is:
In the above equation, “i” is current, Q is charge, and “t” is time. Cirris testers monitor the amount of current delivered to an assembly during the ramp phase and will shut down with an overcurrent error if the specified amount of current is exceeded. This is why some cable specs will specify a ramp rate for high voltage testing. It will ensure that the voltage is raised slow enough to avoid excessive current due to capacitance errors.
On a Cirris 1100H+ or Easy Touch Pro, the charge and overcurrent errors are both reported under the overcurrent error. This error occurs because the current surpassed the specified level or the charge exceeded the safe limit.
When testing with a Cirris CH2 tester, the charge errors and overcurrent errors are reported separately. An overcurrent error occurs when the current surpasses specified levels. The charge error occurs when the tester calculates that more charge has accumulated on the device than is safe.
On Cirris testers the ramp rate is controlled automatically by default and thus should not be a setting you need to adjust. If you are using test equipment which does not have automatic ramp rate or you have chosen a manual ramp rate on the Cirris CH2 tester, lengthening the ramp time could help reduce capacitance related errors.
How to Solve High Voltage Capacitance Errors
Reducing capacitance depends on what aspects of the device you have control over changing. The more wires in a net, the more the capacitance will increase. The thickness and material of the insulation, and the length and width of the wires can also affect capacitance. Shield nets are electrically quite wide and thus typically have much higher capacitance than other nets in the cable. Most of these are physical aspects of the cable and you will unlikely be able to change them. In general, the more metal you have, and the closer that metal is to other metal, the higher the capacitance. You may not be able to change the shields or cable length, but there are a couple of things you might be able to do to reduce capacitance in your device.
Make sure all electrical conductors (wires, shields, metal connector shells, etc.) are connected to test points. Then make sure the test points are attached to the test program. This will present what we refer to as ‘floating metal’ in your assembly. Floating metal can cause erratic and non-reproducible test errors. Also try uncoiling the harness to reduce capacitance. Remember that capacitance increases the closer the two conductors are to one another. If connecting the floating metal and uncoiling the cable does not solve the capacitance-related error, there are other ways to try and solve this issue.
Since you may not have the option of changing the physical aspects of a cable, you may have to adjust the voltage. This can be done using Cirris easy-wire Software. There are several ways you can do this:
- Turn the voltage off for a particular net with the error. During hipot testing, each net is tested with respect to every other net. The net is first tested against the other nets in one direction. The direction is reversed when the other nets are tested against the first net. Turning off the voltage for a given net will only affect one direction of the test. You will still test this net at full voltage from the direction of every other net as long as no other nets are also disabled. The excluded net will be held at ground during the test.
- Lower the voltage on the particular net with the error to allow the net to pass. Similar to turning off the voltage, this only affects one direction of the test. The net will still be tested at full voltage from the direction of the other nets.
- Change the settings to High Cap for the capacitive net using HV Control.
HV Control High Cap for the 1100H+/Easy Touch Pro
Even if a cable has no critical errors, the cable may fail a test due to an overcurrent error caused by a capacitive shield. The 1100H+/Easy Touch Pro has two different ways of solving this issue.
- If testing on a standalone 1100H+ tester not connected to easy-wire, Cirris has provided a solution to a single overcurrent error. When setting up the high voltage test parameters, select the option “High Capacitance Shield Allowed.” This will allow the test to pass the cable even if it finds a single overcurrent error. The test will still fail if any error other than a single overcurrent error occurs. The drawback with this method is that the test will continue regardless of the location of the overcurrent error. This means that if you turn on this setting because you suspect a shield will cause the overcurrent error, but the overcurrent error occurs in a different net, the test will continue and not report the overcurrent error. This situation is rare, but still prevents a risk when using this setting.
- If using an 1100H+ connected to a PC or an Easy Touch Pro, you can use easy-wire to solve an overcurrent issue by turning on HV High Cap for one net. (The 1100H+/Easy Touch Pro will not allow HV High Cap for multiple nets.) For example, turn on HV High Cap for the net containing the cable’s shield. Unlike the “Shield Allowed” method, which ignores one overcurrent error anywhere in the cable, turning on HV High Cap tells the tester to ignore the overcurrent error associated with a particular net. Errors found in other nets or multiple errors in the High Cap net will still cause the test to fail. Cables that would normally fail due to an overcurrent error will be able to pass.
HV Control High Cap for the CH2
With a normal HV test, the net is raised to the specified voltage while all the remaining nets are held to ground. All nets marked as High Cap, however, get treated differently according to the following procedure:
- Segregate these nets from the other nets.
- Measure the capacitance of each net to each other (other nets are held at ground).
- Sort the networks according to capacitance with the highest capacitance first.
- The first net in the sorted group will be energized with high voltage while all other points and nets are floating (not at ground or HV).
- The second net in the sorted group will be energized with high voltage while the first net is held at ground and all other points and nets are floating.
- The third net in the sorted group will be energized with high voltage while the first and second nets are held at ground and all other points and nets are floating.
- The tester processes these high cap nets in this manner until all that are marked as high cap are processed.
- The tester proceeds with the remaining points and nets following the normal algorithm.
With this procedure, the highest capacitance net is raised to the full voltage per the spec. Since the other nets are not part of the same circuit, the raised net does not have the capacitance issue. This also means that since the other nets are floating there is no path to ground, and thus the raised net is unlikely to find an error to those other nets. However, as the other nets are tested later, they are tested against the more capacitive nets and thus any failures that exist should be found (they will just be found only in one direction and not the other.)
This method works best when marking more than one net as High Cap. If only one net is marked as High Cap, there little difference between the high cap method and just turning off HV testing on that net altogether (in terms of catching legitimate errors). However, if you have multiple capacitive nets, it would be better to mark them all as High Cap rather than Off because then they will all be tested against each other in at least one direction.
If capacitance in cables is causing issues in your shop, you may try some of the methods suggested in this article. To learn more about capacitance, Cirris has another useful article you can check out called Capacitance and Cable Testing.