To "hipot test" a cable assembly means to apply a High Potential between each wire and all other wires (and shields) to assure that they are properly insulated, (or isolated) from each other. For cables that contain only wires (and/or un-terminated points), this is a fairly simple task. However, if your cables have embedded components (resistors, diodes, capacitors, coils, IC's etc.) you may be concerned about damaging components with high voltage testing.
There are two elements of a high voltage test (For the purposes of this document "hipot" refers to both);
- A DWV (dielectric withstand voltage) test is done with DC or AC voltage.
- An IR (insulation resistance) test always done with DC voltage.
Safely test assemblies with components on Cirris Hipot testers
Cirris testers are designed to perform high voltage testing safely on assemblies containing components. This is possible because:
- Cirris testers do a low-voltage continuity scan first, before doing the high voltage test.
- Cirris testers can selectively apply voltage on multiple test points at a time.
What is a net?
Since a continuity test is performed first, the exact wiring pattern of the cable is known before the hipot test. Each group of test points within the wiring pattern is called a net. A net can be:
- Two points connected by a wire.
- A group of points connected by wires.
- A single point net (sometimes called a non-connected or NC point). *
*Single point nets, while not included in the net list, are treated as separate nets for the hipot test since they must be checked for isolation from all other points/nets.
After performing a continuity test and learning the wiring pattern, the hipot test can intelligently apply high voltage by:
- Testing each net against every other net that should be isolated from each other.
- Combining nets that are "linked together" with components so that voltage doesn’t drop across, or current flow through, the components.
Sample Wire List
Let's look at a simple cable assembly as an example.
The test program for this type of cable would look like this:
The net list would look like this:
Nets joined by components are displayed this way:
Remember: The single point nets (J2-4 and J2-6), while not included in the net list, are treated as separate nets for the hipot test since they must be checked for isolation from all other points/nets.
How hipot tests could damage components
Because of their test point architecture, many hipot cable testers can only apply voltage to one point at a time as follows:
- One point (the first point of a net) and all points lower
- One point and all other points (only works for single point nets)
- One point (the last point of a net) and all points higher
Also, point-order is always defined as raw system point number, so there may be no correlation to the order of points in the way the tester is wired to the fixture.
Since they can't apply voltage to multiple points at a time, with this type of test, the capacitor linking nets 3 and 4 would be damaged when high voltage was applied to only point J1-5 (the head point of net 3) or point J2-2 (the end point). Also, the resistor in net 5 could be damaged, depending on the amount of capacitive coupling between it and the other wires in the cable, when high voltage is applied to only one side of the net.
How a Cirris Tester would safely hipot this Cable
First, the tester would perform a complete continuity scan of all connections, including components.
Note: If the continuity test fails, the test is finished, as there is no need to hipot a bad cable.
Step-by-Step High Voltage Test Sequence:
- Net 1 (green - J1-1 J1-4 J2-1) is charged to high voltage while all other nets are tied together and held at 0 Volts (ground).
- Net 2 (red - J1-2 J1-3) is charged while all others are held at 0 Volts (ground).
- Net 3 (blue - (J1-5 J2-2) and Net 4 (J1-6 J2-3 J2-5) are tied together with a component, so all points of both nets are charged to high voltage simultaneously while all other nets are held at 0 Volts (ground).
Note: Although the component is at a high potential, with respect to the other nets, both sides of the component are at the same voltage so there is no voltage drop across, or current flow through the component.
- Net 5 (orange - J1-7 J2-7) is a component, so again, both points are charged simultaneously. As before there is no voltage drop across, or current flow across the component, therefore there is no chance of damaging it.
- Net 6 (black - J2-4) is a Single Point, but is charged to high voltage while other nets are tied together and held at 0 Volts (ground).
- Net 7 (black - J2-6) is also a Single Point, but (like the others) is charged to high voltage while other nets are tied together and held at 0 Volts (ground).
High Voltage Testing Precautions:
Although Cirris hipot testers can self-learn the components in a cable, it is a good idea to verify that the test program is correct before performing hipot tests to assure that no damage occurs to components in your cables. Here is the process Cirris recommends:
Cirris hipot testers can self-learn an assembly including most but not necessarily all components. It is your responsibility to make sure that all components are correctly listed in the test program.
Verify and Edit if necessary:
Again, be sure to verify that the test program is correct and includes all components. If the self-learn feature of the tester missed a component, or if you have a component that isn't identified correctly, you must manually add it to the test program to assure that it won't be damaged during the hipot test. If you have non-standard components you can add a link command to the test program to protect them from voltage.
(see LINK description below)
Once you've verified that your test program properly lists all connections and components, you can safely hipot test it without the risk of damaging any components.
The Link Command
When a Learn containing components is set up properly, Cirris testers automatically Learn most common components (resistors, diodes, and capacitors) and automatically link them together during the hipot test, as described above. You can also add a Link instruction to link together any points or groups of points that you don’t want to be separated during a high voltage test.
To do this, add a Link instruction to the test program in the Test Program Editor (you only need to link one point in each net for this to work properly).
EXAMPLE: You don't want high voltage applied between nets 3, 4 and 5 of the following test program.
Edit the program, adding Link instructions like this:
Note that the new commands link the first point in Net 3 (J1-3) with the first point in Net 4 (J1-5), and the first point in Net 4 (J1-5) with the first point in Net 5 (J1-7).
The following option has the same effect.
This links the first point in Net 3 (J1-3) with the first point in Net 4 (J1-5), and the first point in Net 3 (J1-3) with the first point in Net 5 (J1-7).