Creepage and clearance measure distances of the shortest path between electrically conductive components. Creepage refers to the shortest distance along the surface of an insulator. Clearance measures that distance through the air. Creepage can also take short circuits into account for gaps and grooves across surfaces, where the air has less resistance.
The repercussions of violating minimum distances can be extremely expensive. (And, of course, anytime you are dealing with electricity, poor designs can cause injuries or worse.) If discovered too late, problems can result in a product recall, which damages your company’s reputation and market share. When discovered early, issues can be mitigated by modifying the harness routing or implementing a physical barrier between components.
I’ve been an engineer for three decades and I’ve seen many companies that have products with cable harnesses, but they don’t design their harnesses in CAD.
Harness development often is performed in a practical manner, laying the wires out in a prototype. The companies that do perform cable harness design in CAD often do not perform analysis on their harnesses. As companies move more toward electrification, especially for electric vehicles like cars and drones, it’s important that they both design and analyze harnesses and components in their mechanical CAD assemblies.
There are many kinds of electrical analyses and simulations that you can perform to validate and optimize your electrical cable harnesses in CAD. Here’s how you would proceed with creepage and clearance analysis.
The general process for analyzing your assemblies for clearance and creepage is as follows:
1. Set up the components. When initiating a creepage and clearance analysis, we start by defining a special parameter called the comparative tracking index (CTI) for our components. CTI has a value of zero for conductors and a value greater than zero for insulators. The higher the value, the better the material is at insulating. (Parts can be made of multiple materials, so different bodies and surfaces can be assigned different CTI values.)
To improve results, we can also define individual parts as rivets, screws, springs, and convex hulls (flexible parts like clamps). These classifications result in more accurate calculations.
We can also “cement” insulators that are adjacent to each other so that they are treated like a continuous component to take the full creepage path into account.
2. Define electrical nets. All adjacent conductive components are automatically combined to a common net. We then assign the type to each net:
3. Set the analysis parameters. We can specify the minimum allowable distances for clearance and creepage either manually or using a table. The latter method allows users to comply with industry standards based on voltage and insulation classes, such as IEC (International Electrotechnical Commission) 60664-1 or UL (Underwriters Laboratories) 60950-1.
4. Evaluate the results. After the analysis has been set up, it can be run to view the results. For each combination of sources and target nets, we can see the computed distances for clearance and creepage and whether there was a violation. A tolerance can be added to the distance in order to distinguish between minor and major violations.
The path for the shortest distances can also be displayed on the screen. This helps with the next step.
5. Improve the design. Now that we have identified where we have issues and violations, we can improve our model. Then we simply run the analysis again to ensure that we meet our requirements.
By performing electrical creepage and clearance analysis, we can increase the safety and quality of our products. If you design assemblies with electrical harnesses, you might also like the video demo, Multibody--Creepage & Clearance Analysis.
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