Can Standard Cirris Cable Testers Detect Broken Wire Strands?
Many Cirris customers have asked if our standard cable testers can detect broken strands in their cables. If you’ve ever seen a cable assembly with broken wire strands or strands outside of a crimp, you might think that normal resistance testing would reveal these faults. But is that really the case? The results might surprise you!
Test This! – The Empirical Results
The Setup
- A 38″ length of 7 stranded, 26 AWG wire.
- An HP 34401A multi-meter capable of measuring very small resistance changes using 4-Wire (Kelvin) measurements.
- A Dino-Lite Digital Microscope.
First, we measured the end-to-end resistance of the 38″ wire to establish a baseline. Then, we cut one strand at a time and recorded the resistance readings after each cut. We also took images of the wire using the microscope.
The Results
Wire Image | 4 Wire Measurement | Condition of Wire |
| 00.1290 | No strands are cut |
| 00.1290 | One strand has been cut |
| 00.1291 | Two strands have been cut |
| 00.1294 | Three strands have been cut |
| 00.1293 | Four strands have been cut |
| 00.1297 | Five strands have been cut |
|
| 00.1334 | With Five strands cut we decided to experiment. We warmed up about 8" of the wire by holding it in our hands to see how it would effect the resistance readings. |
| 00.1320 | Six strands have been cut. |
| 00.1351 | All Seven strands have been cut, but we then simply smoothed down the wires with our fingers and pushed the two ends together to take our last reading. |
What have we learned?
Our baseline resistance (all wires intact) is 0.1290 Ω. When all but one strand was cut, the resistance was 0.1320 Ω—a difference of only 3 milliohms! To put this in perspective, when we measured the resistance with 5 strands cut, we measured 0.1297 Ω. When we warmed up a small portion of the wire in our hands, the resistance changed to 0.1334 Ω—a difference of 3.6 milliohms. There was more change in resistance from warming the wire than from cutting all but one strand!
Comparisons
- Length Variation: The difference of 0.0030 Ω (no strands cut vs. all but one strand cut) would be similar to a small change in cable length. In this example, if the cable was was 0.9″ longer, the change in resistance would be roughly equivalent to the change from the broken strands.
- Cross-Sectional Variation: Manufacturer’s specs allow for a 2% variation in stranded wire construction. This variation is similar to the 0.0030 Ω difference we observed.
Conclusion
Detecting broken wire strands in your cable assembly using resistance measurements is not practical. The variations in wire length, cross-sectional measurement, and room temperature overshadow the resistance changes caused by broken strands. It’s physics!
Many manufacturers look for cut strands introduced during the wire cutting, stripping, and terminating process and only find them during visual inspection. Komax ACD and Schleuniger SmartDetect technology can detect when the stripping blades come in contact with any of the strands or conductors. This is particularly important when making IPC/WHMA A-620 Class 3 Assemblies like those found in Aerospace, Defense, and Medical wire assemblies.
Cut strands can be prevented with Komax ACD and Schleuniger SmartDetect technologies.
The Theoretical
A cable is made up of a number of smaller gauge wires:
In this example, our cable is 38 inches long and is made of 7 strands of copper wire that are roughly 34 AWG.
The resistance of 34 AWG copper wire is 0.2609 ohms/ft. at room temperature.
The resistance of each strand is parallel to the resistance of all the other strands, or in other words, the total resistance of the cable is the parallel resistance of all the strands.
Parallel resistance is figured using the formula below:
In our theoretical cable, the resistance is: 0.1180262 ohms.
Let’s suppose we have a broken strand and the gap made by the broken strand is 0.005 inches.
Now, our cable is 6 parallel paths for 0.005 inches that is in series with 7 parallel paths that are 37.995 inches long.
The 0.005 gap is 1/7600th of the length of the cable.
The resistance of 6 strands for a length of 38 inches is 0.1376972 ohms. Divide this by 7600 and you get the resistance of 6 parallel strands over a length of 0.005 inches or 0.0000181 ohms. This little bit of resistance is in series with our 7 parallel paths that are 37.995 inches long.
This 37.995 inches is 7599/7600th of our length or 0.999868 of 38 inches. Our original resistance of 0.1180262 ohms for 38 inches times 7599/7600 gives us 0.1180106 ohms for 37.995 inches.
Series resistance simply add up so the total resistance of our cable with the broken strand is: 0.0000181 ohms + 0.1180106 ohms giving us a total of 0.1180287 ohms.
The difference in resistance for the two conditions (0.1180262 ohms for no broken strand vs. 0.1180287 for one broken strand) is 2.58 micro ohms or 0.00000258 ohms.
We think you will agree that this is too small to measure in any kind of real-world scenario, and very difficult to measure even in a laboratory environment!
