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Electrical Troubleshooting Quiz

Dec 8, 2009 10:53 AM

You just came off a facility outage and are looking at the cable testing trends. In a large percentage of these, you can see the telltale slope change that indicates insulation integrity has entered the rapid decline phase.

You know this means the cables need to be scheduled for replacement, and you requested an outage for that purpose. Management responded by “asking” you to explain why these cables have suddenly become such a problem. A production manager even stopped by your office and remarked, “All those cables have been working just fine for a long time. Your people must have done something during that last outage to wreck them.”

How can you answer management's question, and how can you answer the sentiment expressed by this production manager?

The root cause is probably just age. You happen to see many “bad” cables at once, due timing.

Similar cables installed at the same time in the same environment likely will show similar changes in insulation resistance characteristics. If you test cables annually, you'll get X percentage that show the slope change trend breakover. With longer intervals, the percentage is higher and the “problem” looks worse.

To prevent misunderstandings, make sure your boss and the various department heads understand the basics of how cable test trending works. Although you essentially have an education problem here, you still need to address some technical issues.

First, review the test voltages used to ensure they are within the prescribed limits for the specific applications of the cables under test. Then, send a brief explanation to the various managers. You want them to understand that cable testing doesn't cause insulation deterioration, it merely identifies it. The exception, of course, is when the test voltage is too high.

Next, look at the cable environment for contributing causes. For example, solvents can migrate into raceway and, once there, act upon cable insulation from the outside. Power anomalies can act upon cable insulation from the inside (they also can result in excess heat, which attacks insulation from all directions).

 

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