Crane Safety Revisited

Just when you think you have all the bases covered, the “impossible” happens. During a lift on one of our recent outages, the idler pulley and mounting bracket for the drive on the overhead crane fell approximately 25 feet. Fortunately no one was injured. We have discussed the need for pre-outage crane inspections in previous Safety Tips, identifying the need for an OSHA-compliant inspection before the outage. The customer had conscientiously performed the inspection, and we had examined the report. In a post-incident report, the company’s preferred crane inspection vendor discovered the root cause was an alignment issue with the driven sprocket for the trolley. The company implemented two corrective actions: the crane vendor installed a safety cable to prevent the bracket from falling in the event of a failure, and an alignment protocol was added to the inspection checklist. Discussions are continuing on additional preventive measures.

For TGM’s part, we recognize that we have a responsibility to not only request the inspection report, but to read and analyze it. We have discovered that this crane vendor includes an Inspection Report Key along with the report which gives a Priority Code and a Condition code for each finding. These codes help identify the importance of the finding and the need for corrective action. These codes are specific to this particular vendor. Others may use a different key or not have one.  We will now be asking for an Inspection Report Key in addition to the report so we can double check that all deficiencies have been corrected. We will also be asking for the report with enough lead time such that any corrections can be made before the outage.

AC & DC High Potential Testing Fact & Fiction

Which is better: AC or DC Testing? Will these tests hurt my generator? Here are the facts:

  1. DC high potential test equipment is relatively small and easy to transport. AC equipment is comparatively difficult to transport (large and heavy).
  2. DC testing stresses the dielectric in a different manner than AC. DC tends to stress the end windings more. DC tends to be more sensitive to low resistance contaminates on the surface of the windings.
  3. AC high potential testing stresses the stator insulation system much as it would during normal operation. AC tends to stress the straight section of the stator windings more. AC is more apt to flush out high resistance insulation weaknesses that DC might not.
  4. DC can be used in a controlled over-voltage testing methodology where the test can be terminated prior to winding failure. AC is basically a go/no-go proof test.
  5. AC and/or DC high potential tests will not fail an otherwise sound insulation system. Sufficient research has been performed within the industry to conclude that windings which fail hipot already had either systemic and/or specific issues.
  6. AC and/or DC high potential testing does age the stator winding insulation. However, a one minute AC or DC high potential test equates to only a 0.0004% reduction in service life per test.*

In conclusion, AC high potential testing is better suited for acceptance proof testing of new equipment (i.e. coils, bars, and stator windings). DC testing is better suited for in-process proof testing, maintenance proof testing, and controlled over-voltage testing of new and/or used equipment. AC or DC high potential testing should not be used as the sole source of diagnostic or acceptance data. An experienced and qualified generator testing specialist will recommend a testing protocol specifically suited to your machine’s life cycle and operating history.

* A one minute AC or DC high potential test equates to approximately 11 hours of useful life expended. Assuming that a stator winding has a nominal 30-year life expectancy (263K hours), this would equate to only a 0.0004% reduction in service life per test.