Severity Guide lines for establishing repair priorities.

  
RECOMMENDED ELECTRICAL REPAIR PRIORITY TABLE MEASURED TEMP. RISE PHASE TO PHASE REPAIR PRIORITY ASTM E1934 - 07

7. Procedure

7.10 A method to determine a repair priority should be agreed upon by the thermographer and owner. Prioritization of findings should take into account any observed thermal differences and should also consider the impact on personnel safety, the criticality of the equipment, present and future thermal relationships, and productive operation of the equipment and/or the facility.

7.11 An example approach to prioritizing findings for repair is to ask the following questions. If any of them are answered in the affirmative, the finding is categorized as a Priority 1, requiring immediate action; this might include such actions as repair, schedule for repair, reduction of temperature by increased cooling or load reduction, or further testing to determine root cause, etc.

7.11.1 Is the phase-to-phase temperature difference greater than 24C (43F)1? 7.11.2 Is the absolute temperature greater than 100C (212F)1? 7.11.3 Is convection greater than 24kph (15mph)? 2 7.11.4 Is physical damage, such as melting or discoloration, evident? 7.11.5 Is component shiny flat or tubular bus? 7.11.6 Thermal gradients are large or moderate3 7.11.7 Are loads likely to increase by 3X or more prior to repairs? 7.11.8 Notes: 7.11.8.1 1For measurements that are reliable, i.e. high emissivity surfaces 7.11.8.2 2Convection measured at the component surface 7.11.8.3 3Massive, solid connector, indirect connections such as bus stabs, or any oil-filled or gas-filled equipment 7.12 If none of these questions are answered: YES, component should be scheduled for further investigation and/or more frequent thermographic inspection cycle.

THERMAL ITEM: ELECTRICAL-MAINTENANCE SCHEDULING CRITERIA:

 

Scheduling Temperature Rise Criteria over Reference

1

Critical 90 Deg. F. and Above / (50 Deg. C. and Above)

2

Serious 45 - 90 Deg. F. / (25 - 50 Deg. C.)

3

Important 20 - 45 Deg. F. / (11 - 25 Deg. C.)

4

Minor 20 Deg. F. / (1 - 11 Deg. C.)

This criteria has been used to determine the suggested repair priority of the Thermal / Electrical data. The repair priorities are based on the measured temperature rise on the component in question against a similar component with the same load. The final decision as to the criticality of the repair priority for each problem and the scheduling of maintenance and repair rests solely with the client. The above table is only a guide to help you determine the order to schedule your electrical repairs. You must keep in mind what the equipment operates and the loads that the equipment will be subjected to when determining the priorities and scheduling of your repairs.  With each documented thermal item (where applicable) information is provided on the tested load and also on the measured temperature rise at the time of the inspection. This information is offered so that you may better evaluate your electric repair priorities based on their temperatures and anticipated loads. There are no rules for the assessment of excess temperatures that are measured on indirectly overheated surfaces. Indirect overheating can be caused by hidden faults, e.g. cracks inside a breaker where the temperature is measured from the outside. For every problem always inspect for physical damage to determine repair or replacement of the particular component identified. An infrared inspection should also be made after a problem has been fixed, to insure that it has been corrected properly. It is important to understand that a piece of equipment may fail at any stage, not just at the critical level. In the particular case of electrical inspections, a temperature rise is measured from a normal phase with a similar load to the observed hot phase to accurately determine the severity of the potential problem. Ambient air temperature is not used as a base line for a reference, because this does not accurately represent the correct measurement of the temperature rise of the component. Ideally, thermographic electrical inspections should be carried out under normal or full load.

In order to allow you to assess projected/normalized temperature rises, a modified Joules Law is used for recalculation to 50% and 100 % load conditions. In addition this allows you to project a worst-case scenario if the measured temperature rise load is less than 50% and likely to increase. As you double the load on a problem you must consider that the measured temperature rise will be quadrupled. The magnitude of the problem may be seriously understated if the piece of equipment is minimally loaded and the load is going to increases later on. This provides the best way to correct the severity classifications of the potential problems. It should be noted however, that the measured and projected/normalized temperature rises and severity classification must be viewed along with type of equipment and the process that the equipment operates under to truly assess the severity of the problem.

 

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