Up Time Magazine - February/March 2009 - (Page 26) insulation board and insulators are all prime areas where problems are likely to occur. Typically, the effects of corona on rubber-based insulators, tape, and insulation board will leave a white powder residue or dust (Figure 4). This white residue is thought to be the combination of the physical breakdown of rubber based insulation and the accumulation of dust attracted to the conductive air surrounding the corona activity. Figure 5 - 13kV power feed cables showing corona powder and a small but intense area of tracking (shown at arrow). finishes and micro-crack stains on cable insulation will be evident. In worst case scenarios, cables will be severely deteriorated (Figure 5). Unusual weathering patterns on copper bus and conductors are also good indicators of corona (Figure 6). Humid conditions inside of switchgear cabinets will allow nitric acid to form which attacks the copper surface leaving unusual weathering patterns. Cabinets lacking heaters or poor weather seals, and cabinets that are poorly sheltered from wet industrial processes, are especially vulnerable to these conditions. Figure 6 - Unusual weathering pattern on 13kV copper bus under attack by corona produced nitric acid. As the corona condition advances, carbon tracks will start to develop on conductors and insulators. The distance between the affected phase and a grounded object will ultimately determine the time for a flash-over to occur. Finally, it should be stated that early stages of corona may not show any visible signs. Mild cases of corona that are caused by metal edges in switchgear cabinets may never be apparent by visual inspection alone. Figure 4 - White powder/dust residue formed on 13kV powder cables that are tie-wrapped together forming tight air spaces between each other; ideal locations for corona to form. Other indicators include discoloration and spider cracking on cable insulation. Usually dull WANT A LITTLE SOUND ADVICE? The only way to make money in a down market is through Introducing EFFICIENCY. A new CMMS/EAM software package that gives companies a complete and accurate maintenance picture. Since 1973, ue Systems has been helping thousands of companies get more efficient through ultrasound predictive maintenance. learn how to spend less and save more in 2009 through early detection of bearing wear. Watch this Free informational webinar on bearing inspection through ultrasound. Our software helps you stay competitive by identifying where time and money is being lost and where you should focus your efforts in order to maximize operations efficiency and minimize maintenance costs. This allows you to focus on delivering customer satisfaction. Go to: www.uesoundadvice.com/17n and watch right now. Contact us today at 312.863.6500 for more information or to schedule a demo. UESoundAdvice.com THE ULTRASOUND APPROACH The premiere resource for plant savings. energy experTiSe www.cmmsdatagroup.com 312.863.6500 info@CMMSdatagroup.com Training Technology SupporT 26 february/march 2009 http://www.uesoundadvice.com/17n http://www.UESoundAdvice.com http://www.UESoundAdvice.com http://www.cmmsdatagroup.com http://www.cmmsdatagroup.com
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