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1、nally been treated as a drying operation can become a rewind job. An insulation tester (commonly referred to as a “Megger,” although that is in fact a registered AVO trademark) overcomes these problems. Rigorous limiting of output cur- rent by a resistor network in the test circuit means that under
2、the lowered resistance as- sociated with deteriorated insulation, the test voltage will automatically drop to non-inju- rious levels. And an insulation test, instead of a simple pass/fail indication, provides an accurate reading of the resistance value. There can be no “near misses.” If the reading
3、he plethora of insulation test methods and procedures that appear in the lit- erature and product descriptions can seem bewildering to someone trying to establish a testing regime or meet an application. A thorough explanation of all of them, of course, is well beyond the scope of a single article,
4、but for a conve- nient organizing principle, one can refer to the Law of Diminishing Returns. That is to say, no single test can tell everything about a particular section of in- sulation or its associated apparatus, so that additional tests may be re- quired to isolate a problem or make a determina
5、tion. But the- ory must be balanced against practicality in order to avoid the wasted time and expense of mak- ing more and more tests for fewer and fewer added results. A proce- dure issued to customers by a lead- ing manufacturer of electric locomotives, GM Electro Motive Di- vision, illustrates t
6、his concept very well. Locomotives returned to shop for rou- tine scheduled maintenance were tradi- tionally tested by either/or 1 kV Megger insulation tests or high-potting, typically at 1080 VAC. While these tests caught most problems, the practice did not catch them all. There persisted annoying
7、and costly failures after some locomotives were returned to service. Why? High-potting is basically a crude test. Many rely on it because its dramatic re- sults (failure light and buzzer) tend to in- spire confi dence in the test, but therein lies the shortcoming. A high-pot makes a pass/fail determ
8、ination based on a trip set- ting for either breakdown or leakage cur- rent, or both. This may be set by the operator, or it may be factory-set by the manufacturer. It gives only an indication of the condition of the test item (in this case, a locomotive) at the time of the test. It has no predictiv
9、e value. If the insula- tion has a pinhole that arcs current to the frame, the high-pot indicates failure, and the problem is recognized. But if, say, moisture ingress has seriously lowered the resistance of the insulation to a value just above the trip setting, its a case of “a miss is as good as a
10、 mile to a blind horse.” The lo- comotive goes back into service, but may fail after the fi rst heavy rain. Furthermore, be- cause a high-pot will output higher currents and sustain an arc, it can further damage the insulation. A failure that could have origi- is above bare minimum but close to unac
11、- ceptable, further cleaning and drying may well restore it to a condition that will keep the equipment running satisfactorily well into the next scheduled maintenance period. Then why did EMD still experience fail- ures in the fi eld? Because an insulation tester is commonly thought of as a 1 kV pi
12、ece of test equipment. Many experienced and ca- pable operators have used insulation testers for years without going above a 1 kV test. This is, of course, generally adequate. But there are problems that a 1 kV test may not see. Pinholes and cuts in insulation posed a big problem for EMD. Depending
13、on their positioning with respect to ground, a given test voltage may or may not “pull” an arc. Remember, air is an effective insulator, so the more voltage that is applied, the greater the separation that can be arced. Carbon tracks and similar physical damage will also allow greater amounts of cur
14、rent at higher voltages. Such damaged spots may “pass” at 1 kV, but rapidly deteriorate to breakdown when put back into service. Does this mean that testing must be repeated at ever higher voltages? Fortunately, theres a practical limit, as EMD discovered through rigorous experimental testing. They
15、conducted an extensive and rigorous study of tests at increasing voltages from 1 kV up to 5, and were able to document that each increase in test voltage revealed addi- tional problems in the tested insulation. Fortunately, by the time they reached 5 kV, theyd achieved “close to 100% suc- cess” in r
16、ecognizing problems. As a re- sult of the study, EMD has recommended the implementation of a 5 kV testing regime to anyone servic- ing their products. The selection of test voltage is a critical fi rst step in applying an insulation test. For most common situations, the familiar lower voltages will
17、work just fine. But fortu- nately, as the EMD study has revealed, for bigger equipment and more demanding re- quirements, enormous test voltages are not the order of the day. A convenient compro- mise between the theoretical and the practical has been “proofed out” at 5 kV. 5kV Testing How Much Is Enough? BYJEFFJOWETT, SENIORAPPLICATIONENGINEER, AVO INTERNATIONAL TEST & MEASUREMENT Questions? For answers, give us a call: MeterCenter 1-800-230-6008 Reprinted with permission from Utility Products Showcase 2002. RadioFans.CN 收音机爱 好者资料库
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