Application Note www.megger.com Use the appropriate size test clips for the arrester under test and attach the black lead to the top of the arrestor and the red lead to the bottom. Finally, attach the blue lead to the middle of the arrester using a conductive tape or wire (see Figure 4). The three terminals play an important role in the measurement. The test voltage is applied between the black and red terminals, while the blue terminal is for measuring surface leakage to discount it from the final result. The blue terminal is the guard terminal, and it is vital for getting an accurate measurement of resistance. Figure 5: The flow of test current and effectiveness of guard connection Test procedures The test must be done with consistency, so use the same test voltage setting and apply the test voltage for the same amount of time each time. It is recommended that the test voltage is applied for at least 15 s, but optimally 60 s. Repeat the test 3 times, taking the average reading as the result. A repeatable result is indicative of a good measurement process. A good medium voltage (1 kV to 30 kV) arrester should have a very high resistance of gigaohm to terohm range at its operating voltage range and arresters of the same type and voltage rating should have very similar resistance values. Any arrester that shows a low resistance value is faulty and must be replaced or refurbished. If a test voltage is applied that is greater than 120 % of the arrester’s rated voltage, then it will result in a lower-than-expected resistance value even when the arrester is not faulty. Therefore, it is best to stay within the recommended test voltage range of 50 % to 120 % of the arrester’s rated voltage.
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