Test Instruments & Measuring DevicesTEST EQUIPMENTFigure 17 Simple Megger Circuit DiagramCoil A is wound in a manner to produce a clockwise torque on the moving element. With theterminals marked "line" and "earth" shorted, giving a zero resistance, the current flow throughthe Coil A is sufficient to produce enough torque to overcome the torque of Coil B. The pointerthen moves to the extreme clockwise position, which is marked as zero resistance. Resistance(R1) will protect Coil A from excessive current flow in this condition.When an unknown resistance is connected across the test terminals, line and earth, the opposingtorques of Coils A and B balance each other so that the instrument pointer comes to rest at somepoint on the scale. The scale is calibrated such that the pointer directly indicates the value ofresistance being measured.Rev. 0 Page 29 ES-14
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