Fluke 1550B FE 1550B Users Manual - Page 14
Using the Guard Terminal, Surface Leakage Current - cables
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1550B Users Manual Using the Guard Terminal Note Insulation resistance is measured between the + and − output connections. The Guard terminal (G) is at the same potential as the negative (−) terminal but is not in the measurement path. For most tests, only two test leads are used, connecting the positive (+) and negative (−) terminals on the Meter to the circuit under test. The Guard (G) terminal is left unconnected. When measuring very high resistances, you can obtain more accurate readings by making a three-wire measurement using the Guard terminal. The Guard terminal is at the same potential as the negative (−) terminal, and can be used to prevent surface leakage or other unwanted leakage currents from degrading the accuracy of the insulation resistance measurement. Figure 5 shows how to measure the resistance from one of the conductors to the outer shield. In this case, there is a leakage current along the surface of the inner insulation near the end of the cable. This leakage adds to the current that the negative terminal detects, and will cause the Meter to read a lower resistance than it should. Shield ( ) ( ) Surface Leakage Current Figure 5. Surface Leakage Current ASW13F.EPS 8