Price Guard Wire Method
Some form of guard circuits is generally used to eliminate the errors caused by leakage currents over insulation. Fig. 3.14 illustrates the operation of the guard circuit. In fig 3.14(a), a high resistance mounted on a piece of insulating material is measured by the ammeter voltmeter method. The micro-ammeter measures the sum of the current through the resistor (IR) and the current through the leakage path around the resistor. The measured value of resistance computed from the readings indicated on the voltmeter and the microammeter, will not be true value but will be in error.
Figure 3.14 Application of guard circuit for measurement of high resistance In fig, 3.14 (b), the guard terminal has been added to resistance terminal block. The guard terminal surrounds the resistance terminal entirely and is connected to the battery side of the micro-ammeter. The leakage current IL now bypasses the micro-ammeter which then indicates the current IR through the resistor and thus allows the correct determination of the resistance value from the reading of voltmeter and micro-ammeter. The guard terminal and resistance terminal are almost at the same potential and thus there will be no flow of current between them.