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.