Question

Can resistor be measured in live circuit and what will happen? Will the reading be accurate?

Answer 1

No the resistor cannot be measured in the live circuit because of the following reasons :

1. Resistance is a static value.  

2. Ohmmeters sends a constant known current into a circuit and measuring the voltage that is dropped across the probes. If you do not disconnect the component being measured, that current is split into all available circuit paths, so less current will flow through the resistor. As a result, less voltage will drop across the resistor and the ohmmeter will measure it as a lower resistance, based on ohms law.

3. Another resistor or components in the circuit can be in parallel with the resistor whose value has to be calculated.

4. ohmmeter has a low impedance that adds loading effect which disturbs the actual measurement.

5. It is not recommended to remove resistor in the live circuit, which affects the entire circuit operation by sending heavy currents in the parallel branches.

Reason for inaccuracy of the measurement :

The most common and simplest way to measure resistance is with a digital multimeter. When you connect the terminals of the resistor to the two probes, it supplies a known current, measures the resulting voltage drop, and calculates the resistance. The trouble is, this approach only works if you can take your resistor out of the circuit; the DMM’s reading cannot be trusted if the resistor’s terminals are connected to other components. So knowing the value of a resistor that cannot be isolated easily from other components in the live circuits gives inaccurate results.