What is a typical method to measure loop current without significantly loading the loop?

What is a typical method to measure loop current without significantly loading the loop?

• A. Use a high-impedance meter or HART communicator; measure current through a low-value shunt resistor.
• B. Use a large shunt resistor to drop significant voltage.
• C. Use a current transformer around the loop.
• D. Break the loop and measure with a multimeter directly.

Answer: (A)

Measuring loop current without loading the loop uses a sensing method with very high input impedance and a small series shunt to convert current into a tiny voltage. In a 4–20 mA loop, you place a low-value resistor in series so the current creates a small voltage drop V = I × R. A high-impedance meter or a HART communicator can then read that voltage without drawing significant current, so the loop current stays essentially unchanged. For example, with a 10-ohm shunt at 20 mA, the drop is only 0.2 V, which the measurement device can capture without appreciably altering the loop. The other approaches would load or disturb the loop—using a large shunt drops more voltage and changes current, a current transformer isn’t practical for DC loops like this, and breaking the loop to measure with a multimeter would interrupt operation.

Measuring loop current without loading the loop uses a sensing method with very high input impedance and a small series shunt to convert current into a tiny voltage. In a 4–20 mA loop, you place a low-value resistor in series so the current creates a small voltage drop V = I × R. A high-impedance meter or a HART communicator can then read that voltage without drawing significant current, so the loop current stays essentially unchanged. For example, with a 10-ohm shunt at 20 mA, the drop is only 0.2 V, which the measurement device can capture without appreciably altering the loop. The other approaches would load or disturb the loop—using a large shunt drops more voltage and changes current, a current transformer isn’t practical for DC loops like this, and breaking the loop to measure with a multimeter would interrupt operation.