Shunts
A shunt is a known resistance used to measure current indirectly. The field meter reads millivolts across the shunt, and the current is calculated from the shunt rating or Ohm’s law.
Shunts are common in rectifiers, bond boxes, anode junction boxes, coupon stations, and current-monitoring circuits.
Basic Shunt Logic
If the shunt rating is 50 mV / 50 A, then a 25 mV reading represents 25 A. If the shunt rating is 100 mV / 5 A, then a 20 mV reading represents 1 A. The calculation is simple, but only if the shunt rating is known and the meter is connected across the correct terminals.
For a general calculation, use I = V / R, with volts and ohms in consistent units.
Common Shunt Mistakes
- Reading millivolts but reporting the number as amps.
- Using the wrong shunt factor.
- Measuring from a shunt terminal to ground instead of across the shunt.
- Ignoring polarity when determining current direction.
- Trusting a corroded or loose connection without verifying continuity.
Field Checks
Before interpreting shunt current, inspect the shunt, terminals, labels, conductor routing, and meter range. In junction boxes, verify whether the shunt represents one anode, one circuit, a structure lead, or the total system output.