4.4 Shunts: Types, Ratings & Current Calculations

Shunts: Types, Ratings & Current Calculations

Measuring direct current (DC) by breaking a circuit and inserting an ammeter in series is often dangerous, disruptive, and inaccurate due to the meter's internal resistance. To safely and accurately measure current in Cathodic Protection circuits, we use shunts. Understanding how to read and calculate current from a shunt is a mandatory skill for any CP1 Tester.

What is a Shunt?

A shunt is a precision resistor with a known, very low resistance value. It is permanently installed in series with the circuit you wish to measure. Because its resistance is precisely known, you can determine the current flowing through it by simply measuring the voltage drop (in millivolts) across the shunt using a high-impedance digital multimeter (DMM) and applying Ohm's Law.

The core equation for using a shunt is always Ohm's Law: I = E / R

• I = The unknown current (Amps)
• E = The measured voltage drop across the shunt (in Volts, requiring conversion from mV)
• R = The known resistance of the shunt (Ohms)

Common Types of Shunts in CP

There are several distinct types of shunts used in the CP industry, each with specific ratings and typical applications.

1. The Holloway Shunt (RS Shunt)

Holloway shunts (often called RS shunts) are widely used in junction boxes and test stations. They are usually small, wire-wound, or stamped metal plates.

• Standard Rating: The most common Holloway shunt has a resistance of 0.01 Ohms (Ω).
• Calculation: Because the resistance is 0.01 Ω, the math is incredibly straightforward. 1 millivolt (0.001 V) of drop equals 0.1 Amps of current. (I = 0.001 V / 0.01 Ω = 0.1 A).
• Shortcut: A rapid mental shortcut for a 0.01 Ω shunt is to take the measured millivolt reading and divide by 10 (or move the decimal point one place to the left).
  • Example: You measure 45 mV. Current = 45 / 10 = 4.5 Amps.

2. The J.B. Shunt (SS Shunt)

J.B. shunts are larger and designed for higher current applications, often found in rectifier cabinets or large bonding boxes. They are typically made of thick manganin metal strips.

• Standard Rating: The most common J.B. shunt has a resistance of 0.001 Ohms (Ω).
• Calculation: 1 millivolt (0.001 V) of drop across this shunt equals exactly 1 Amp of current. (I = 0.001 V / 0.001 Ω = 1.0 A).
• Shortcut: For a 0.001 Ω shunt, the measured millivolt reading is exactly equal to the current in Amps.
  • Example: You measure 12 mV. Current = 12 Amps.

3. Panel/Rectifier Shunts (Amp/mV Rating)

Large shunts installed inside rectifier cabinets to drive the panel meters are rarely labeled with their resistance in Ohms. Instead, they are stamped with a rating ratio, such as "50A / 50mV" or "100A / 100mV".

To find the current flowing through these shunts, you must first calculate the "Shunt Factor" (Amps per millivolt), and then multiply that factor by your measured millivolt drop.

Step 1: Determine the Shunt Factor (Multiplier) Shunt Factor = Rated Amps / Rated Millivolts

• Example for a 50A/50mV shunt: 50A / 50mV = 1 Amp per mV (Multiplier = 1)
• Example for a 100A/50mV shunt: 100A / 50mV = 2 Amps per mV (Multiplier = 2)

Step 2: Calculate the Current Current = Measured mV × Shunt Factor

• If you have a 100A/50mV shunt (Factor = 2), and you measure a 15 mV drop:
• Current = 15 mV × 2 = 30 Amps.

Alternatively, you can calculate the Ohmic resistance of the shunt and use Ohm's law directly. For a 50A/50mV shunt: R = E / I = 0.050V / 50A = 0.001 Ω.

Proper Measurement Techniques

When taking readings across a shunt, technique is critical to avoid errors:

1. Set the DMM to Millivolts DC (mV DC). Shunt voltage drops are very small. Setting the meter to standard Volts may not provide enough resolution.
2. Measure across the dedicated measuring screws. Many shunts have large bolts for the main power cables and smaller screws specifically designed for meter leads. Always measure across the small screws to avoid incorporating contact resistance from the large cable lugs into your reading.
3. Note the polarity. By noting which side is positive and which is negative, you can determine the direction of current flow. Current always flows through the shunt from positive to negative. This is essential when checking interference bonds to ensure current is draining in the intended direction.
4. Never measure Amps. Never switch your DMM to the 'Amps' setting and probe across the shunt. This creates a parallel path with near-zero resistance through your meter, which can blow the meter's internal fuse and will give you a completely erroneous reading.

By mastering shunt identification and the corresponding Ohm's Law calculations, the CP1 tester can safely and accurately record current outputs across complex networks without ever breaking the circuit.