How to Safely Check the mA Current of an Instrument Loop Using a Multimeter

When measuring the mA current in an instrument loop, it’s crucial to follow proper safety and measurement protocols to maintain process integrity and ensure personnel safety. 

The current can be measured at four common locations: at the transmitter in the field, the field analog junction box, the marshalling panel knife edges, and the analog input card of the control system (PLC or DCS). 

This guide provides a step-by-step approach for safely conducting these measurements.

Instrument Loop Wiring Diagram – Analog Input

This below image shows an instrument loop wiring diagram for an analog input signal in an industrial control system. 

We will explain the key components and signal flow:

Field Transmitter:

This is the source of the analog signal, typically a sensor or measuring device located in the field (e.g., a temperature or pressure transmitter). It has positive (+) and negative (-) terminals.

Analog Junction Box:

Located in the field, this connects the field device to the main cable run. It maintains signal continuity and provides a convenient connection point.

Marshalling Cabinet:

This is the interface between field wiring and the control system. It organizes and terminates field cables before connecting to the PLC input modules.

PLC Cabinet:

Contains the Programmable Logic Controller (PLC) and its input/output modules. The analog input (AI) module receives the signal.

Wiring Path:

The red line represents the positive signal wire, while the black line is the negative or return wire. The signal passes through each component, maintaining polarity.

Terminal Numbering:

Consistent terminal numbering (1 for positive, 2 for negative) is used throughout to ensure proper connections.

PLC Backplane:

The analog input module connects to the PLC’s backplane, allowing the controller to read the analog value.

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Separation:

A dashed blue line divides the diagram into “Field Side” and “Cabinet Room Side”, showing the physical separation between outdoor and indoor equipment.

This diagram illustrates how an analog signal from a field instrument is wired through various connection points before reaching the PLC for processing. It’s a typical setup in industrial automation for monitoring analog process variables.

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How to Check the mA Current of an Instrument in a Loop with a Multimeter

Safety Protocols

Safety is paramount when working with instrument loops. This section highlights essential safety practices to ensure a secure working environment.

Lockout/Tagout (LOTO)

• To prevent accidental startup or release of energy follow the Lockout/Tagout procedure
• Determine all energy sources related to the instrument loop, including electrical, hydraulic, and pneumatic systems.
• Use switches, valves, or other devices to isolate the energy sources.
• Attach locks and tags to the isolation devices to prevent reactivation.
• Attempt to energize the circuit to confirm it is de-energized.

Follow the Work Permit System

• Ensures that hazardous work is authorized and that all necessary precautions are taken.
• Obtain necessary Work Permits  from the site safety officer or operation department.
• Carefully review the permit for specific safety requirements.
• Adhere strictly to the permit conditions throughout the task.

Safety Precautionary Measurement

To avoid disruptions in process operations and ensure safe measurement, take the following precautions:

Bypass the Loop

Maintain process continuity during working on the loop.

• Determine if interrupting the loop will affect process operations.
• Coordinate with Control Room operators to switch to manual operation or enable a bypass.
• BY using the  Bypass Tools like jumpers or bypass switches to keep the loop active

Interlock Inhibition

• Purpose of this to prevent unintended process shutdowns during testing.
• Determine which interlocks are associated with the loop.
• Follow site procedures to temporarily disable interlocks, ensuring supervisor approval.
• Log the inhibition and ensure interlocks are reactivated after testing.

Inhibition of Alarm Related to Loop

• Purpose of this to avoid false alarms during loop interruption.
• Identify alarms that may be triggered by loop testing.
• Temporarily inhibit alarms as per site protocols.
• Restore alarm functionality after completing the measurements.

Required Tools and Equipment for Checking the Loop Signal

Ensure you have the right tools for the task to perform measurements safely and accurately:

• Multimeter: Ensure the mulit is calibrated and set to DC mA measurement mode.
• Insulated Tools: Use screwdrivers and test leads with proper insulation to prevent electric shock.
• Bypass Jumpers: For maintaining loop continuity if bypass is required.
• LOTO Kit: Contains locks, tags, and isolation devices for safety.
• Personal Protective Equipment (PPE): Includes gloves, safety glasses, and flame-resistant clothing for protection.

Methods for Measuring mA Current at Different Locations

This section details the step-by-step process for measuring mA current at various points in the loop.

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Method :1 Measuring at the Transmitter in the Field

1. Break the Loop: Locate the transmitter in the field. Identify the loop wires connected to the transmitter and carefully disconnect one of the wires. This action breaks the circuit, allowing for measurement.
2. Connect the Multimeter: Take a digital multimeter and set it to measure current (mA). Connect the multimeter in series by attaching one lead to the disconnected wire and the other lead to the transmitter terminal where the wire was previously connected.
3. Measure and Record: Power up the loop so that the transmitter is operational. Observe the current reading on the multimeter. Compare this reading with the expected value based on the transmitter’s output (e.g., 4-20 mA corresponds to a certain range of measurement).
4. Restore the Loop: After recording the measurement, carefully remove the multimeter and reconnect the wire to its original terminal. Ensure the connection is secure, and the transmitter is functioning normally within the loop.

This procedure helps verify the correct operation of the transmitter and the accuracy of the output signal in a live field environment.

Method :2 Measuring at the Field Analog Junction Box

1. Break the Loop: Locate the junction box in the field where the loop wires are connected. Carefully disconnect one of the loop wires to break the circuit, ensuring the power is off or the loop is in a safe state before doing so.
2. Connect the Multimeter: Set your multimeter to measure current (mA) and connect it in series with the loop by attaching one lead to the disconnected wire and the other to the terminal where the wire was removed.
3. Measure and Record: Power up the loop and observe the current reading on the multimeter. This reading should fall within the expected range for the loop, typically 4-20 mA for many industrial transmitters.
4. Restore the Loop: After recording the measurement, remove the multimeter and reconnect the wire to its original position in the junction box. Double-check that the connection is secure and that the loop is functioning correctly, with the transmitter providing accurate readings.

This method allows for the verification of current signals without directly interacting with the transmitter, making it useful in various field conditions.

Method :3 Measuring at the Marshalling Panel Knife Edges

1. Break the Loop: Identify the appropriate knife-edge terminal in the marshalling panel. Open the knife-edge terminal to break the circuit, ensuring the loop is de-energized or in a safe state before proceeding.
2. Connect the Multimeter: Set the multimeter to measure current (mA). Attach the multimeter leads across the knife edge in series with the loop, effectively completing the circuit through the multimeter.

3. Measure and Record: Energize the loop and observe the current reading on the multimeter. Record the reading and compare it with the expected signal range (e.g., 4-20 mA).
4. Restore the Loop: After taking the measurement, remove the multimeter and close the knife-edge terminal to restore the loop to its original condition. Ensure the loop is functioning correctly and that the transmitter is providing accurate readings.

This method is very beneficial when working in a centralised control environment where signals from numerous field equipment are combined at the marshalling panel.

Method :4 Measuring at the Analog Input Card of the Control System (PLC or DCS)

1. Break the Loop: Locate the analog input card in the control system where the loop wire is connected. Carefully disconnect the loop wire from the input terminal to break the circuit.
2. Connect the Multimeter: Set the multimeter to measure current (mA) and connect it in series with the loop by attaching the leads to the disconnected wire and the input terminal.
3. Measure and Record: Power the loop and observe the mA reading on the multimeter. Ensure the reading matches the expected range for the signal, typically 4-20 mA.
4. Restore the Loop: After recording the measurement, remove the multimeter and reconnect the wire to the input terminal. Verify that the control system (PLC or DCS) correctly recognizes the signal and that the loop is functioning normally.

This method ensures that the signal reaching the control system is accurate, confirming that the analog input card receives the correct current for process monitoring and control.

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Re-enable Interlocks, Alarms, and Remove Bypass

• Restore any interlocks that were inhibited during the measurement process to ensure the process is safeguarded against any operational issues.
• Reactivate any alarms that were temporarily inhibited to avoid missing any critical alerts.
• Remove any bypasses or jumpers used to maintain loop continuity, ensuring the loop operates as intended.

Verify Loop Operation:

• Perform a thorough check to confirm that the instrument loop is fully operational. 
• This includes verifying that the transmitter is sending accurate signals, the control system (PLC or DCS) is receiving correct readings, and there are no disruptions in the loop’s functionality.

Document the Measurement:

Record essential details of the measurement process, including:

• The specific location where the measurement was taken (e.g., transmitter, junction box, marshalling panel, or analog input card).
• The exact mA reading obtained during the measurement.
• Any actions taken, such as enabling bypasses, inhibiting interlocks, or adjusting settings.
• Any issues encountered and the steps taken to resolve them.

Normalizing Interlock Inhibition, Alarms, and De-isolation:

• Ensure that all interlocks that were inhibited are normalized back to their operational state.
• Verify that alarms are re-enabled and functioning correctly.
• Complete the de-isolation process, ensuring that all systems are reconnected and functioning as they should.

Closing Work Permit:

• Review the work permit to ensure all conditions have been met, and all safety measures have been restored.
• Sign off on the work permit, indicating that the task is complete and the system is back to normal operation.
• Inform the site safety officer or operations department that the work has been completed and the system is safe for regular use.

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