How to do troubleshooting of a 4-20mA loop?

Components in the 4-20mA

• The 4- 20mA loop consists of three main components: power supply, transmitter, and passive loop devices.
• The power supply generates the energy needed to operate the loop.
• The transmitter regulates the current flowing through the loop.
• Passive loop devices provide feedback to the system, such as indicators displaying measured quantities or relay output control devices.
• Loop devices can be either passive (loop powered) or active (with their own power supply) signal transformation devices, like isolators converting 4 – 20mA to other electrical signals, such as 0 – 10V.

Required tools for the troubleshooting of 4-20mA loop

• To troubleshoot a 4 – 20mA loop electrically, a multimeter is required.
• The multimeter should be capable of reading Volts and milliAmps DC.
• Proper knowledge and usage of the multimeter are essential to avoid damaging the loop, multimeter or causing harm to oneself.
• Voltage measurement involves attaching the multimeter’s leads across the device, effectively connecting the multimeter in parallel with the device.
• Current measurement requires disconnecting the loop wire and attaching the multimeter’s in series with the circuit, causing all the current to flow through the multimeter.

Troubleshooting a 4-20mA Loop

• Malfunctioning 4/20mA loop can have various causes: power, wiring, or loop device issues.
• To troubleshoot, first identify the area of the problem: power, wiring, or loop device.
• If the entire loop is not functioning, start by checking the power supply and wiring.
• If the loop appears partially functional, focus on understanding the specific nature of the malfunction.
• For incorrect readings on the display, refer to the display manual or setup documentation.
• Otherwise, begin the investigation by examining the loop device section for potential issues.

1. Power 

• The first step in troubleshooting is to check the power supplies of the 4/20mA loop.
• Measure the loop power supply voltage and ensure it is at the correct level.
• If the supply output voltage is zero:
  1. Check if the supply is receiving power.
  2. Verify if a fuse is blown or if the supply itself is damaged.
• If the supply voltage is slightly low (unregulated supply):
  1. Variation of output voltage with load is normal for an unregulated supply.
• If the supply is regulated, and the output voltage is low:
  1. Disconnect the loop and measure the voltage output separately.
  2. If the voltage remains low, the supply might be faulty.
  3. If the voltage returns to the specification, a high loop load could be the cause.
• To measure the loop current, connect a milliamp meter between the supply and the disconnected wire.
• A loop current significantly larger than 26mA indicates an excess load on the loop.
• Possible causes of excessive loop current include miswiring, ground loop, or issues with the transmitter.
• Verify the proper polarity installation of the transmitter.
• If the current is less than 22mA and the supply output voltage is still low, the power supply might be faulty.

2. Wiring 

• Check the wiring of the 4 – 20mA loop.
• Ensure that the power supply’s + terminal is connected to the + terminal of the first item in the loop.
• Connect the – terminal of the first item to the + terminal of the second item, and so on until the wiring returns to the – terminal of the power supply.
• Measure the voltages across each loop device while the loop supply is powered.
• The voltages on the loop devices should match the specifications for those devices, and the voltage polarity should agree with the + and – terminals on the terminal block.
• There can be a break in the loop if the voltages across all loop devices are zero and the loop supply is operating within specifications.
• If most or all of the power goes across one of the loop devices, there may be something wrong with that device.

3. Loop Devices

• Troubleshooting a loop depends on the devices used in the loop.
• The most crucial step is to ensure that the wiring is correct, as many issues caused from improper wiring.

4. Problem loop power devices

• Problems with loop-powered indicators are often caused by improper scaling, incorrect wiring, or electronic failures.
• An improperly scaled display will react to the 4-20mA signal but won’t display the correct values for the loop. To resolve this, scale the display to match the range of the 4-20mA transmitter.
• Improperly wired displays may not turn on. Check the voltage across the terminals using a multimeter and compare it to the markings to verify correct polarity.
• Electronic failures in indicators usually result in the display not turning on, making proper wiring identification crucial.
• Obvious faults like missing digits, segments, or constant indications are easier to diagnose.

5. Problem AC power devices

• AC powered indicators may face issues due to improper scaling, incorrect wiring, or electronic failure.
• Improperly scaled displays react to the 4 – 20mA signal but do not show the correct loop values. Scaling correction involves identifying the range of the 4 – 20mA transmitter and adjusting the display accordingly.
• Another fault state is brought about by the indicator’s power. The loop current is not flowing through the indicator if the display shows a value that is around -25% of the span below the 4mA scaling point.
• Electrical problems within the indicator, such as a blown power supply, soft power supply, or malfunctioning electronics, are not part of troubleshooting a 4-20mA loop, unless the loop is powered by the indicator’s supply. In such cases, repairing the indicator can resolve these issues.

6. Problem with transmitter

• Transmitter problems are often caused by wiring issues between the loop and the sensor.
• Improper wiring can lead to damage if power is attached to the wrong terminals.
• To troubleshoot, ensure proper wiring between the loop and the sensor.
• Measure the voltage across the transmitter to check polarity and ensure sufficient voltage for operation.
• Compare the measured voltage to the minimum specified in the transmitter’s specifications.
• Properly attach the sensors according to the wiring diagram provided for the transmitter.

7. Problems with signal isolation barrier

• Isolator problems are often due to wiring issues.
• Confusion may arise regarding where to apply power and how to wire the isolator into the circuit.
• Isolators come in various configurations:
  1. Some require power on both the input and output sides of the isolation barrier.
  2. Some transfer energy across the barrier and need power on only one side.
  3. Power can be supplied from AC powered equipment on either side of the isolation barrier..
• Properly connecting both sides of the isolation barrier is essential for some isolators to operate correctly.
• Power can be supplied from AC powered equipment on either side of the isolation barrier.
• An issue on the opposite side of the barrier could be the root of a malfunction on one loop.

8. Problems with PLC

• Problems with PLCs in a 4/20mA loop can vary in nature.
• Incorporating a PLC into a loop requires checking several aspects.
• PLCs often have different terminals for attaching the 4/20mA loop, depending on power sources (internal or external).
• Non-isolated PLC inputs can connect the 4/20mA loop to earth ground, leading to issues when other loop points are also connected to earth ground, creating alternative pathways for loop current flow.
• Certain sensors, like pH and thermocouples, may be inherently electrically connected to the measured values.
• Most loop designs ground the loop at the power supply to reduce noise.
• A thorough analysis of the loop design is essential to avoid potential problems when setting up the system.