Many factors can lead to a malfunctioning 4-20mA loop. The problem can be caused by a poor selection of power, wiring or loop devices. In the event that the loop does not appear to work properly, it is recommended that you first check the power and wiring. 

Multiple loops must be installed if we want to monitor all three (3) channels available. Running so much wire could lead to problems with ground loops if independent loops are not properly isolated or there is a bad wiring connection. 

Figure 1. Device diagram in a 4-20mA current Loop

SYMPTOMS

Remember Rheonics sensors provide 3 non-isolated analogs outputs, below we are going to describe what could happen with bad wiring, unproperly earthing and bring out possible solutions for these issues by adding 4-20mA isolators. We have 3.5 mA that can be read as error signal.

- Unpredictable 4-20 mA signal fluctuations; are a sign that something is interfering with your current loop. This is likely a result of electrical noise or a ground loop.

- Variation in display signal; the signal may also experience an addition or subtraction by some value from one point in the loop to another. This is caused because bad scaling in the instrument.

- Shared commons causing signal averaging; Problems with shared, non-isolated commons will commonly average the processed signal causing the same value to be registered on devices that should be receiving different process variables. 

-Physical damage to the components; Overload, poorly wiring and dimensioning the devices in the loop. Power supply voltage must always be at least as high as the worst-case sum of all of the voltage drops in the loop.

GROUND LOOPS

Ground loops are our main issue to fix with the usage for non-isolated analog outputs. A ground loop is a flow of current from one signal ground to another because of a voltage differential between the two grounds. 

This can happen if two devices in the network are grounded at separate locations and one of the locations causes the signal ground there to experience a higher voltage potential, so it is recommended to be the only ground on the control side.

                                     

Figure 2. Ground Loops

These isolation requirements become exponentially harder to accomplish as the number of loops increases. When the conductive path between the different voltage's potential is isolated, a current cannot form between the two voltages point.

A ground loop forms when at least these conditions are present: 1. There are two grounds; 2. The grounds are at different potentials; 3. There is a galvanic path between the grounds. 
 

Isolation methods for Galvanic Isolation:

The main purpose of a signal isolator is to interrupt or break the galvanic path between circuits that are created by grounding of various potentials.   See figure 3.

Figure 3. Isolated system diagram.

Optical Isolation An optical isolation circuit has as operating principle two basic parts: a light source (usually a LED- Light Emitting Diode, working as the transmitter) and a photo-sensitive detector (usually a phototransistor, acting as the receiver). The output signal of the optocoupler is proportional to the light intensity of the source see Figure 4

 

Opto-isolator - Wikipedia

Figure 4. Galvanic Signal Isolation via an Opto-Coupler


Transformer Isolation- The electromagnetic isolation uses a transformer to electromagnetically couple the required signal across an air gap or non-conductive isolation gap. The electromagnetic field intensity level is proportional to the input signal applied to the transformer. Figure 5

Figure 5. Galvanic isolation via Transformer

DISADVANTAGE OF USING ISOLATION, the isolation adds a burden in the circuit so the original 720 Ω will drop to the technical specification of the isolator resistance or voltage.

Example for commercial isolator compatible with Rheonics Sensor:

Figure 6. Commercial Isolator for 2 wire Self-powered transmitter

POWER

The first step we should take is test the power supply, so we must measure the loop power supply and ensure that it has the proper power level(18-36VDC). 

  1. If the power supply is reading as zero, check if the power source is powered, a fuse is blown, or if the power supply is damaged.
  2. If we have a little drop in the power supply, check if the power supply is unregulated. This little variation is normal for an unregulated supply.  
  3. If the power supply is regulated, and the output is below the value expected, it may be caused by a high loop load. Disconnect the loop and measure the voltage output. If the voltage output remains with low level, the supply is broken.

 

WIRING

All wires in the system should be shielded twisted pair in which both wires are used. All sensor signals should be isolated if possible, using devices with isolated inputs and outputs. Lastly, always be aware of non-isolated multi-loop devices and take care while planning your wiring.

Figure 7. Twisted pair cable

The benefits of employing a twisted pair cable are those: Magnetic field cancellation, electric field pickup and radiation, keeps wire close together, balances common mode impedance.

A common test for Wiring troubleshooting:

  1. Check the installation of each cable based on our wiring diagram. 
  2. With the loop supply powered, measure the voltages across the devices within the loop. The voltages on the loop devices should agree with the specifications in technical sheet, and also the voltage polarity must agree with the + and – of the terminal block. If the voltages across all the loop devices are zero, then wire is broken.
  3. Verifying transmitter compliance margin, the wiring and receiving devices are essentially resistive elements so each voltage drop will be just the resistance of the device across its terminals or the wire´s resistance multiplied by the loop current so consider this at maximum when the loop current is 20mA.  The total of all the wiring and device voltage drops must sum to exactly the loop power supply voltage. For our Rheonics sensor, the maximum load impedance is (0 to 720 Ohms), so be careful with how many devices are installed in series so the voltage drop cannot affect our device to ensure proper operation first, calculate and sum all the voltage drops in the loop when the loop current is at 20mA. For the wiring, add up the wire lengths and use Table 1 below to find the total wire resistance.

 

Table 1. AWG Resistance copper wire

For receiving devices, the manufacturers should provide the input internal resistance (typically between 50 and 500 Ohms.). Finally sum the voltage drops of the wiring, the transmitting device, and the receiving device(s.) if the sum exceeds the power supply voltage, the transmitter will be unable to source the full 4-20mA range, working properly only up to some current value below 20mA. 

LOOP DEVICES

Troubleshooting a loop depends in each device that is connected in the loop and the wire. The most important troubleshooting step is to make sure that is it wired properly.

-A display that is improperly scaled will react to the 4-20mA signal, not displaying the proper values for the loop.

-Usage of non-insulated PLC inputs can also carry plenty of issues since each of those can create a path that is connected to earth ground.


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