Maintaining the various components that comprise a Supervisory Controls and Data Acquisition (SCADA) system isn’t always easy.  And troubleshooting them can be even trickier. 

While your system manufacturer is a good source for answers, you’re sometimes the one who has to figure out what to do.  Below are some do-it-yourself tips for maintaining and troubleshooting your SCADA system’s power supply, battery backup system, analog loops, discreet digital inputs/outputs, and communications. 

Power Supplies
 
Power supplies are the heart of your panel.  Just as the heart pumps blood through the body, power supplies provide your panel with the correct voltage for operating equipment. 

When troubleshooting equipment, it’s important to identify the UL-approved panel’s incoming voltage.  In addition to appearing on the wiring diagrams, this information is usually noted on a sticker inside the panel door.  Most control panels are typically 480 Vac three-phase, 240 Vac three-phase, or 120/240 Vac single-phase.  The first two are typical incoming voltages if the motor starter circuits are included inside the enclosure or the voltage from the electrical power supplier isn’t brought into a lighting panel prior to going to the equipment enclosure.

Controller equipment typically operates on 120 Vac, 24 Vac, 24 Vdc, or 12 to 15 Vdc.  You absolutely must know which voltage your equipment requires, as connecting the wrong voltage to the power inputs could permanently damage equipment.  If the panel is to contain controller equipment, typically a transformer is used to convert 480 or 240 Vac three-phase to 120 Vac single-phase.  Transformers come in different sizes, depending on the panel’s incoming voltage and load.  The incoming power is typically wired to a main circuit breaker before being wired to the transformer.  This transformer will supply the panel’s control circuit and starter section with a 120 Vac single-phase circuit that will be wired to the control circuit breaker and starters. 

You can typically find controller input voltage information where the input voltage terminates on a UL-approved controller.  If you’re unable to find this information on the controller, consult the wiring diagrams.  Some controllers will run on 120 Vac, 24 Vac, 24 Vdc, or 12 to 15 Vdc while other available controllers can operate on any combination of these voltages. 

It’s important to have the correct sized power supply and fuse for the current load of all of the equipment on that circuit.  When the controller operates on 120 Vac, the control circuit breaker is typically wired directly to the controller.  But when the controller operates on 24 Vac, the control circuit breaker is usually wired to a transformer that converts the 120 Vac to 24 Vac before being wired to the controller. 

Similarly, when the controller operates on 24 or 12 to 15 Vdc, the control circuit breaker is wired to a DC voltage supply that converts the 120 Vac to the required DC voltage which is then wired to the controller.  Some DC power supplies are adjustable to their output voltage while others are fixed at a certain output voltage.  The incoming voltage, the desired output voltage and the amount of current needed to operate the equipment on the circuit all factor into determining the right size power supply and fuse.

Power supplies are typically attached to 120 Vac, which can be the incoming voltage, one leg of an incoming 240 Vac single-phase voltage system or the output side of a transformer on an incoming 240/480 Vac three-phase voltage system.  You can use an AC meter to verify 120 Vac voltage.  If you don’t detect 120 Vac, check the fuses, circuit breakers, and service entry.  Contact an electrician if you’re still unable to determine the reason for lack of 120 Vac. 

After you verify 120 Vac is on the input of the power supply, use a volt meter to determine if the correct controller voltage (typically 24 Vac, 24 Vdc, or 12 to 15 Vdc) is present on the output of the power supply.  If you have a 120 Vac on the input but no output voltage on the power supply, even after disconnecting the load (output circuit), the power supply is likely bad and needs to be replaced.  But before swapping it out, check that all fuses on the power supplies are still good.   

And if the correct control voltage is present on the power supply output, trace the control circuit through to the controller’s power input terminals and make sure that all fuses and surge protectors are in good working order. 

Battery Backup Systems

Using a battery backup system is a low-cost solution for continuous monitoring of your system’s status during a power failure.  Panels that typically use a form of battery backup provide levels and/or alarms such as AC power fail, phase fail, high/low levels and intrusion alarms.  

An uninterrupted power supply (UPS) is used to back up the circuit for controller circuits using 120 Vac.  In such instances, the 120 Vac is typically wired before the control circuit breaker while the UPS uses electronic circuitry and battery packs to maintain a constant 120 Vac output wired to the control circuit breaker.  A good solution to use for “dirty” or unstable power, the UPS will ride dips in the incoming power while supplying a reliable steady output.  Even if a power failure occurs, the UPS will continue to supply 120 Vac.  The amount of current load on the control circuit and the desired length of backup operation will determine which size UPS to use.

A battery circuit is usually used to back up the circuit for controllers that use 12 or 24 Vdc.  Some DC voltage controllers have battery terminals for wiring in a backup battery.  When there are no battery input terminals for wiring the battery directly to the controller, the DC power supply will wire to a battery charging circuit that is, in turn, wired to the controller.  The battery charging circuit will monitor the incoming voltage.  If there’s a power loss, the charging circuit will switch to battery voltage.  The DC circuits typically require less current.  One gel cell battery will supply from four to 12 hours of backup power, depending on the amount of DC current load on the control circuit.  As an UPS is single-phase, it’s typically sized to provide roughly 15 minutes of backup. 

Note that during a power failure, pumps won’t be able to run unless they’re backed up by a generator.  By combining a battery-backed control panel with the alarm notification, you’ll continue to be notified of any issues at the site should the control panel lose power. 

To test the battery circuit, simply remove the incoming power.  The controller should continue to operate despite the loss of AC power.  If there’s a battery backup circuit and the controller doesn’t continue to operate during a power loss, remove the battery leads and check the battery terminals for DC voltage.  A good single cell battery typically has 12 to 13.5 Vdc on it.  When using a controller that requires 24 Vdc, you can usually hook two batteries up in series for approximately 24 Vdc on the battery terminal leads. 

If there’s not enough voltage on the battery terminals, check the battery charging circuit for proper operation.  You can do this by leaving the battery leads disconnected and reapplying the incoming power.  Put the meter across the battery charging leads and look for the presence of a charging voltage.  Typical battery charging voltage is slightly greater than the battery type voltage (12 or 24 Vdc). 

If the battery charging leads have sufficient voltage to charge the battery but the battery voltage isn’t enough to operate the controller during an AC power loss, replace the battery.  And if the battery leads don’t have sufficient voltage to charge the battery, then replace the battery charging circuit as well as the battery. 

A gel cell battery typically lasts five to seven years, depending on usage.  However, changing them out every two years will help ensure the battery backup system remains reliable in times of AC power loss.

Analog Loops

Analog inputs are typically either a current 4-20mA or a voltage 1 to 5 Vdc signal.  When analog inputs are involved in the system, a DC power source needs to be used. 

Typical 4-20mA current loops are powered with 24 Vdc.  An external source typically supplies voltage input loops to the controller.  When troubleshooting analog input circuits, identify where the DC source is coming from.  For instance, it could be the controller or the sensor being monitored.  Or sometimes there will be a DC power supply if neither the controller nor the sensor can provide the source. 

Occasionally, the DC power supply for the controller circuit is also used to power the analog circuits.  To provide better isolation and prevent interference through ground loops, use a separate loop power supply and ground only one end of the loop. 

When the controller analog input is 1 to 5 Vdc and the sensor is a current 4-20mA, use a 250-ohm resistor across the controller’s analog inputs to convert the 4-20mA into 1 to 5 Vdc. 

Before troubleshooting, determine if the analog signal for the circuit is a 4-20 mA or a 1 to 5 Vdc.  Then figure out how the loop is being powered.  Find the source of the loop supply voltage and use a meter to determine if the supply voltage is present.  The loop supply voltage is typically 24 Vdc.  If you can’t find the loop voltage, disconnect the circuit and recheck for the supply voltage; sometimes a bad device on the circuit will draw down the loop supply voltage. 

If after isolating the loop power supply and determining that it’s not putting out the required loop voltage, swap out the device.  If you determine the source is putting out the required loop voltage, add the next device into the loop and recheck for the loop supply voltage.  Continue adding loop devices back into the circuit and rechecking for the supply voltage.  Replace any device that causes the loop supply voltage to disappear. 

If the loop supply voltage is present on the intact circuit (with all the loop devices attached) but there’s still no reading, use a mA current meter in the circuit to determine the loop mA signal.  If possible, change the signal (level, flow, etc.) that is being monitored and see if the mA signal also changes. 

If the mA signal doesn’t change and there’s sufficient loop voltage present on the circuit, then swap out the monitoring device (transducer, flow meter, etc.).  If, however, the mA signal does change yet there’s no reading on the controller, swap out the analog input module.

A signal generator is useful when troubleshooting analog loops.  A controller’s 4-20mA analog input can be verified by disconnecting the actual transmitter and inserting a signal generator in its place.  The signal generator will provide a reliable known good signal to simulate the loop signal to the controller.  If the controller still doesn’t display the correct known level, then swap out the analog output module.  If the controller now displays the correct known level, then the problem likely resides with the transmitter or analog loop.

Discreet Inputs and Outputs (I/O)

Digital I/Os can be 120 Vac or DC voltage inputs.  When the digital I/Os are a DC voltage input, they’re usually the same voltage as the controller’s power voltage. 

When troubleshooting low DC voltage discreet inputs, identify if they’re sinking or sourcing.  A sinking input occurs when a DC voltage is present on an inactive input that is then taken to ground when activated.  With a sourcing input, no DC voltage is present on an inactive input, resulting in it being sourced with a voltage when activated. 

Controller I/Os typically have indicator lights to show they’re active.  A volt meter can also indicate if they’re active.  Determine if the inputs are sourcing (where voltage is applied to the input when active) or sinking (when the voltage is taken to ground when active).   Then use the volt meter to test for a voltage signal as you alternate the status of the input signal.  If the volt meter indicates a change in voltage but the controller doesn’t display the change, then swap out the controller’s input module.  If the volt meter doesn’t reflect the change in voltage, the issue is likely with the input wiring or the device being monitored.

If you don’t have access to a meter, you can use a test wire to simulate the input signal contact closure by putting the wire between the input and the supply voltage (for sourcing circuits) or to ground (for sinking circuits).  If using the jumper doesn’t activate the input, replace the controller input.  However, if using the jumper activates the input, use the meter to test the input circuit wiring from the controller to the device that’s being monitored.  The volt meter should be set to ohms, to check for continuity or resistance.  If you twist one end of the wires together and put the meter leads across the other end, there should be minimal resistance as opposed to open line.  If the wires are good, the device being monitored may be bad. 

To troubleshoot low DC voltage discreet outputs, determine if the controller is supplying a voltage output for the circuit when active or if the controller is supplying a dry contact with the voltage coming from an alternate source.

Output circuits have either the load between the supply voltage source and the controller output (sourcing) or the load between the controller output and ground (sinking).  Use a meter to see if the voltage changes state when the controller’s output is activated.  If the voltage doesn’t change state when the output is activated, replace the controller input.  But if the voltage does change state and the controlled device doesn’t respond, use the volt meter to check for continuity or resistance of the wiring between the controller and the device being controlled.  If the wire is good and voltage is being supplied to the device, then the device being controlled might be bad and should be swapped out. 

Communications

When troubleshooting SCADA systems, just remember that all you’re doing is communicating.  Regardless of the equipment brand, you’re simply taking data from an input, and then monitoring, manipulating, and sometimes sending it to another place.  Even if your system is a stand-alone controller, it’s still communicating and manipulating local data.

The term “SCADA system” normally refers to a computer and several control panels that communicate yet are located at different sites throughout a system.  Communication is typically done via leased or dedicated lines, dial-up phone line, radio and/or network (via fiber optics, DSL, broadband) communications.  The SCADA system uses one or several of these modes to pass data back and forth.  When troubleshooting system communications, think of which control panels are communicating with one another.  (There should be a system overview that gives you this information.)  Also consider which mode of communication is being used.

A lease line refers to one or two pairs of wires that connect two or more locations.  All locations hear the same data and talk when it’s their turn.  The distance between some sites may require that the lease lines go through a central office operated by a phone company. 

A dial-up line refers to using a standard phone line supplied by a phone company.  The central telemetry unit (master) will call each station and collect that station’s data.  Some remote stations have the ability to dial the master up to report data.

Radio mode uses licensed and/or unlicensed radios to communicate.  When line-of-site exists, unlicensed low-watt radios can be used between locations.  A recent technology utilizes network radios to form wireless local area networks (LANs).

Network communication involves establishing LANs that allow for faster data communications.  Typically LANs allow all stations on a local network to share data instantly.  Common means of network communications can be fiber optics, Cat 5 cable, broadband and DSL. 

Several indicators show if the controller is communicating.  Two of the most common are transmit data (TD) and receive data (RD) lights that are located on the controllers, on the controller’s communication cards, on the radios or on the various types of different converter boxes.  Familiarize yourself with how these lights look during normal communications.  Note that the TD light located on a master telemetry unit (MTU) changes state as the controller attempts to transmit data.  The RD light should then change state as the remote terminal units respond and the controller receives data.

If the MTU’s TD light doesn’t change state and the controller is enabled and trying to communicate, that typically indicates there’s a problem with the controller’s communication board. 

Conclusion

Knowing how to maintain and troubleshoot your SCADA system is essential to protecting your investment.  Becoming familiar and comfortable with how your system’s power supplies, battery circuits, analog loops, discreet I/Os, and communications work will not only simplify your life but also save you valuable time and money. 

Part of effectively troubleshooting your SCADA system involves continually dividing your system into smaller sections until you’re able to identify what is and isn’t working.  This “divide and conquer” method makes complex problems easier to work on.  

Once you’ve reduced the problem down as far as you can and you’ve tested the circuits and confirmed various add-on devices aren’t the problem, consider swapping out the potentially troublesome device in the circuit.

Useful Tools (Sidebar)

• Keeping some of the following key items on hand should make maintaining and troubleshooting your SCADA system a little easier. 
• A spare set of drawings or wiring diagrams inside or next to the panel.
• A notebook for reminders, proper readings, observations, and a history of problems and resulting corrective steps.
• Spare replacement parts for critical components (needed to keep your system up and running).
• A meter capable of measuring AC and DC voltages, ohms and current (4-20 mA) circuits.
• A current signal generator.
• A list of people to contact for additional support or questions on your system (SCADA provider, electrician, manufacturer, etc.).

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