7.1 Instrumentation, Electrical Fundamentals & SCADA

7.1 Instrumentation, Electrical Fundamentals & SCADA

Every North Carolina needs-to-know document lists Instrumentation and Applied Electricity as required subjects — the drinking water tables score instrumentation on the A and B Surface and Well exams, and the Biological Grade 1 course devotes an hour each to applied electricity and to instrumentation and controls.

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1. Anatomy of a control loop

  [ PRIMARY ELEMENT ] -> [ TRANSMITTER ] -> [ CONTROLLER ] -> [ FINAL CONTROL ELEMENT ]
   sensor in the water     converts to a      compares to      valve, VFD, chemical
   (mag meter, probe)      4-20 mA signal     setpoint         pump, blower
                                     ^                               |
                                     +-------- process feedback -----+

Why 4–20 mA. A live zero at 4 mA means a healthy loop always carries current. A broken wire, failed transmitter, or lost power reads 0 mA, which the controller distinguishes immediately from a legitimate zero-flow or zero-level reading at 4 mA. The linear span makes conversion simple: on a 0–10 MGD meter, 12 mA is midscale, or 5 MGD.

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2. Electrical fundamentals

Practical points that appear on exams and in the field: motors are protected by thermal overloads sized to nameplate full-load amps and service factor; single phasing on a three-phase service will burn a motor quickly and is guarded against with phase-loss relays; megger testing of motor windings to ground detects insulation failure before it becomes a burnout; and variable frequency drives save energy on centrifugal loads because power varies with roughly the cube of speed — but they require shaft grounding and cable practices that prevent bearing fluting.

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3. Primary elements by measurement

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4. Calibration discipline

1. Calibrate against a traceable standard on a defined schedule (formazin for turbidity, buffers for pH, known standards for chlorine).
2. Verify online analyzers against a grab sample analyzed by a bench method — daily or as the permit and rules require.
3. Document every calibration and verification in the log with date, standard used, as-found value, and as-left value. Under 40 CFR 122.41(j)(2) calibration and maintenance records are part of the records an NPDES permittee must retain for at least three years.
4. Investigate divergence. When the online reading and the grab sample disagree, the answer is not to adjust the online instrument until it matches — it is to find out which one is wrong.

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5. Control modes

• On/off (two position). A float starting a pump. Simple, but the process oscillates between the on and off points.
• Proportional. Output moves in proportion to the error, leaving a small permanent offset.
• Proportional-Integral (PI). Integral action removes the offset — the normal choice for level and flow.
• PID. Adds derivative action for processes with long lag; over-tuned derivative makes a loop twitchy and is rarely needed on water processes.
• Flow pacing versus residual (compound loop) control. Chemical feed paced to flow alone assumes constant demand; compound-loop control paces to flow and trims to a measured residual, which is standard for chlorine and dechlorination systems.

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6. What SCADA can and cannot do

SCADA collects, trends, alarms, and controls. It gives an operator the ability to see a filter's head loss curve, a wet well's cycle times, or a chlorine residual's drift. It does not satisfy the requirement to be there:

• Wastewater visitation minimums in 08G .0204(2) are physical visits.
• Drinking water oversight under 18C .1303 requires on-site presence, including the 20 percent rule at surface plants.
• Under 15A NCAC 02T .0305(h)(1)(E), a pump station not connected to telemetry must be inspected every day, while one connected to telemetry is inspected once per week — telemetry reduces the frequency but never eliminates the visit.

Cybersecurity is now an operational duty: unique credentials for each user, no shared or default passwords, remote access through a managed VPN with multifactor authentication, network segmentation between the business network and the control network, tested offline backups of PLC and HMI programs, and a written procedure for running the plant in manual when the control system is unavailable.