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newSchoolWorks.com › VoTechWorks › Manufacturing & Marine › Industrial Maintenance › Programmable Logic Controllers for Industrial Maintenance › Lesson 12
MFG-INDU16-06 · Lesson 12 · 11.5 clock hours · field

Preventive and Predictive Maintenance of PLC Systems

Objective. Students perform a documented PM inspection of a PLC control panel including firmware version check, battery test, terminal-torque check, enclosure cleaning, and environment measurement, then write a maintenance work order.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed engineer or certified journey-level tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why PLC Preventive and Predictive Maintenance Matters

Programmable Logic Controllers are the nerve centers of modern manufacturing and marine automation. A PLC that fails unexpectedly can stop an entire production line or disable critical shipboard systems. Preventive maintenance (PM) means performing scheduled tasks before failure occurs. Predictive maintenance means using measured data — battery voltage trends, scan time drift, temperature readings — to spot trouble early. Together they reduce unplanned downtime, extend equipment life, and protect the workers who depend on that equipment.

As an industrial maintenance technician you will perform PLC PM inspections on a regular cycle, document every finding, and write maintenance work orders that become part of the equipment's permanent history. This lesson walks through each step of a complete inspection.

Safety First: Lockout/Tagout Before You Touch

Before opening any control panel, you must de-energize it and apply lockout/tagout (LOTO) per OSHA's Control of Hazardous Energy standard. Steps:

1. Notify operators and supervisors.

2. Identify all energy sources (electrical, pneumatic, hydraulic).

3. Shut down the equipment using the normal stopping procedure.

4. Isolate each energy source at its disconnect or valve.

5. Apply your personal padlock and tag to each isolation point.

6. Release or restrain stored energy (discharge capacitors, bleed pneumatics).

7. Verify zero energy with a calibrated meter before reaching inside.

Wear appropriate PPE throughout: safety glasses, insulated gloves rated for the voltage present when verifying zero energy, and hearing protection if the environment requires it. Never rely on a control-panel power switch alone — always lock out at the disconnect.

Step 1: Firmware Version Check

With the PLC powered (before LOTO, panel closed) connect the programming laptop and open the manufacturer's programming software. Navigate to the controller properties or online status screen. Record the firmware version exactly as displayed — for example, "v21.011" — and compare it to the manufacturer's current recommended version. Outdated firmware can contain known bugs that cause intermittent faults. Note the version in your PM data table and flag it if it differs from the reference in the PM checklist.

Step 2: CPU Diagnostics — Memory, Scan Time, and Fault Codes

While still online, use the software diagnostics screen to record:

Capture all values in the PM data table on your checklist before disconnecting.

Step 3: Battery Test

Most PLC CPUs use a lithium battery to retain program memory and clock data during a power outage. After applying LOTO and verifying zero energy, locate the battery. With a digital multimeter set to DC volts, measure the battery voltage at its connector or terminals. Compare the measured voltage to the manufacturer's minimum acceptable voltage (typically specified in the hardware manual). A battery reading at or below the threshold must be flagged for immediate replacement. Record date, measured voltage, and battery part number on the work order. Never short the battery terminals.

Step 4: Terminal-Torque Check

Loose terminals cause intermittent faults, arcing, and heat damage. With the panel locked out and verified de-energized, use a calibrated torque screwdriver set to the manufacturer's specified torque value for each terminal type. Work systematically from top to bottom, left to right on each rail. Mark each terminal with a felt-tip pen after checking so you do not double-count. Record any terminals found loose in the findings section of the work order and note the corrective action taken (re-torqued to specification).

Step 5: Enclosure Cleaning

Dust accumulation blocks ventilation, traps heat, and can conduct electricity if it becomes contaminated with moisture or metallic particles. With the panel locked out:

Do not use shop compressed air directly on circuit boards — it may contain oil or moisture.

Step 6: Enclosure Temperature Measurement

PLC enclosures should be maintained within the temperature range specified by the manufacturer (check the hardware datasheet). Use a calibrated thermometer or infrared temperature gun to measure ambient temperature inside the enclosure near the CPU module. Record the value. An over-temperature condition can indicate a failed cooling fan, blocked vents, or a change in the plant environment. Flag any reading outside the specified operating range.

Writing the Maintenance Work Order

A maintenance work order is a legal and operational document. A complete work order includes: date and time, equipment ID and location, work-order number, technician name, all tasks performed in sequence, every measured value, any out-of-tolerance findings, parts replaced, and recommended corrective actions with a suggested timeframe. Write clearly and completely — the next technician who opens this panel will rely on your record.

Connecting PM Data to Predictive Maintenance

When work orders accumulate over time, trends become visible. Battery voltage dropping 0.1 V per year, scan time creeping upward, or enclosure temperature rising each summer — these trends allow you to schedule corrective work before failure. Keep all PM records in the facility's computerized maintenance management system (CMMS) or a controlled paper log so trends can be tracked.

Key terms9

Preventive maintenance (PM)Scheduled inspection and servicing tasks performed at fixed intervals to prevent equipment failure before it occurs.
Predictive maintenanceUsing measured data and trend analysis to anticipate equipment failure and schedule maintenance only when indicators show it is needed.
FirmwareThe embedded operating software stored in a PLC CPU that controls how the processor executes the user program and communicates with other devices.
Scan timeThe time a PLC CPU requires to complete one full cycle of reading inputs, executing the program, and updating outputs; measured in milliseconds.
Lockout/tagout (LOTO)An OSHA-required procedure for de-energizing and isolating equipment, then applying personal locks and warning tags before performing maintenance, to prevent unexpected re-energization.
Torque screwdriverA calibrated hand tool that applies a precise, measurable rotational force to terminal screws, ensuring they are tightened to the manufacturer's specification.
Memory backup batteryA small lithium cell inside the PLC CPU that preserves the user program, data registers, and real-time clock when main power is removed.
Fault logAn internal record stored in the PLC that lists error codes generated by hardware problems, communication failures, or program execution errors, used during diagnostics.
Maintenance work orderA formal document recording the tasks, measurements, findings, parts used, and recommendations from a maintenance activity; becomes part of the equipment's permanent history.

Worked examples4

1. The PLC hardware manual states the memory backup battery must be replaced when its voltage drops to 2.8 V or below. You measure 2.75 V. What do you record and what action do you recommend?
  1. Compare the measured value (2.75 V) to the manufacturer's replacement threshold (2.80 V).
  2. 2.75 V is below 2.80 V, so the battery is at or past its end-of-service voltage.
  3. Record on the work order: 'Battery voltage measured 2.75 V DC; manufacturer minimum is 2.80 V DC — out of tolerance.'
  4. Recommend corrective action: 'Replace battery with the correct part number before next scheduled PM or at earliest opportunity to prevent program loss on power failure.'
Answer: Battery voltage is out of tolerance (2.75 V < 2.80 V threshold). Record the finding and recommend immediate battery replacement.
2. The PLC software shows total program memory of 512 KB and current memory usage of 448 KB. What is the percentage of memory used, and how should you record it?
  1. Divide used memory by total memory: 448 ÷ 512 = 0.875.
  2. Multiply by 100 to express as a percentage: 0.875 × 100 = 87.5%.
  3. Record on the PM data table: 'CPU memory usage: 448 KB used / 512 KB total = 87.5%.'
  4. Note that 87.5% exceeds a common advisory threshold of 80%, so flag it as a finding and recommend a program audit to review unused rungs or data tables.
Answer: Memory usage is 87.5%. This should be flagged as a finding with a recommendation for a program audit.
3. You are checking terminal torque. The manufacturer specifies 0.5 N·m for the I/O terminal block screws. Your torque screwdriver is marked in inch-pounds. What is 0.5 N·m expressed in inch-pounds (to two decimal places), and how do you set the tool?
  1. Use the conversion factor: 1 N·m = 8.8507 in·lb (a standard engineering conversion).
  2. Multiply: 0.5 N·m × 8.8507 in·lb/N·m = 4.4254 in·lb.
  3. Round to two decimal places: 4.43 in·lb.
  4. Set the torque screwdriver to 4.43 in·lb before checking the terminal screws, and record the specification and setting used on the work order.
Answer: 0.5 N·m equals approximately 4.43 in·lb. Set the torque screwdriver to 4.43 in·lb.
4. During three consecutive quarterly PM inspections the enclosure internal temperature was recorded as 38 °C, 41 °C, and 44 °C. The manufacturer's maximum rated operating temperature is 55 °C. Based on this trend, how many more quarters remain before the temperature is projected to reach the limit, and what should you recommend?
  1. Identify the trend: temperature is rising by 3 °C each quarter (38 → 41 → 44).
  2. Calculate remaining headroom: 55 °C − 44 °C = 11 °C.
  3. Divide headroom by the rate of increase: 11 ÷ 3 = 3.67 quarters.
  4. Round down to be conservative: approximately 3 full quarters remain before the limit is reached if the trend continues.
  5. Recommend investigating and correcting the cause (blocked vents, failed cooling fan, increased ambient plant temperature) before the next PM cycle rather than waiting for the limit to be exceeded.
Answer: Approximately 3 quarters remain at the current rate of increase. Recommend immediate investigation of cooling to prevent exceeding the 55 °C limit.

The math in this lesson2

Learners compute percent of PLC memory used and project the enclosure temperature trend against a limit.
7.RP.3N-Q.1
Brush up in the math halls at newSchoolWorks.com:
In class: Students follow a printed PM checklist to inspect the training panel: record firmware version from the programming software, measure battery voltage with a multimeter, check terminal screws for torque, remove dust with compressed air (panel de-energized and locked out), and record enclosure temperature. · Students use programming software diagnostics to check CPU memory usage, scan time, and any stored fault codes, recording all values in a preventive maintenance data table. · Students write a completed maintenance work order (using a standard industrial form) describing all tasks performed, measurements taken, any out-of-tolerance findings, and recommended corrective actions — reviewed against a model answer.
Materials: PLC trainer panel, PM checklist, multimeter, torque screwdriver, canned air (electrical-safe), thermometer or IR temperature gun, work-order form, programming laptop, NCCER Industrial Maintenance controls PM reference
Check: PM checklist must be fully completed with real measured values, and work order must include all required fields (date, equipment ID, tasks, measurements, findings, recommendations) — instructor grades work order using a 10-point rubric; 8/10 to pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Safety and Health
1. What is the first action a technician must take before opening a PLC control panel to perform a PM inspection?
Maintenance and Troubleshooting
2. A technician measures PLC backup battery voltage at 2.70 V. The manufacturer's hardware manual specifies a minimum acceptable voltage of 2.80 V. What is the correct next step?
Maintenance and Troubleshooting
3. During a PM inspection, the PLC diagnostics screen shows that the CPU scan time has gradually increased over three consecutive quarterly inspections. Which of the following is the most likely cause to investigate first?
Maintenance and Troubleshooting
4. Why must electrical-safe canned air rather than standard shop compressed air be used to clean PLC circuit boards?
Quality, Continuous Improvement, and Manufacturing Operations
5. A technician records the following on a PM work order: date, equipment ID, tasks performed, and measurements taken. Which required field is still missing?
Maintenance and Troubleshooting
6. The PLC hardware manual specifies terminal-block torque as 0.6 N·m. A technician's torque screwdriver is calibrated in inch-pounds. Approximately what value should the tool be set to? (Use 1 N·m ≈ 8.85 in·lb.)
Maintenance and Troubleshooting
7. Which PLC diagnostic value, when found to be unusually high, most directly indicates that the controller may need a program audit or a memory expansion?
Maintenance and Troubleshooting
8. A technician reviews the PLC fault log and finds several stored fault codes, none of which are currently active. What is the correct action?
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