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MFG-MECH15-07 · Lesson 1 · 10 clock hours · theory

Structured Fault-Isolation Methodology and Work-Order Documentation

Objective. Students will describe and sequence a seven-step fault-isolation process and complete a standard work order for a given fault scenario with no missing required fields.

⚠ 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.

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Why a Structured Approach Matters

When a robot cell goes down or a conveyor stops unexpectedly, the pressure to restore production is immediate. Without a repeatable method, technicians often jump to the most familiar fix — swapping parts, resetting faults, or guessing — which wastes time, masks the real cause, and can create new hazards. A structured fault-isolation methodology gives you a logical path from symptom to verified repair every time, regardless of the system's complexity. It also produces the documentation that maintenance supervisors, engineers, and auditors depend on.

The Seven-Step Fault-Isolation Process

Step 1 — Observe. Before touching anything, watch and listen. Note indicator lights, alarm codes, sounds, smells, and the state of the machine when the fault occurred. Observe without disturbing the scene. This is also when you confirm that all required lockout/tagout (LOTO) procedures under OSHA 29 CFR 1910.147 are in place before you approach energy-isolated components. Put on required PPE — safety glasses, cut-resistant gloves, arc-rated clothing as needed under NFPA 70E — before getting close to any panel or junction box.

Step 2 — Define the Symptom Precisely. Vague descriptions like "it doesn't work" are not acceptable on a work order or in a root-cause analysis. Write a specific, measurable statement: "Conveyor belt on Line 3 stops within 90 seconds of start-up; motor drive faults with F-007 overcurrent alarm at approximately 8.2 A on phase B." Precise symptom definition narrows the search space immediately.

Step 3 — List Probable Causes. Brainstorm every plausible cause that could produce the defined symptom. Use your knowledge of the system — electrical, mechanical, pneumatic, and control elements. Rank causes from most likely to least likely, considering recent changes, environmental conditions, and maintenance history. For example, an overcurrent fault might be caused by a mechanical jam, a failing motor winding, a worn bearing, an incorrect drive parameter, or a shorted sensor cable. Document all of them before testing any.

Step 4 — Test the Most Likely Cause. Starting with the highest-ranked probable cause, perform a targeted test. Use the correct instrument — a calibrated digital multimeter, clamp-on ammeter, or pressure gauge — and record the actual measurement. Compare it to the specification. A single, focused test either confirms or eliminates a cause. Do not replace parts before you have evidence. If electrical measurements are required on energized circuits, follow NFPA 70E shock and arc-flash boundaries, use appropriate PPE, and obtain an energized-work permit if required by your site policy. Verify absence of voltage at all relevant points before any physical intervention on de-energized circuits.

Step 5 — Repair. Once the root cause is confirmed, make the repair using approved parts, torque specifications, and procedures. Follow ANSI/RIA R15.06 / ISO 10218 safeguarding requirements before re-entering a robot cell. Restore all guards, barriers, and interlocks before re-energizing. Remove lockout/tagout devices only after the area is clear and only the authorized employee who applied each lock removes it, per 29 CFR 1910.147.

Step 6 — Verify. Restart the machine and confirm normal operation through a complete duty cycle, not just a momentary jog. Check that the symptom is gone and that no new faults have appeared. Measure key parameters — motor current, air pressure, cycle time — and compare to baseline specifications. Verification is not complete until the system runs normally under real operating conditions.

Step 7 — Document. Complete the work order fully. Incomplete records are as harmful as no records. Documentation drives predictive maintenance, spare-parts planning, and regulatory compliance.

Completing a Standard Work Order

A work order is a legal and operational record. Required fields typically include:

Downtime calculation is straightforward: subtract the time the equipment was returned to production from the time the fault was first reported. If a fault was reported at 09:15 and the line restarted at 11:40, downtime is 2 hours 25 minutes (145 minutes). Record both the clock times and the total duration.

Connection to PLC and Robot Fault Codes

Modern PLC systems and FANUC robot controllers generate fault codes that are part of your observation in Step 1. A FANUC alarm code appears on the teach pendant with an alarm number and description — always record the full code, not just the alarm number. PLC diagnostic screens in ladder diagram or function block view can show which rung or block is forcing a false condition. These digital clues guide your probable-cause list and save significant troubleshooting time.

Why Documentation Protects You

A fully completed work order demonstrates that the repair was performed correctly and safely. In industries governed by OSHA, NFPA, and ANSI standards, missing or vague records can expose you and your employer to citations. Good records also show professionalism and are expected by every PMMI Mechatronics and FANUC CERT assessor reviewing your competency portfolio.

Key terms9

Fault isolationThe systematic process of identifying the specific component or condition responsible for a machine malfunction, as distinct from identifying only the symptom.
Symptom definitionA precise, measurable statement of the observable condition that indicates a fault, including what is wrong, where, and under what conditions.
Probable-cause listA ranked inventory of all plausible reasons that could produce a defined symptom, developed before any testing begins.
Root causeThe fundamental, underlying reason a failure occurred; the condition that, if corrected, prevents recurrence of the same fault.
Work orderA formal maintenance record documenting the reported fault, test measurements, corrective action taken, root cause, and downtime for a single repair event.
DowntimeThe elapsed time from when a fault is reported until the equipment is returned to normal production; used to calculate overall equipment effectiveness (OEE).
Lockout/tagout (LOTO)The hazardous energy control procedure required by OSHA 29 CFR 1910.147 that ensures machines are de-energized and cannot be restarted before maintenance personnel complete their work.
Failure modeThe physical mechanism by which a component failed, such as insulation breakdown, mechanical wear, corrosion, or incorrect parameter setting.
VerificationThe step in the fault-isolation process in which the technician confirms the repair resolved the original symptom and that no new faults have been introduced, typically by running the machine through a complete operating cycle.

Worked examples3

1. A fault was reported at 06:45. The technician completed the repair and the line restarted at 09:10. Calculate the downtime in hours and minutes.
  1. Step 1 — Find the difference in hours: 09:10 minus 06:45. Hours portion: 9 − 6 = 3 hours, but the minutes portion requires borrowing because 10 minutes is less than 45 minutes.
  2. Step 2 — Borrow one hour from the hours portion: 3 hours − 1 hour = 2 hours remaining; add 60 minutes to the minutes portion: 60 + 10 = 70 minutes.
  3. Step 3 — Subtract the minutes: 70 − 45 = 25 minutes.
  4. Step 4 — Combine: 2 hours and 25 minutes total downtime. Convert to minutes only if the work order requires it: 2 × 60 + 25 = 145 minutes.
Answer: Downtime = 2 hours 25 minutes (145 minutes).
2. During Step 4 of fault isolation on a conveyor motor circuit, you measure phase-to-neutral voltage at the motor terminal block with a calibrated DMM and read 108 V on one phase, 121 V on the second phase, and 122 V on the third phase. Nominal supply is 120 V single-phase split from a 240 V panel. Assess whether the low-voltage reading is a significant deviation and what it tells you.
  1. Step 1 — Identify the acceptable voltage tolerance. Most industrial equipment specifies ±10% of nominal. Ten percent of 120 V = 12 V, so the acceptable range is 108 V to 132 V.
  2. Step 2 — Compare each reading to the range: 121 V — within range. 122 V — within range. 108 V — exactly at the lower boundary.
  3. Step 3 — A reading at exactly the tolerance limit under load warrants further investigation. Note that 108 V is 10% below 120 V (120 × 0.10 = 12; 120 − 12 = 108), meaning this phase is at the outer edge of acceptable.
  4. Step 4 — Record the actual readings with units, the instrument used, and the nominal specification on the work order. Flag the low phase as a finding and investigate the connection, conductor, or breaker on that phase as a probable cause contributing to the fault.
Answer: 108 V is exactly at the −10% tolerance limit; it is a flagged finding. The other two phases are within normal range. The technician should inspect the low-voltage phase connection and record all three readings on the work order.
3. A FANUC robot work order requires calculating downtime in minutes. The alarm was logged by the controller at 13:22 and the verified restart occurred at 14:05. What is the downtime?
  1. Step 1 — Both times are within the same hour block (13-hour and 14-hour). Find total minutes from 13:22 to 14:05.
  2. Step 2 — From 13:22 to 14:00 is 38 minutes (60 − 22 = 38).
  3. Step 3 — From 14:00 to 14:05 is 5 minutes.
  4. Step 4 — Total: 38 + 5 = 43 minutes.
Answer: Downtime = 43 minutes.

The math in this lesson1

Learners compute downtime in hours and minutes from fault and restart clock times for work orders.
N-Q.1
Brush up in the math halls at newSchoolWorks.com:
In class: Lecture and guided notes on the seven-step process: observe, define symptoms, list probable causes, test most likely cause, repair, verify, document; students annotate a printed flowchart as instruction proceeds. · Small-group case-study exercise: each group receives a one-page fault narrative (e.g., conveyor stops intermittently) and must produce a ranked probable-cause list and a test plan using only the information provided. · Individual practice completing a standard maintenance work order (digital form) for two scripted fault scenarios, including failure mode, test measurements, corrective action, and downtime calculation; instructor reviews entries against a rubric.
Materials: Seven-step fault-isolation flowchart handout, sample work-order template (digital and paper), fault scenario packets, whiteboard, projector, PMMI Mechatronics study guides (Industrial Electricity 1, PLC 1)
Check: Each student submits one completed work order; pass requires all required fields populated, fault description specific and measurable, and corrective action logically consistent with stated cause — instructor rubric, 80% minimum score.

Practice check7

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

Workplace Safety / Industrial Electricity
1. In the seven-step fault-isolation process, what must a technician do BEFORE physically inspecting components on a de-energized machine?
PMMI Mechatronics — Industrial Electricity 1
2. A technician writes the following fault description on a work order: 'Motor on Line 2 is bad.' Why is this description insufficient?
PMMI Mechatronics — General Troubleshooting
3. Which step of the seven-step fault-isolation process comes IMMEDIATELY after confirming the root cause and completing the physical repair?
PMMI Mechatronics — Industrial Electricity 1
4. A conveyor stops intermittently. During Step 3, a technician's probable-cause list includes: worn drive belt, overloaded motor, loose terminal connection, and incorrect PLC timer setpoint. What is the BEST reason to rank 'loose terminal connection' highly on the list?
Work-Order Documentation
5. A fault is reported at 07:15 and the equipment is returned to production at 10:00. What is the downtime in minutes?
PMMI Mechatronics — Industrial Electricity 1 / Workplace Safety
6. A technician is about to take a voltage measurement on a live 480 V panel to check for an unbalanced phase. According to NFPA 70E, which action is required before proceeding?
PMMI Mechatronics — General / Work-Order Documentation
7. On a completed work order, the corrective action field reads 'Replaced motor.' The root cause field reads 'Unknown.' Why would an instructor or supervisor reject this work order?
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