Faults surface throughout commissioning — in pre-commissioning checks, electrical testing, CCR and transformer testing, fixture and photometric testing, ALCMS and SAT. Division 21 provides the structured method for taking a fault from first detection through to a formally closed record, holding the line at every stage that a symptom is not a diagnosis and a diagnosis is not yet a root cause. It governs identification, classification, isolation, investigation, equipment-specific diagnosis, root-cause analysis, corrective action, retesting, and deviation/non-conformance management.
Philosophy: do not treat the symptom as the fault. Identify the actual root cause, correct it, retest the affected system, verify the fault has not propagated to another function, and formally close the deviation. This is the commissioning fault-management and problem-solving division — the bridge between commissioning failures found under Divisions 01–20 and final acceptance.
Every fault discovered during commissioning moves through the same underlying sequence, regardless of which system or division it was found in.
A fault is only closed once every stage in the chain is evidenced — skipping a stage leaves the closure unsupported.
| Stage | What It Confirms |
|---|---|
| Fault Identified | The fault is captured with location, equipment, circuit, and test reference |
| Fault Classified | The fault type is assigned so the correct diagnostic sequence is used |
| Fault Isolated | The fault is narrowed to the smallest practical section of the system |
| Investigation | Evidence is gathered and the actual sequence of events is established |
| Root Cause Identified | The underlying cause is confirmed, not just the immediate symptom |
| Corrective Action | An action addressing the cause is defined and implemented |
| Retest | The original test and every related function are re-verified |
| Result Verified | Retest results confirm the fault has not propagated elsewhere |
| Documentation Updated | Drawings, schedules, configuration and test records reflect the fix |
| NCR/Deviation Closed | Any raised NCR or deviation is formally dispositioned and closed |
| Fault Closed | Objective evidence demonstrates the problem is fully resolved |
Two principles govern how Division 21 is applied, whatever system the fault was found in.
Grouped by diagnostic focus, in the order a fault is normally worked through — click any topic to expand its checks and acceptance criteria.
Acceptance Criteria: every fault discovered at any stage of commissioning is captured on an initial record with location, equipment, circuit, and test reference before investigation proceeds.
Acceptance Criteria: the fault is assigned a classification before detailed investigation begins, so the correct diagnostic sequence is selected.
Hold Point: do not authorize component replacement until the fault has been isolated to the smallest practical section of the system.
Acceptance Criteria: the investigation produces a documented, evidence-based account of what happened before diagnosis begins.
Hold Point: do not repeatedly energize a circuit with an unresolved electrical fault where doing so could create a safety or equipment hazard.
Acceptance Criteria: the cable fault is located to a specific section, with both cause and location confirmed by measurement.
Hold Point: "Fixture OFF" does not automatically mean "fixture fault" — verify the complete circuit path before condemning the fixture.
Acceptance Criteria: measured transformer behaviour is confirmed to match expected behaviour under the tested condition, or the deviation is fully explained.
Hold Point: do not declare a fixture defective until the complete supply chain ahead of it has been verified.
Acceptance Criteria: the CCR fault is traced through the diagnostic flow to a confirmed cause before any adjustment is made.
Acceptance Criteria: the fault is correctly attributed to one of electrical, optical/alignment, or calibration/configuration cause — not treated as a single undifferentiated issue.
Hold Point: "no illumination" does not necessarily indicate an LED-module failure — verify the complete electrical and control chain first.
Acceptance Criteria: the point in the command chain where the fault occurs is identified and confirmed by trace-through, not assumed.
Acceptance Criteria: the communication fault is located to a specific point between source and destination, supported by a source/destination/expected/actual record.
Acceptance Criteria: the identified cause is the underlying root cause, not merely the immediate symptom.
Hold Point: corrective action must address the identified cause — an action that only removes the symptom does not close the fault.
Hold Point: a temporary corrective action must never be allowed to silently become the final installation.
Acceptance Criteria: the final approved solution is installed, verified, and reflected in every affected record.
Hold Point: do not close a fault on the original failed test alone — retest every related function the correction could have affected.
Acceptance Criteria: every commissioning deviation is formally recorded and dispositioned — never absorbed into general commissioning remarks.
Acceptance Criteria: the non-conformance is fully documented from identification through disposition and formal closure.
Acceptance Criteria: a fault is closed only when objective evidence demonstrates the problem is resolved — the full identification → classification → isolation → investigation → root cause → corrective action → retest → documentation → closure sequence is complete.
Division 21 runs as a single continuous sequence per fault, from first detection through to formal closure.
A test failure or abnormal condition is observed during commissioning.
Capture location, equipment, circuit, test reference, and description.
Assign the fault to its correct type before investigation.
Narrow the fault to the smallest practical section of the system.
Gather evidence and establish what actually happened.
Apply the equipment-specific diagnostic sequence.
Trace from symptom through to the underlying root cause.
Define an action that addresses the cause, not the symptom.
Implement and track through to the final approved solution.
Re-verify the original test and every related function.
Confirm the fault has not propagated to another function.
Formally record and disposition any deviation or non-conformance.
Update drawings, records, and reports to reflect the fix.
Close the fault only once objective evidence confirms resolution.
Quick answers about Division 21 — Troubleshooting During Commissioning in the AGL Testing & Commissioning Manual.
Division 21 of the AGL Testing & Commissioning Manual covers Troubleshooting During Commissioning. Faults surface throughout commissioning — in pre-commissioning checks, electrical testing, CCR and transformer testing, fixture and photometric testing, ALCMS and SAT. Division 21 provides the structured method for taking a fault from first detection through to a formally closed record, holding the line at every stage that a symptom is not a diagnosis and a diagnosis is not yet a root cause. It governs identification, classification, isolation, investigation, equipment-specific diagnosis, root-cause analysis, corrective action, retesting, and deviation/non-conformance management. It contains 22 topics (TC-TSC-001 to TC-TSC-022) organised into 7 groups (A–G), each topic with its own verification checks.
Do not treat the symptom as the fault. Identify the actual root cause, correct it, retest the affected system, verify the fault has not propagated to another function, and formally close the deviation. This is the commissioning fault-management and problem-solving division — the bridge between commissioning failures found under Divisions 01–20 and final acceptance.
Every fault discovered during commissioning moves through the same underlying sequence, regardless of which system or division it was found in. Steps: Identify → Classify → Isolate → Investigate → Diagnose → Root-Cause Analyze → Correct → Retest → Verify → Document → Close. Focus areas: Electrical & cable faults; Connector & equipment faults; Control & communication faults; Root-cause & corrective action; Deviation & NCR management.
Division 21 has 22 topics in 7 groups. A — Identification & Classification: Commissioning Fault Identification, Fault Classification. B — Isolation & Investigation: Fault Isolation, Fault Investigation. C — Electrical & Cable/Connector Diagnosis: Electrical Fault Diagnosis, Cable Fault Diagnosis, Connector Fault Diagnosis. D — Equipment-Specific Diagnosis: Transformer Fault Diagnosis, Fixture Fault Diagnosis, CCR Fault Diagnosis, PAPI Fault Diagnosis, Sign Fault Diagnosis. E — Control & Communication Diagnosis: Control-System Fault Diagnosis, Communication Fault Diagnosis. F — Root Cause & Corrective Action: Root-Cause Analysis, Corrective Action, Temporary Corrective Action, Permanent Corrective Action. G — Retest, Deviation & Closure: Retesting After Correction, Commissioning Deviation Management, Non-Conformance Management, Fault Closure.
A fault is only closed once every stage in the chain is evidenced — skipping a stage leaves the closure unsupported. Fault Identified: The fault is captured with location, equipment, circuit, and test reference; Fault Classified: The fault type is assigned so the correct diagnostic sequence is used; Fault Isolated: The fault is narrowed to the smallest practical section of the system; Investigation: Evidence is gathered and the actual sequence of events is established; Root Cause Identified: The underlying cause is confirmed, not just the immediate symptom; Corrective Action: An action addressing the cause is defined and implemented; Retest: The original test and every related function are re-verified; Result Verified: Retest results confirm the fault has not propagated elsewhere; Documentation Updated: Drawings, schedules, configuration and test records reflect the fix; NCR/Deviation Closed: Any raised NCR or deviation is formally dispositioned and closed; Fault Closed: Objective evidence demonstrates the problem is fully resolved.
Two principles govern how Division 21 is applied, whatever system the fault was found in. Prove the Fault Before Replacing: Never replace an AGL component merely because it appears faulty; First prove the fault, isolate its location, and determine the root cause; Correct the actual cause, then retest the complete affected circuit or function; Example: "Runway light OFF" should not immediately become "replace runway light" — trace CCR Output → Primary Series Circuit → Isolation Transformer → Secondary Lead → L-823 Connector → Fixture → Optical/Mechanical Condition first.
Division boundary: Divisions 01–20 perform the testing and commissioning itself; Division 21 governs what happens when any of those tests fails; Division 21 is the problem-solving bridge between commissioning failures and final acceptance, maintaining the traceability needed for Divisions 18–20; A temporary corrective action used to keep commissioning moving must always convert to a permanent one before final acceptance; Every deviation and non-conformance is recorded and dispositioned here — never absorbed into general commissioning remarks.
Division 21 runs as a single continuous sequence per fault, from first detection through to formal closure. (1) Fault Detected — A test failure or abnormal condition is observed during commissioning; (2) Identify Fault — Capture location, equipment, circuit, test reference, and description; (3) Classify Fault — Assign the fault to its correct type before investigation; (4) Isolate Fault — Narrow the fault to the smallest practical section of the system; (5) Investigate — Gather evidence and establish what actually happened; (6) Diagnose — Apply the equipment-specific diagnostic sequence; (7) Root-Cause Analysis — Trace from symptom through to the underlying root cause; (8) Corrective Action — Define an action that addresses the cause, not the symptom; (9) Temporary/Permanent Correction — Implement and track through to the final approved solution; (10) Retest — Re-verify the original test and every related function; (11) Verify Performance — Confirm the fault has not propagated to another function; (12) NCR/Deviation Management — Formally record and disposition any deviation or non-conformance; (13) Documentation Update — Update drawings, records, and reports to reflect the fix; (14) Fault Closure — Close the fault only once objective evidence confirms resolution.
Troubleshooting during commissioning is where engineering judgment matters most — under time pressure, with a system not yet proven.
Build the technical knowledge to understand systematic fault-finding methodology, root-cause analysis, and how to resolve issues without introducing new ones into a system still being verified.
Diagnose with precision. Resolve with confidence. Commission with certainty.
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