BAS-302 Visual Aid Library
Figure-use rule
Each figure is an orientation and reasoning aid, not a project drawing. Learners must pair it with the current sequence, point list, wiring/piping documents, manufacturer instructions, and as-found evidence.
Normal-rule anchor
Troubleshooting begins by controlling risk and meaning, not by making the first available change.
Case overlay
An occupied clinic AHU has an intermittent low-temperature alarm and the operator asks for an immediate reset.
Calculation/evidence callout: Rank candidate tests by risk, discriminatory value, elapsed time, disruption, reversibility, and restoration effort.
Normal-rule anchor
Actual behavior cannot be called abnormal until expected behavior is sourced for the current mode and conditions.
Case overlay
A VAV damper is nearly closed during heating, and a technician assumes the actuator is failed.
Calculation/evidence callout: Calculate expected state from supplied schedule, mode, deadband, load, and sequence conditions.
Normal-rule anchor
A cause explains the mechanism that produced the evidence; a component name alone usually does not.
Case overlay
The graphic says supply fan failure, the fan is audible, and current proof is off.
Calculation/evidence callout: Create a claim-evidence table and score support, contradiction, and unknown status for each statement.
Normal-rule anchor
Diagnose at the evidence boundary: command is not output, output is not action, action is not proof, and proof is not process result.
Case overlay
A valve command and position both change, but coil discharge temperature does not respond.
Calculation/evidence callout: Compare expected and observed values at each link and identify the first statistically or physically meaningful divergence.
Normal-rule anchor
A useful hypothesis predicts evidence that differs from competing explanations and can be rejected by a safe test.
Case overlay
Zone temperature oscillates after a sensor was replaced; possible causes include tuning, location, scaling, and real load cycling.
Calculation/evidence callout: Use a hypothesis matrix to count unique predictions, contradictions, assumptions, and unresolved evidence.
Normal-rule anchor
The best next test is not the easiest reading; it is the safe authorized test that most reduces uncertainty.
Case overlay
An AI is wrong at the front end; the field reference, raw controller value, and network object can be compared.
Calculation/evidence callout: Estimate test value as information gained divided by combined risk, disruption, time, and restoration cost.
Normal-rule anchor
Trend interval must be faster than the event being diagnosed and all compared clocks must share a known time basis.
Case overlay
A fan proof drops for ten seconds once per day, but the existing trend records every fifteen minutes.
Calculation/evidence callout: Compare event duration to sampling interval; calculate clock offset and minimum capture rate for the supplied case.
Normal-rule anchor
Multiple simultaneous changes may restore operation while destroying the evidence needed to know why.
Case overlay
A technician changes PID gains, sensor offset, and setpoint before waiting for the system response.
Calculation/evidence callout: Calculate elapsed stabilization time from the supplied time constant and compare before/after windows under equivalent conditions.
Normal-rule anchor
A root-cause statement must account for the evidence and survive a changed-condition verification.
Case overlay
A scaled sensor caused a false low-static alarm while an unrelated dirty filter increased fan demand.
Calculation/evidence callout: Reconcile evidence explained by the primary cause, secondary defect, and remaining unknowns; none may be double-counted.
Normal-rule anchor
A repair is not verified by disappearance of one symptom; test the causal path and affected operating cases.
Case overlay
The alarm clears after a controller restart, but no causal evidence was preserved and the override register is incomplete.
Calculation/evidence callout: Reconcile every temporary state and required test; critical restoration failures force remediation regardless of average score.