The Hazard of 'Troubleshooting by Replacement'
Why blindly swapping PLC cards or relays without understanding the root cause often introduces new faults, clears evidence, or inadvertently bypasses safety interlocks.
1. The Urgency Trap
When a critical process goes down, the pressure on the E&I technician is immense. Every minute of downtime costs the facility money. In this environment, the temptation to bypass a methodical root-cause analysis and jump straight to “troubleshooting by replacement” is high.
This practice—blindly swapping out PLC cards, control relays, or sensors based on a guess—might occasionally get the process running faster, but it introduces massive safety and reliability risks.
2. The Dangers of Blind Swapping
Replacing components without proving they are failed leads to several dangerous outcomes:
- Destroying Evidence: The original component holds the key to the failure. If a relay coil burned up due to a downstream short, removing it before checking the circuit destroys the evidence. If you put a new relay in, it will immediately burn up again.
- Introducing New Faults: Swapping complex components like PLC I/O cards or VFD control boards often requires moving dozens of wires or re-seating delicate pins. Each intervention is an opportunity to roll a wire, bend a pin, or drop a screw into a live bus.
- Bypassing Interlocks: In older, undocumented relay logic panels, a technician might swap a failed multi-pole relay with whatever is in the parts bin. If the new relay has a different contact configuration (e.g., normally open instead of normally closed), a critical safety interlock might be permanently bypassed without anyone knowing.
3. The Methodical Alternative
Effective troubleshooting requires proving the fault before breaking out the screwdriver.
- Use the Schematics: Start at the prints, not the panel. Trace the logic and identify the test points that will isolate the problem in half.
- Test Before You Pull: Use your multimeter to confirm that a component is receiving the correct input but failing to produce the correct output. Only then is it a confirmed failure.
- Ask “Why did it fail?”: Components rarely die of old age. Before installing the replacement, you must answer what killed the original component—was it a voltage spike, a mechanical jam, or water ingress?
4. Actionable Takeaways
- Ban the “Parts Cannon”: Establish a maintenance culture — aligned with NFPA 70E 110.5 (host/contract employer responsibilities) and CSA Z462 Section 4.1 (electrical safety program) — where technicians must explain why a component failed before requesting a replacement from stores.
- Post-Incident Review: If a system required three different parts swapped to get it running, the root cause was not found. Schedule a post-mortem to analyze the original removed parts.
- Document Changes: If a component is replaced, ensure the new part number matches the schematic perfectly. If a substitution is made, it must be documented through the Management of Change (MOC) process.