Sourcing · 7 min read · Updated June 2026
Lead-Time Risk: Dual Sourcing and the Critical Spares List
The part that stops your plant is rarely the expensive one. How to build a defensible critical-spares strategy: classification by downtime, stock levels by formula, qualified alternates and lifecycle monitoring.
After the shortages of recent years every plant manager knows the feeling: a 200-dollar module with a 26-week lead time holding a line hostage. The fix is boring and effective — classify, stock and qualify alternates before the failure.
Classify by downtime, not by price
For each installed component ask: if it fails today, when does production stop, and how long until a replacement is running?
- A — production stops immediately, no workaround.
- B — degraded operation or a workaround exists.
- C — no production impact.
The A-list is usually shorter than feared — and full of cheap parts: a 24 V power supply like the QUINT4 24 V/10 A, a communication module, a safety relay, a specific sensor on the only filler.
Set stock levels with a formula, not a feeling
For A-items: expected failures during the replenishment lead time (from failure history or vendor MTBF data) plus a safety margin for lead-time variance. In practice: minimum one of anything electronic on the A-list whose lead time exceeds your tolerable downtime; more for wear parts and for units installed many times over. Store the PLC program with the spare CPU — a CPU 1214C on the shelf without the project backup is only half a spare.
Qualify alternates before you need them
- Cross-reference each A-item: manufacturer successor part, competitor equivalent, and an authorized-surplus source.
- Define what "qualified" means — form, fit and function tested, firmware and configuration verified — and record who signed it off.
- For obsolete electronics decide per item: last-time buy, repair partner, or an engineered retrofit. Plants still running the S7-300 CPU 315-2 PN/DP need this decision this year, not after the next failure.
Watch lifecycle proactively
Subscribe to manufacturer product change and end-of-life notifications for the A-list. An EOL notice with twelve months of runway is a plan; discovered after a failure, it is a crisis. This is the monitoring we run for customers as part of sourcing services.
Make it audit-proof
One list, reviewed yearly, with owner, stock level, storage location, qualified alternate and lifecycle status per line — attached to your maintenance system, not to someone's personal spreadsheet. Include the ordering code, not just the family name: "Cerabar" is not a spare, "PMC51B-xxxx" is.
A note on buffer stock
For recurring requirements we can hold agreed buffer stock in Hai Phong against a blanket order and release it on call-off — useful for consumable sensors and the parts your maintenance team replaces every quarter.
Send us your installed-base list, even a rough one. We return criticality flags, current lead times, lifecycle status and alternate options per line — the raw material of a spares strategy. Start here.
Need engineering help with this?
Our engineers apply this to real projects every week — selection, specification, commissioning. Send your question or a part number.
Quick answers
How many spare PLC CPUs should we keep?
At least one per CPU type on the A-list whose lead time exceeds your tolerable downtime, with the current program backed up on a memory card and stored with it. More if the same CPU is installed many times or the plant runs continuously.