In this guide
Holding every spare is expensive and holding none is a gamble. A simple ranking gets you most of the way to a sensible shelf, and it can be done in an afternoon with a list of your machines.
Step 1: List what stops the line
Walk each machine or line and list the components that, if they failed, would stop production or put people at risk. Include the PLC CPU and its power supply, key I/O modules, main drives, operator panels that cannot be bypassed, safety relays, and the 24 V DC supplies that feed all of them.
Step 2: Put a cost on an hour of downtime
Use lost output, idle labour, scrap and any contractual penalties. An approximate figure is enough. The downtime and critical spares calculator compares the price of a spare with what waiting for one would cost.
Step 3: Find out how long a replacement takes to get
Lead time matters as much as failure rate. A part you can buy the same day is a lower risk than one that takes weeks, even if both fail equally often. Parts that are costly to lose and slow to source belong on your shelf. Ask suppliers for current lead times rather than assuming, because they change.
Step 4: Classify each part
| Class | What it means | Stock policy |
|---|---|---|
| A | Stops a line or creates a safety risk, and is slow to replace | Hold at least one spare, tested and labelled |
| B | Stops a line, but a replacement is quick to get | Hold a spare if downtime is costly. Otherwise know your supplier and the lead time |
| C | Minor impact, or easy to substitute | Buy when needed |
Look for single points of failure
- One PLC CPU controlling a whole line
- A single power supply feeding an entire control panel
- A drive with no like-for-like replacement on site
- An operator panel with no backup of its application
Record exactly what you hold
- The full order number and the hardware or firmware version of each spare
- Where it is stored and who can release it
- A backup of the program or drive parameters that goes with it
- The date you last checked it, and whether the manufacturer has announced a phase-out
A spare that is the wrong version, or whose program you cannot load, is not much of a spare. Store spares in anti-static packaging in a clean, dry place.
The critical spares register template on the resources page is a spreadsheet with these columns. Fill it in, then send us the list and we will tell you what we can source, what has long lead times and what looks risky.
Review it
Review the list at least once a year, after any equipment change, and after every failure. Failures are the best evidence you will get of what actually goes wrong on your site.