The importance of spare parts in industrial automation

The cheap spare that costs a full shift of production

In industrial automation there is an asymmetry that surprises anyone who has not lived through it: the cheapest component in an installation can be the one that costs the most money when it fails. A €60 power supply, a €120 digital input module or a €90 safety relay are modest items in a project budget. But if that component fails and there is no equivalent in the storeroom, the line does not restart until the spare arrives. And that lead time is, in many cases, measured in days or weeks.

The real cost of a stop is not set by the price of the part, but by everything that stops around it: raw material in process, undelivered orders, energy spent on restart and cool-down, SLA penalties and, in regulated sectors, batches that have to be scrapped. In food, pharmaceutical or ceramics facilities, an extended outage can cost between €2,000 and €20,000 per hour. Against that figure, keeping a stock of critical spares stops being an expense and becomes an insurance policy with an obvious return.

Why lead times are now the main risk

Until a few years ago, the "I'll order it when it breaks" model worked reasonably well because supply times from the major manufacturers (Siemens, Schneider, Wago, Beckhoff, ABB) were short and predictable. That has changed. Disruptions in the semiconductor and electronics supply chains have driven up and, above all, made unpredictable the lead times of many automation components.

Today it is perfectly normal to run into situations like these:

• Variable frequency drives from specific ranges with lead times of several weeks or months depending on configuration.
• PLC CPUs from recent generations with fluctuating availability.
• Special I/O modules (high-resolution analogue, high-speed counting, specific communications) with intermittent stock.
• HMI panels with reference changes that force a migration of the visualization project if the original model has disappeared.

The problem is not only the lead time, but the uncertainty about that lead time. Not being able to give production a reliable restart date is, in management terms, almost as bad as the stop itself.

Obsolescence: the spare that no longer exists

The second major reason to have a spare parts strategy is obsolescence. A PLC installed 10 or 15 years ago still works perfectly, but its CPU may have been discontinued for years. The day it fails, there is no equivalent new spare: you have to turn to the second-hand market, to refurbished equipment or, in the worst case, face a full emergency PLC migration, under the pressure of a line that is already stopped.

A planned migration is a controlled project; a migration forced by a failure with no spare is a crisis. Having the CPU, the modules and, above all, a versioned and verified copy of the program in the storeroom turns that crisis into a one-hour swap. That is why spare parts management and the management of software documentation and backups are, in practice, the same problem.

Which components are worth keeping in stock

This is not about duplicating the entire installation in the storeroom — that would be absurd and extremely expensive. It is about identifying the components whose combination of criticality (what happens if it fails) and replenishment lead time (how long it takes to get another) justifies keeping them on hand. The usual candidates are:

• PLC CPU and I/O modules for the references present in the plant, especially if they are from older generations or have tight availability.
• 24 VDC power supplies: cheap, easy to store and a frequent cause of stops.
• Variable frequency drives for critical motors, or at least one for each common power range.
• Safety relays, contactors and interface relays, which are wear components.
• HMI panels for the main operator stations.
• Communication modules and gateways (Profinet, EtherCAT, Modbus, IO-Link) specific to the project.
• Sensors and encoders with uncommon references or long lead times.
• Batteries, memory cards and consumables of the control system itself.

How to decide what to keep: criticality analysis

The most sensible way to build a spare parts plan is to cross two variables for each component: the impact of its failure and the difficulty of replacement. This produces a very clear decision matrix:

This approach avoids the two typical mistakes: tying up capital in spares that will never be used and, at the same time, running out of the part that actually stopped the plant. The goal is not to have everything, but to have the right things.

The physical spare is useless without its "digital spare"

A common mistake is to focus only on hardware. Replacing a failed PLC CPU with a new one does not bring production back if you cannot load the correct program into it. That is why a spare parts plan is only complete when it also includes:

• A versioned and verified copy of the program for each PLC, HMI, SCADA and parameterizable drive.
• Drive parameter sets saved and documented (not just "in the head" of whoever did the commissioning).
• Licences and programming software needed to load or adjust each device.
• Up-to-date electrical schematics and signal lists, so any technician can perform the swap without rebuilding the knowledge.
• A documented replacement procedure for critical components.

When the physical spare and the "digital spare" are aligned, a failure that used to stop the plant for half a day is resolved in an hour. And if remote support is also in place, an engineer can guide the swap and reload the program without having to travel to site.

Spare parts and predictive maintenance: getting ahead of failure

Having spares solves the "when it fails"; predictive maintenance helps you know when it is going to fail. Combining both is the most efficient strategy: monitoring the wear of critical components lets you replenish and plan the replacement before the failure, turning an unplanned stop into routine maintenance. The safety stock then stops being a reaction and becomes part of a plan.

How we approach spare parts management at Bluemation

At Bluemation we treat spare parts management as part of the project life cycle, not as an afterthought. In the installations we design and maintain:

• We build a critical component list with its criticality and real replenishment lead time, so the decision on what to stock is backed by data.
• We keep a versioned copy of the program and electrical schematics for every device, available immediately when an incident occurs.
• We document the parameter sets of drives and equipment, so that a spare is commissioned with the correct configuration first time.
• We flag obsolescence and propose orderly migration plans before the component becomes impossible to source.
• We combine the spare parts plan with remote support and preventive maintenance to reduce both the frequency and the duration of stops.

If you want to review which critical spares your plant should keep, or you need to put order into the documentation and backups of your control equipment, get in touch with us. We carry out a no-commitment analysis of the current state of your installation and propose a spare parts plan tailored to your real criticality.