Critical spares: making sure you are not caught short
The right spare, on the right shelf, is the difference between an SLA met and an SLA breached. Here is how to decide what "critical" means.
Critical spares are the replacement parts essential to keeping critical infrastructure running without interruption. To make sure they can be fitted quickly, they are held on site or kept in stock at the supplier, so they can be sourced and installed within the timeframe the operation needs.
Why you need them
You keep critical spares to keep critical systems running, resilient and within a customer SLA. Take a chilled-water pump system running N+1 — one duty pump, one standby. If a pump fails, you are now running at N. Your SLA might give you hours, not days, to restore the N+1 state, so the spares — seals, bearings, or in a remote location the entire pump set — need to be close at hand.
This matters most in remote markets, where the supply chain is not as deep: fewer parts are carried locally, and the service skills may not be available. By contrast, you would not stock a toilet cistern — it is not part of a critical system and unlikely to sit in any SLA, so it gets fixed once the part turns up.
How you decide what is "critical"
Getting the list right is the whole game. Too short, and a system can fail or you breach your SLA. Too long, and you are paying to hold inventory you do not need. Whether a spare is critical depends on the component's reliability, the system design, and the supplier's supply chain in your area.
Defining critical spares is normally done by the operator and the design consultant during design, and confirmed when the equipment is procured. The consultant defines the system topology, talks to the supply chain, confirms the expected life of key parts, then sets out what is critical and a reasonable time to repair each component — discounting suppliers whose supply chain cannot meet the requirement.
When to do it
Ideally at tender, alongside procurement. That is when you have the most leverage: you can negotiate the best price, assess total cost of ownership, and rule out a supplier whose supply chain falls short. Leave it to during or after construction and that leverage is gone — the supplier has no incentive to keep the list tight, which invites a drawn-out debate about what is and is not a critical spare.