In short. List every load, ask what happens if it loses power for a second, a minute and an hour, and assign a tier. The tiers decide which loads get UPS, which sit on the generator, which are shed and in what order loads restart. A single page of agreed classification drives many design decisions.
A method you can use today
- List loads from the load list. See load list guide.
- Define consequence categories: safety, regulatory, data or process damage, revenue, comfort.
- Define tolerable outage for each load: none, seconds, minutes, hours.
- Assign a tier from the combination.
- Check dependencies: a load inherits the tier of what depends on it.
- Review and approve with operations and safety.
A scoring matrix (illustrative)
| Consequence of loss | Tolerable gap | Tier |
|---|---|---|
| Safety or legal duty | Seconds or less | Tier 1: never shed; fastest support |
| Data loss or major process damage | None to seconds | Tier 1: UPS-backed |
| Serious production or service loss | Minutes | Tier 2: generator, staged restart |
| Moderate loss, deferrable | Hours | Tier 3: shed, restore later |
| Comfort or convenience | Hours or more | Tier 4: shed first |
Dependencies
Trace support loads. Cooling for a data hall, ventilation for a hazardous area, lubrication pumps for a turbine, control power for a process and lifts to a critical floor all inherit their parent’s tier.
Using the result
- Tier 1 total sets the UPS size. See UPS capacity.
- Tiers 1 and 2 set generator capacity. See capacity requirement.
- Tiers 3 and 4 form the shedding list. See load-shedding planning.
- Tier order defines the restart sequence. See restart sequencing.
Worked illustration
A plant has 1,000 kW connected. Classification gives Tier 1: 80 kW, Tier 2: 420 kW, Tier 3: 300 kW, Tier 4: 200 kW. The UPS is sized near 80 kW plus margin; the generator for 500 kW plus margins; the other 500 kW is shed. Without classification, the generator would have been sized near 1,000 kW. With it, the cost is reduced and the outage behaviour is explicit.
Governance
- Keep a register with approver and date.
- Review after process changes.
- Link to the single-line diagram so each feeder’s tier is visible.
Classification worksheet
| Question | Answer sets |
|---|---|
| What happens if this load stops for 1 second? 1 minute? 1 hour? | Class and backup type |
| Is there a safety, legal or contractual requirement? | Mandatory backup and start time |
| Is the load a heavy starter? | Restart sequence position |
| Does another critical load depend on it (such as cooling)? | Priority linked to dependency |
Priority builder (illustrative)
| Priority | Typical loads | Treatment |
|---|---|---|
| 1 | Life safety, control power, IT core | UPS plus generator, never shed |
| 2 | Cooling for critical areas, fire pumps, lifts | Generator, restore early |
| 3 | Production loads | Generator, restore in sequence, shed in shortage |
| 4 | Comfort and convenience | Shed first, restore last |
Dependencies
A priority-1 server room is not safe if its cooling is priority 4. Check every critical load for its supporting loads.
Output
A table with each load, class, backup source, restoration order and shedding order, approved by the operator and reviewed yearly.
See critical-load classification and critical and mission-critical systems.
Common mistakes
- Classifying by equipment type.
- Putting every load in Tier 1.
- Forgetting support loads.
What to send us
Send your load list with tolerable outage for each load. Send your application details. See also critical and essential classes.
Questions engineers ask
Who decides the tier?
The process or facility owner with safety and engineering input.
How many tiers should I use?
Three or four is usually enough. More becomes hard to maintain.
Can tiers change?
Yes. Revisit when operations change.
What about loads of unknown criticality?
Ask operations. Treat unknown loads cautiously until classified.
This guide gives general principles. Classification must be agreed for your facility.