In short. Group loads by the interruption they can tolerate. No-break loads need UPS or battery support. Short-break loads accept seconds and can ride a generator start and transfer. Long-break loads can wait minutes or hours and may restart in stages or manually. Each class gets a different architecture and cost.
The three classes
| Class | Tolerable interruption | Typical architecture |
|---|---|---|
| No-break | None to a few milliseconds | Online UPS, BESS, static transfer switch |
| Short-break | Seconds up to about a minute | Standby generator with ATS |
| Long-break | Minutes to hours | Generator after higher classes, or restart after restoration |
The boundaries are set by the loads, not by a code. A drive may ride through a one-second dip, a process computer may not.
Examples
- No-break: servers, safety and control systems, medical equipment in critical care, communications.
- Short-break: emergency lighting, fire pumps, ventilation, lifts, many process loads with ride-through.
- Long-break: comfort cooling, general lighting, workshop equipment, deferred production.
How to decide
- For each load group, ask what happens at 0.1 s, 10 s, 1 min and 10 min of interruption.
- Test or confirm ride-through of contactors, drives and controls, which often drop out at short interruptions.
- Assign the class and the supporting source.
Contactor drop-out is a common surprise: a motor starter may open during a one-second break even if the motor and process could cope, causing a longer outage than the supply break itself.
Designing around the classes
- Put no-break loads on a UPS or BESS-backed bus with its own distribution. See UPS architecture.
- Put short-break loads on the generator bus with an ATS. See ATS, STS and UPS coordination.
- Put long-break loads on a shed or delayed section. See restoration and restart sequencing.
Why not UPS everything?
UPS and battery capacity are costly, take space, and need maintenance and replacement. Loads that can tolerate a short break do not need them. Splitting loads into classes lets the expensive protection be spent only where it is needed.
Worked illustration
A plant has 60 kW of controls and IT (no-break), 300 kW of process pumps and fans (short-break) and 200 kW of general loads (long-break). A UPS of about 60 kW with allowance for growth serves the first group. The generator is sized for the first two groups, 360 kW plus starting and growth allowances. The third group is shed on outage and restored after the generator is stable. A UPS sized for the whole 560 kW would cost far more and add little.
Acceptance
- Perform an outage test and record what each class did.
- Check that contactors and drives ride through as assumed, or add hold-up devices.
- Confirm the long-break loads restart in the planned order.
Three classes by interruption tolerance
| Class | Interruption | Typical loads | Backup |
|---|---|---|---|
| No-break | None (under about 10 ms) | IT, controls, medical devices | UPS or BESS |
| Short-break | Seconds up to a minute | Lighting, small process, lifts | Generator via ATS, sometimes with fast start |
| Long-break | Minutes up to hours | Comfort HVAC, non-critical equipment | Generator after higher classes, or no backup |
Why separate them
A UPS sized for the whole building is expensive and wasteful. Putting only no-break loads on the UPS, short-break loads on the generator via ATS and long-break loads on late restoration cuts capital and keeps generator loading orderly.
Practical split (illustrative)
A site has 800 kW total, of which 150 kW is no-break, 250 kW short-break and 400 kW long-break. UPS: 150 kW plus margin. Generator: 400 kW plus margin for starting, with the long-break load restored in steps. The generator is sized for 400 to 800 kW depending on policy.
Design consequences
- Separate switchboards or sections for each class.
- Clear labelling.
- Restoration order defined in control logic.
See critical and essential systems and restoration sequencing.
Common mistakes
- Assuming a process rides through without testing the contactors.
- Putting short-break loads on the UPS to be safe, using up capacity.
- Failing to plan the restart of long-break loads.
What to send us
Send your load list with the tolerable interruption for each load. Send your application details. See also critical and essential classes.
Questions engineers ask
What interruption can a standby generator system hide?
None. It has a gap while the engine starts and the transfer happens, usually seconds. Only UPS or battery systems bridge that gap.
Do all loads need a defined class?
For design purposes yes. Unclassified loads default to the most expensive assumption or are overlooked.
What is contactor drop-out?
A starter that opens when voltage dips or is lost, even briefly, so the load stays off until restarted.
Can a load change class?
Yes, if the process changes or the load is given hold-up. Update the register when it does.
This guide gives general principles. Confirm ride-through values with equipment data.