A UPS (Uninterruptible Power Supply) is a device that, when an outage or momentary voltage sag occurs, immediately supplies power from its internal battery so that critical loads such as servers and production equipment are not damaged by a sudden loss of power.
| Type | Characteristics | Application |
|---|---|---|
| Standby (Off-line) | Connected directly to utility power in normal operation, transfers on outage (delay of a few ms) | Small loads such as PCs |
| Line-interactive | Includes voltage correction, short transfer time | Small to mid-size server rooms |
| Online (double conversion) | Continuous AC→DC→AC conversion, transfer time effectively zero | Data centers, industrial production lines |
The view from the plant floor — On a production line, even a momentary outage can cause process defects or large equipment restart costs, so the online type, with no transfer time, is the standard. UPS batteries have recently been shifting from lead-acid to lithium-ion; this reduces installation space and extends life, but it also raises initial investment and the requirements for fire safety design, and both should be weighed together.
Where each topology sits in the power path
The three types differ less in their batteries than in where the load sits relative to the utility. In the standby type, the load runs directly on utility power, and the UPS only acts after it detects a failure; the few ms of transfer delay is a gap the load must tolerate. In the line-interactive type, the utility still feeds the load, but the voltage correction function absorbs part of the disturbance before a transfer is needed. In the online type, the load is always fed from the inverter, so a utility disturbance never reaches the load directly and there is no transfer event to ride through.
This is why “transfer time” is the wrong single figure to compare. What matters for a production line is whether the control power for PLCs, servo drives, and sensors ever sees the disturbance at all. With a standby unit, it does, briefly. With an online unit, it does not, as long as the inverter and battery are healthy. The price of that isolation is continuous conversion: the AC→DC→AC path is always running, so its losses and heat are present every hour of operation, not only during outages.
Interlock points to confirm first
Before any discussion of topology or battery chemistry, the safety interlocks around the UPS have to be fixed, because they decide how the unit can be maintained and shut down safely.
- □ The EMO (Emergency Off) circuit is hardwired and opens the UPS output and battery path directly; it is not implemented only as a software command through the PLC or building management system.
- □ The maintenance bypass switch is interlocked so that the load cannot be transferred between inverter and bypass in a way that parallels two unsynchronized sources.
- □ The battery circuit breaker status is wired back as a signal, so an open battery breaker is visible before the next outage, not during it.
- □ The UPS alarm and “on battery” signals are mapped into the PLC or SCADA I/O map, so the line can start a controlled stop sequence rather than waiting for the battery to run out.
- □ For lithium-ion installations, the fire detection and ventilation interlocks required by the battery room design are confirmed before commissioning.
Character comparison by topology
| Aspect | Standby | Line-interactive | Online |
|---|---|---|---|
| Load exposure to utility | Direct until transfer | Partly corrected | Isolated |
| Transfer event | Yes | Yes, shorter | None |
| Continuous conversion loss | Minimal | Low | Always present |
| Fit for control power | Weak | Conditional | Strong |
The takeaway from this table is that the choice is a trade between isolation and continuous losses. Where a momentary disturbance on control power means scrapped product or a long restart, the online type’s isolation is worth its conversion losses; where the load simply needs time to shut down cleanly, a standby or line-interactive unit can be sufficient.
Lead-acid or lithium-ion: what the shift actually changes
The move from lead-acid to lithium-ion changes the facility around the UPS more than the UPS itself. Smaller installation space frees floor area in the electrical room, and longer life reduces how often the battery string has to be replaced. On the other side, the higher initial investment has to be justified over that longer life, and the fire safety design requirement brings in detection, ventilation, and separation items that a lead-acid room may not have had. The decision is therefore a facility decision, and it should be reviewed together with the room layout and fire protection plan, not as a battery purchase alone.
Execution checklist
- □ List the loads that must never see a disturbance (PLC, servo, sensor, and network power) separately from loads that only need time to stop.
- □ Assign the online type to the first group, and decide the second group case by case.
- □ Put the UPS transfer behavior and the upstream breaker behavior on the same time chart and confirm which acts first.
- □ Define the controlled stop sequence the line runs when the “on battery” signal arrives, and confirm the battery covers it with margin.
- □ For lithium-ion, confirm the fire safety design items with the room layout before placing the order.
- □ Schedule periodic discharge testing and record the results, because a battery that has never been tested is an assumption, not a backup.
- □ Do not assume the momentary voltage change that actually appears on control power during transfer; it requires waveform analysis and field measurement.
Boundary conditions
The online type’s advantage reverses when the protected load does not need isolation and simply needs time to stop: in that case, its continuous conversion losses buy nothing, and a standby or line-interactive unit does the job. The lithium-ion advantage reverses where floor space is not scarce and the site cannot meet the added fire safety design requirements, because the space and life benefits then do not offset the higher initial investment and the extra design burden.
The single selection criterion is this: must the load never see the disturbance, or does it only need enough time to stop safely?
