UPS Input and Output Breakers: A Practical Coordination Checklist

An output branch fault should not turn one failed circuit into an outage for every load on the UPS. Yet a breaker schedule showing only ampere ratings cannot tell you which device will open first. The answer changes with the source supplying the fault: the inverter is current-limited, while an available static-bypass path can bring in a much higher fault current from the utility or generator.

For an industrial online UPS, breaker coordination is a system study, not merely a choice between two catalogue trip curves. The engineer needs the installed one-line diagram, the UPS model's fault and bypass behavior, the source fault levels, the cable paths and the actual protective-device curves. This checklist identifies the decisions to make before anyone approves a breaker schedule.

Start at the UPS terminals. Label the rectifier input, the static-bypass input if it is separate, and the UPS output distribution. A common-input unit still has distinct functional rectifier and bypass paths inside the UPS. Add any generator, ATS and maintenance-bypass connection shown in the project design. Then mark the input feeder breaker, bypass feeder breaker, UPS output device and each downstream branch device.

This article concerns AC feeder and branch protection around one UPS system. Battery-string DC protection and the isolation sequence for an external maintenance-bypass cabinet need their own design reviews. Our maintenance-bypass breaker and short-circuit guide addresses the alternate maintenance path.

Before choosing settings, decide where selectivity is required. For a short circuit in one load branch, should only that branch open while the other UPS loads stay energized? The answer belongs on the one-line diagram and in the protection study. It may call for a different design from a fault on the common UPS output bus, where multiple branches cannot remain unaffected.

An input breaker must carry the UPS's specified input demand under the relevant operating states, including battery charging and any permitted overload or bypass conditions. The rectifier and bypass input ratings need separate attention on a dual-input model. A downstream breaker must suit its circuit and load. Continuous current is only the first check.

At each installation point, compare the device's rated breaking capacity and the assembly's short-circuit rating with the maximum prospective fault current. Then review the trip unit or fuse curve against the available fault current and the conductor-protection requirement. The lowest credible fault current at a remote branch also matters: it may be too small to operate an instantaneous element as someone expected. Use the actual device model, setting and manufacturer curve, including tolerances and let-through data where the study requires them.

The earlier C-curve versus D-curve UPS input guide deals with one specific input issue: nuisance tripping at energization. A breaker that avoids an inrush trip still has to protect the cable, interrupt the prospective fault and coordinate with the upstream system. “Use a D curve” cannot answer those questions by itself.

Here is the part a single time-current plot can hide. With the UPS supplying the load from its inverter, output fault current is limited by that model's inverter behavior. On battery, the static bypass may be unavailable because its AC source has failed. A branch device that trips promptly when fed from utility bypass may not clear in the same way on battery. The UPS may instead limit current, alarm or shut down according to its design.

When an acceptable bypass source is available, some UPS designs transfer a severe downstream fault to static bypass. That changes both the prospective fault current and the stress on the static switch and its protective devices. Schneider Electric's published coordination note distinguishes the current-limited inverter from the higher-capacity bypass path and says downstream settings require a site-specific study. Eaton likewise recommends comparing branch-device clearing behavior with the UPS inverter and bypass capabilities rather than treating the internal bypass fuses as protection for the building distribution.

A generator adds another source case. Its available fault current and voltage response may differ from the utility's, and bypass availability depends on the UPS model and generator power quality. The generator-backed UPS specification guide explains why rectifier acceptance and static-bypass readiness must be checked separately. Include the generator case in the protection review if the protected loads are expected to operate on that source.

The table is a review map, not a set of pass values. Actual clearing times and settings must come from the chosen UPS and protective devices, the fault study and the applicable project requirements.

The UPS protects its own components. Its internal fuses, electronic current limit and static-switch protection do not automatically give each load branch a selective protective device. If a small branch fault operates a common UPS or bypass device first, all downstream loads may be lost even though the fault began on only one circuit.

Coordination also has limits. At a high fault level, two breakers' instantaneous regions or let-through behavior may overlap; at a low fault level, an inverter may not supply enough current to make a branch trip quickly. A qualified designer must check both ends of the fault-current range and state where selectivity is achieved, where it is only partial, and what the UPS does when the bypass path is absent.

Neutral and earthing arrangements affect the fault path. Do not copy a three-pole or four-pole breaker layout from another UPS drawing without checking the site's source, bonding and bypass arrangement. This article does not specify a universal pole count or a field switching procedure.

Give the designer and UPS supplier the same information:

• As-built or proposed one-line diagram, with rectifier, static-bypass, output and maintenance paths identified
• UPS model and its input, bypass, inverter short-circuit and static-switch data
• Utility and generator source data, including maximum and minimum prospective fault levels at relevant points
• Feeder and branch cable sizes, lengths, installation conditions and connected loads
• Exact breaker or fuse models, ratings, adjustable settings and time-current/let-through curves
• Earthing and neutral arrangement, plus the circuits where selective isolation is required

Ask the review to return the device schedule, study assumptions and a short list of operating cases that were actually checked. If a branch cannot be selective in battery mode, that limitation should be visible in the handover documents, not discovered during a fault. The UPS installation acceptance test guide shows how to record the installed configuration and agreed functional tests; a commissioning test does not replace the protection study.

The practical question is not “Which breaker is bigger?” It is whether the right device clears the right fault while the UPS is in each mode the site intends to use.

Schneider Electric, Downstream Relay and Breaker Coordination for Galaxy and Symmetra MW UPS Systems — manufacturer explanation of model and site-specific coordination, inverter and bypass source behavior.

Eaton, Power Source Behavior and Its Effect on Power Distribution — inverter limitation, bypass fault duty and load-branch selectivity.