Refinery UPS Selectivity: Static Bypass, Inverter Limits and Branch Fault Clearing

1. Define the continuity requirement

A fault on one instrument branch should not remove power from every control cabinet sharing a UPS output board. Whether that goal is achievable depends on fault location, UPS mode and protective-device behavior. Define which healthy loads must remain supplied and the disturbance they can tolerate. A board-level fault cannot generally be cleared selectively by its own downstream branch devices; set the boundary honestly.

Eaton's power-distribution paper explains that a UPS inverter behaves as a limited source during faults, while bypass exposes the load to a different source path. Manufacturer and model determine limits and transfer behavior. This makes the UPS part of the protection model rather than an ideal voltage source. Source-behavior paper.

2. Draw every operating path

Include rectifier input, inverter output, static bypass source, bypass protective devices, maintenance bypass and any alternate or generator source. State whether neutral is switched and where earthing references exist; the resulting fault paths must match the installation design. Record backfeed protection and interlocks from the selected product, not a generic UPS symbol.

The Eaton 93E manual illustrates product-specific bypass and backfeed arrangements. Use the actual purchased model's installation manual for the final design; another range may have different internal protection, ratings or switching behavior. Example product manual.

Collect maximum and minimum prospective current at the downstream board in each source case. Long branch cables often make minimum fault current the difficult case. Include faults with bypass unavailable or outside its permitted voltage/frequency/synchronization conditions, plus maintenance bypass operation and battery-only supply.

3. A hypothetical inverter-only case

Assume an illustrative 100 kVA, 400 V three-phase UPS. Its rated output current is calculated below. For this example only, suppose a supplier envelope guarantees 2.5 times that current for 200 ms under a specified output fault, after which output ceases if the fault remains. These are invented case assumptions, not Eaton product ratings.

Suppose the chosen 32 A branch breaker's manufacturer curve permits an instantaneous pickup band of 8–12 times rating: 256–384 A. The hypothetical UPS limit falls inside that band. Therefore instantaneous operation is not guaranteed by this comparison. The non-instantaneous curve must also be examined at the available waveform and clearing window. Remaining healthy-load current can reduce what reaches the fault branch; do not allocate the entire inverter limit to it automatically.

This example identifies missing coordination evidence. It does not recommend a breaker curve or predict selectivity for any real UPS. A current-limited non-sinusoidal waveform may require manufacturer confirmation beyond a conventional time-current overlay.

4. Compare the source cases

In bypass, high current can clear a branch faster, but may challenge the static switch or internal fuses first. Compare branch let-through energy and peak current with the supplier's conditional withstand and approved upstream protection. A continuous ampere rating is not a short-circuit withstand rating. Do not infer selectivity from nominal fuse sizes alone.

5. Coordinate without weakening safety

Ask the UPS and breaker/fuse suppliers for tested combinations or a supported coordination assessment covering the actual settings and source envelopes. Consider smaller branch subdivisions, suitable protective devices or a revised architecture when the inverter cannot clear the fault. Extending a shutdown timer is not a substitute for equipment duty verification or required automatic disconnection.

Account for common output impedance, healthy-load voltage collapse and transfer refusal. If the bypass source is the same bus experiencing the initiating disturbance, its availability cannot be assumed. Review retained commands and recovery using the refinery ride-through chain.

6. Acceptance and later changes

Procurement should identify UPS model/firmware, source cases, guaranteed fault envelopes, branch-device curves/settings, internal protection, coordination evidence and expected healthy-load disturbance. Verify approved modes and interlocks through a controlled commissioning method; do not create short circuits on an operating instrument board. Save the final device schedule with cable lengths and bypass configuration.

Reassess added loads, changed breakers, generator substitution and maintenance-bypass modifications. In a protected electrical room, the UPS battery is also an independent energization path: include it in the pressure-loss interface study.

7. Questions and primary references

Does static bypass guarantee selectivity? No. Transfer availability, source impedance, internal protection and switch duty matter.

Can a utility fault calculation represent battery mode? No. The inverter's supported current-time envelope governs that case.

Does an oversized UPS solve every branch fault? No. Device pickup tolerances, current sharing and fault location still matter.

Primary references: Eaton source behavior; Eaton 93E example manual.