Aug 11, 2025
UPS Maintenance Bypass vs Static Switch vs Manual Bypass Rotary – What to Use and When
Introduction
Bypass paths and source selectors prevent downtime during faults or maintenance. But a maintenance bypass, a static transfer switch (STS), and a manual bypass rotary are not interchangeable. Here’s how to pick the right device for your design and use it safely.
What each device does
Maintenance bypass: A manual path around the UPS so you can service or replace it without cutting power to the load. Often implemented with interlocked breakers or a bypass switchboard.
Static transfer switch (STS): An electronic device (solid‑state) that quickly selects between two live sources (A/B) for a load. Ideal for fast transfers when sources are synchronized and healthy.
Manual bypass rotary: A robust rotary switch that lets operators choose among positions (e.g., Normal → Bypass → Off). Common in small/medium installations; slower than STS and requires operator action.
Where they sit in the one‑line
Maintenance bypass: Parallel to the UPS output, upstream of the critical load panel.
STS: Downstream of two conditioned sources (e.g., UPS A and UPS B) to feed single‑corded critical loads.
Manual bypass rotary: Between UPS and load or as part of the UPS output panel for small systems.
Selection and sizing
Size devices by kW/kVA with headroom for inrush and fault levels.
Confirm voltage, phases (1/1, 3/1, 3/3), frequency, and neutral/earthing scheme.
For STS, ensure source synchronization windows and phase rotation are within vendor limits.
For manual bypass/rotary, use mechanical interlocks to prevent unsafe backfeeds.
Safety and commissioning SOP (printable)
Verify source voltages, phase rotation, and earthing continuity.
Prove interlocks (mechanical/electrical) before energizing.
Perform no‑load functional transfers (Normal ↔ Bypass / A ↔ B).
Perform under‑load transfers with observers and rollback plan.
Test alarms/indications and label positions clearly.
Log results and sign‑off with date, serials, and witness names.
Common pitfalls
Misunderstanding STS as a maintenance bypass substitute.
Feeding STS from poorly regulated generator sources outside sync windows.
Undersizing manual bypass contacts for inductive loads.
Skipping earthing checks and protection coordination.
FAQs
Do I need both an STS and a maintenance bypass? In dual‑bus designs supporting single‑corded loads, yes—STS ensures fast source selection; maintenance bypass allows safe UPS service.
Will an STS guarantee “no‑break”? Only when sources are healthy and synchronized. Under faults, a transfer may be inhibited to protect the load.
Is a manual bypass rotary enough for small labs? Often yes, if downtime during manual operation is acceptable and safety interlocks are in place.
