How to configure a UPS bypass
The bypass is the reason an online UPS can fail without taking the load down, and it is routinely the weakest part of a specification. Two paths exist — one automatic, one manual — and both have hard sizing rules that cost nothing to follow at design time and are expensive to fix afterwards.
The short answer
The static bypass must be rated at no less than the UPS output rating, and in practice at 1.1–1.25 times it so that a momentary overload does not take out the bypass as well. The maintenance bypass must be a physically separate path with a mechanical or electrical interlock that prevents the inverter output and the bypass from being connected to the load at the same time. Neither can be left to the installer to improvise.
Two different devices, two different triggers
Static bypass is inside the UPS. When the inverter reports overload, over-temperature, output short circuit or an internal fault, the control board fires a pair of thyristors and moves the load to the mains in a few milliseconds, without an interruption for most IT loads. It is automatic and not operator-controlled.
Maintenance bypass is an external switch or a bypass cabinet. It exists so the UPS can be powered down completely — inverter off, DC bus discharged, batteries isolated — while the load keeps running from the mains. It requires a human to operate, in a defined sequence, usually with the UPS already on static bypass.
The sequence matters: transfer to static bypass, close the maintenance bypass, open the UPS output isolator, then work on the UPS. Doing it out of order means paralleling the inverter with the mains, which is how output stages get destroyed.
Sizing the static bypass
Match the bypass to the UPS output, not to the load. A 20 kVA UPS with 8 kVA of connected load still needs a 20 kVA bypass, because the transfer happens precisely when the load is at or above rating.
Then add margin. On a 3:1 overload the inverter will hold for a short time and then transfer; if the bypass is exactly at rating, that transfer trips the bypass breaker and the load goes dark — the worst possible outcome, since the UPS just did what it was designed to do.
Also confirm the bypass is rated for the load's power factor. A bypass rated 20 kVA at 0.9 PF cannot carry 20 kVA at 0.7 PF.
Matching the voltage window
The static bypass connects the load directly to the mains, so its input window must cover the actual site voltage. On a nominal 230 V site that fluctuates between 190 V and 260 V, a bypass window of 230 V ±10% will refuse to transfer exactly when the grid is worst — which is usually when the inverter has failed.
For low-voltage export markets the point is sharper. On a 110 V or 127 V system, the bypass must be configured for that nominal, not for 230 V. This is a common specification error when a single model is sold into several markets: the inverter stage adapts, the bypass window is fixed and does not.
Transfer time and what the load will accept
| Transfer behaviour | Typical time | Loads that survive it |
|---|---|---|
| No-break | 0 ms | servers, medical imaging, process control |
| Short break | 4–8 ms | most IT power supplies, telecom transmission |
| Acceptable break | 10–30 ms | equipment with its own holdup, lighting, general office |
Ask for the transfer time with the inverter faulted, not the transfer time from mains failure to battery — the second figure is always 0 ms on a double-conversion unit, and quoting it is misleading.
Breaker coordination
Three protective devices sit in series on the bypass path: the UPS output breaker, the bypass breaker, and the upstream distribution breaker. Their settings must be staggered so that a fault on the load trips the nearest device only. If the bypass breaker is set lower than the UPS output breaker, a downstream short circuit trips the bypass and leaves the load unprotected while the UPS continues to report "normal".
Time-current curves should be checked, not assumed. On small systems a 2:1 ratio between successive devices is a reasonable starting point; on large systems the discrimination study is part of the electrical design.
The interlock is not optional
A maintenance bypass without an interlock is a loaded gun. If the bypass is closed while the UPS output isolator is still closed, mains and inverter are paralleled. Depending on phase relationship, the result ranges from an immediate overcurrent trip to a destroyed inverter bridge.
Acceptable arrangements: a key-interlocked changeover switch, a mechanically interlocked pair of breakers, or a bypass cabinet with a cam-lock sequence. A pair of ordinary breakers with a laminated procedure taped to the door is not an interlock.
Testing and maintenance
Put the bypass on the maintenance schedule with dates, not intentions:
- Simulate a static bypass transfer every 6–12 months, with the load connected, and record whether the load rode through.
- Inspect maintenance bypass switch contacts, terminal temperatures and insulation resistance annually.
- Test the bypass breaker's short-circuit protection annually and confirm the set point has not drifted.
- Thermal-scan the bypass cables and busbars each year; loose joints show up as heat long before they show up as faults.
- Log every transfer event. A rising transfer count means the inverter or the load is degrading, and it is the earliest warning you will get.
FAQ
Can I run the UPS permanently on bypass? Technically yes on most models, and it is sometimes done during a staged commissioning. It removes all protection — the load is on raw mains. Treat it as an emergency state with a time limit and an alarm.
Does a maintenance bypass need its own breaker? Yes, and it must be rated for the full load current plus the same margin as the static bypass. It is a separate circuit path, not a switch on the existing one.
What is a "bypass cabinet"? An enclosure containing the maintenance bypass switch, the interlock, metering and often a distribution board, used when the UPS is far from the distribution room or when the site requires a single operating point.
Why did the UPS transfer to bypass during a motor start? Because the starting current exceeded the inverter's overload curve. The correct fix is to size the UPS on starting current or to move the motor off the UPS, not to widen the overload setting.
Related reading: online UPS architecture, ET6/10KRS PRO long-runtime rack UPS, and the industrial power-frequency range where bypass cabinets are usually specified as part of the package.