Static bypass vs maintenance bypass in a three-phase UPS

Static bypass and maintenance bypass side by side

Vertiv's Liebert APM2 manual and Schneider's Galaxy VS manual describe the same two paths in their own terms. The table sets them side by side. A static transfer switch (STS) is a separate device that selects between two sources; see Where an STS fits, below.

What triggers a static transfer

Vertiv lists the triggers for the APM2: the inverter fails, the inverter is overloaded, or it is shut down manually; the static switch then moves the load to the bypass without interrupting it. Schneider separates two cases on the Galaxy VS. Requested static bypass follows a command from the display, and if a fault is detected the UPS goes back to double conversion or to forced static bypass. Forced static bypass follows a command from the UPS itself, or someone pressing the inverter OFF button.

Overload is one automatic trigger. Schneider's White Paper 75 explains that the points at which a module transfers to static bypass are chosen by the manufacturer to protect the load and the module, and gives a typical rating as an example: a module that carries 125% of its rated load for 10 minutes, or until a component reaches a given temperature, then transfers to static bypass if the load has not returned to normal. The exact limits are in each model's specification.

One mode uses the static bypass path on purpose. In the Galaxy VS eConversion mode, the UPS supplies the active part of the load through the static bypass while the utility is within tolerance, with the inverter running in parallel; that is an efficiency mode, not a fault. Whether a transfer to bypass is allowed at all depends on the bypass tolerance settings covered further down.

What the load is exposed to on bypass

• Unprotected power. White Paper 75 lists it first among the drawbacks of an N (capacity) design: when the module breaks down, the load is transferred to bypass operation, exposing it to unprotected power, and the same happens during maintenance of the UPS, batteries or downstream equipment.
• No battery behind the load. On the Galaxy VS, the batteries are not available as an alternate power source in forced static bypass, internal maintenance bypass or external maintenance bypass.
• A short break if out of sync. The APM2 transfer from inverter to bypass carries a transitory interruption of less than 20 ms if the two are not synchronized.
• The EPO stops the bypass too. On the APM2, the emergency power off shuts down the rectifier, the inverter and the static bypass, but does not disconnect the mains input inside the UPS.

In parallel systems the Galaxy VS adds one more state, static bypass standby: a unit that cannot enter forced static bypass turns its output off while the other units carry the load, and if they cannot, the whole parallel system transfers to forced static bypass.

Maintenance bypass: the order matters

White Paper 75 states the design rule: the maintenance bypass is a normally open circuit that can only be closed when the UPS module is in static bypass, and the design has to prevent it being closed at any other time. Once it is closed, Vertiv notes, the maintenance bypass parallels the UPS static bypass.

Vertiv's APM2 procedure shows the sequence in outline. To go into maintenance bypass: transfer the UPS to internal bypass, release the interlock and close the MBB, then open any battery breakers, then the rectifier input, bypass input and output breakers (RIB, BIB, MIB) to isolate the UPS. To come back: close those breakers, let the UPS start in internal bypass and verify it is there, then open the MBB and transfer to inverter. Vertiv warns that opening the MBB without the UPS on internal bypass drops all output power to the connected equipment.

This is an outline, not a procedure: follow the manual for your model and frame. Schneider, for example, limits the Galaxy VS internal maintenance disconnect to single systems and simplified 1+1 parallel systems without an external maintenance bypass. Bypass transfers are part of the three-phase UPS service we schedule.

Static bypass and maintenance bypass by model

Sources, checked October 3, 2026: values as each manufacturer publishes them for the model shown; “Not stated” means the page we read does not give the value.

Built in or in a cabinet

Schneider lists a built-in static bypass on the Galaxy VS, Easy UPS 3S and Symmetra PX, and sells a maintenance bypass cabinet for the Galaxy VS as an accessory. Vertiv builds the maintenance bypass into the Liebert APM2 up to 150 kVA at 208 V or 300 kVA at 480 V, and offers a cabinet for the ITA2. Socomec's MASTERYS GP4 has an internal maintenance bypass switch, and XPC puts the M90U-80's maintenance bypass in the UPS cabinet. On the large Eaton units the maintenance bypass is external: Eaton lists it as an option for the 9395P and 9395XC, alongside PDU, RPP and STS options.

Bypass in parallel systems

When units run in parallel, the bypass is either centralized in one cabinet or distributed across the units. Eaton allows up to 4 Power Xpert 9395P units with distributed bypass and up to 8 with a system bypass module. Vertiv's Liebert EXL S1 uses a static switch in each module (distributed bypass) for up to 8 units, and Mitsubishi describes a distributed bypass design for multi-module 9900B systems. ABB's Redundant Parallel Architecture on the TLE Series works without a centralized bypass or master control, and Socomec's MODULYS GP has no centralized parallel control and a hot-swappable bypass module.

Bypass settings to check at start-up

The static bypass only takes the load when the bypass supply is inside its tolerance window. Schneider lists a bypass voltage tolerance of plus or minus 10% for the Galaxy VS. Vertiv lets the Liebert APM2 upper limit be set to 10, 15 or 20% (15% by default) and the lower limit from 10 to 40%. On a generator-fed bypass, those settings decide whether a transfer is allowed.

Where an STS fits

An STS sits downstream of two UPS systems or two sources and switches the load between them; it belongs to a dual-path (2N) design rather than to the UPS. Eaton lists STS among the options for the 9395P and 9395XC.

Bypass and transfer tests belong in each preventive maintenance visit, and frames with a built-in static bypass are listed in the three-phase range.

Frequently asked questions

What is the difference between a static bypass and a maintenance bypass?

One is automatic and inside the UPS, the other manual and around it. The static switch takes over by itself when the inverter cannot carry the load; the maintenance bypass breaker is closed by a person, after a transfer to static bypass, so the UPS can be isolated and serviced.

Is the load protected when the UPS is on bypass?

Not by the inverter. The load runs on the bypass source; Schneider's White Paper 75 calls it unprotected power, and on the Galaxy VS the batteries are not available in forced static bypass or maintenance bypass. In requested static bypass, a utility outage sends the Galaxy VS to battery.

Why must the UPS be in static bypass before the maintenance bypass breaker is closed?

White Paper 75 describes the maintenance bypass as a circuit that can only be closed when the module is in static bypass. Vertiv's APM2 procedure transfers the UPS to internal bypass first, and warns that opening the MBB without internal bypass drops the output.

Does the APC Galaxy VS have a static bypass?

Yes. Schneider lists a built-in static bypass on the Galaxy VS (GVSUPS20KFS, GVSUPS100KGS), with a maintenance bypass cabinet sold as an accessory.

Does the Vertiv Liebert APM2 have a maintenance bypass?

Vertiv offers an integrated maintenance bypass up to 150 kVA at 208 V or 300 kVA at 480 V, and the APM2 bypass module is hot-swappable.

How many Eaton 9395P units can share a bypass?

Eaton lists up to 4 units with distributed bypass and up to 8 with a system bypass module.

Is a static transfer switch the same as a static bypass?

No. The static bypass is inside the UPS and transfers the load to the bypass supply; an STS is a separate device that switches the load between two independent sources.

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