“Bypass” on a UPS: When It’s Normal, When It’s Critical (and What to Check On-Site)

Scope, terms and assumptions

This guide covers bypass on online double-conversion UPS systems, from single-phase rack and tower units of about 2 kVA up to three-phase systems in the hundreds of kVA. It explains what each kind of bypass does, why a UPS moves to it, how to tell a planned bypass from a fault, and which checks are safe before a technician arrives. The examples come from APC by Schneider Electric and Vertiv documents; Eaton, Tripp Lite and other brands word their messages differently, but the principles are the same. This guide does not replace your model’s operating manual: where the manufacturer’s procedure differs, the manual wins.

• Static bypass (also called internal, automatic or dynamic bypass): an electronic switch inside the UPS, built from pairs of silicon controlled rectifiers (SCRs), that connects the bypass input straight to the output in a fraction of a cycle.
• Maintenance bypass: a manually operated breaker or rotary switch that carries the load around the UPS so the UPS can be switched off, serviced or removed. It can be built into the unit, fitted to a rear distribution box, or installed as a separate wall-mounted or rack panel.
• Rectifier and inverter: the rectifier turns incoming AC into DC and charges the battery; the inverter turns that DC back into clean AC for the load.
• Bypass available: the UPS has checked that the bypass source is within its voltage and frequency limits and synchronized with the UPS output, so a transfer can happen without a break.

Two assumptions hold throughout. The UPS is an online double-conversion design, because standby and line-interactive units have no static bypass in the sense used here. And the load is IT, telecom, medical or process equipment whose owner wants it to stay powered; where a load must never see raw utility power, bypass may be disabled on purpose, as explained below.

The three bypass paths inside a UPS installation

In normal operation a double-conversion UPS rectifies the incoming AC to DC, keeps the battery charged from that DC bus, and rebuilds AC with its inverter. Vertiv describes this normal mode on its Liebert GXT5 as using the rectifier and inverter to provide voltage and frequency stabilized power to the load. Bypass is any path that skips that conversion.

Figure 1 shows the three paths on one single-line diagram. The static bypass switch sits inside the UPS and is controlled by it; it joins the inverter output inside the cabinet, upstream of the output breaker. The maintenance bypass is a mechanical path, closed by a person, that runs outside the UPS. With the input, bypass-input and output breakers open and the maintenance bypass closed, the whole UPS can be disconnected while the load stays on.

Schneider Electric’s White Paper 96 explains why the middle path has to be electronic. Each phase of a static bypass switch is a matched pair of inversely mounted SCRs, one conducting on the positive half cycle and one on the negative. Electromechanical contactors and breakers are not fast enough for the transfer, so without the SCRs a power disruption could occur. For static switches used as separate transfer devices between two sources, see our guide to the static transfer switch and static bypass.

What your equipment loses on bypass

The load keeps running on bypass, which is exactly why bypass is easy to ignore. What changes is what stands between the load and the next utility event. Table 1 compares the four states an operator is likely to see.

One correction to a common claim: bypass cannot remove “galvanic isolation” from a UPS that never had an isolation transformer, and many do not. What bypass removes on every design is regulation and battery ride-through. On static or maintenance bypass, the load is as exposed as it would be on a wall outlet; Vertiv’s GXT5 manual states that the load is unprotected from disturbances in the power supply while the UPS is on bypass.

The first 15 minutes on site

These checks are safe for a facility or IT person with no electrical qualification, because none of them involves opening a cover, operating a breaker or pressing a UPS control.

1. Read, then photograph, the display and the event log. Record the exact message, the time of the transfer and the load percentage. On the GXT5 the display reads “On Bypass” and, per the bypass-mode description in its manual, the buzzer beeps once every two seconds; other alarms and other brands use different wording and cadences, so photograph rather than paraphrase.
2. Check the load against the rating. If the display shows overload, unplug non-essential plug-in equipment you are responsible for (a printer, a test bench, a space heater). Leave panel breakers and hardwired circuits to a qualified person. Vertiv’s manual warns against connecting devices such as laser printers that would overload the unit.
3. Check the room. Temperature, blocked vents, a failed air conditioner, a door left closed on a small server closet.
4. Protect the servers. On bypass there is no battery time, so a utility outage drops the load at once, without the on-battery warning that shutdown software normally acts on. Plan an orderly shutdown of non-essential systems, or move critical workloads, until the UPS is back on inverter. On the GXT5, if the utility fails or goes out of range during bypass, the UPS shuts down and no output is supplied.
5. Do not cycle the UPS and do not command a transfer back yourself. Switching a unit off and on to “clear” bypass risks dropping the load if the fault is in the output stage, and it erases the state a technician needs to see. The GXT5 returns to normal on its own once an overload or over-temperature clears and normal power is restored; if yours has not, leave it for the technician.
6. Call for service with the photos. A model number, a serial number and a picture of the log shorten diagnosis more than any description. If the unit is under manufacturer warranty or a service contract, contact that provider first; Vertiv’s GXT5 manual, for example, directs owners of a unit in bypass mode to Vertiv Technical Support. Otherwise GDF Technologies answers 24/7 at 514-252-8324.

Why a UPS transfers to static bypass

An automatic transfer is the UPS protecting itself and the load at the same time: rather than shut down on a condition it cannot carry, it hands the load to the bypass source. Table 2 lists the triggers documented in the manufacturer sources cited here.

The opposite condition matters just as much: a UPS can refuse to use its bypass. Schneider Electric describes the SRT “Bypass not in range” event as informational, not a fault: the input is unsuitable for bypass because of voltage or frequency out of range, a distorted waveform, a missing neutral, phase imbalance or short transients (FAQ000275794). Vertiv’s GXT5 specification disables bypass when the input frequency prevents synchronous operation, and re-enables it only when the input voltage returns within plus or minus 10% of the nominal output voltage. While bypass is unavailable the load stays on the inverter, which is fine, but an overload or internal fault in that window has nowhere to go. If that alarm keeps recurring on generator power, our note on testing generator and UPS compatibility when frequency drifts covers the site test.

Not every UPS has a bypass

People often expect a bypass on units that do not have one. Schneider Electric’s FAQ FA335449 states that the Smart-UPS SRT and Easy UPS SRV have bypass capability while the Smart-UPS SMC, SMT and SMX do not. Those three are line-interactive families: when one of them is overloaded or fails, there is no bypass to fall back on, which is why their alarms are covered in our APC UPS error code guide rather than here.

Configuration also matters. The GXT5 can run as a frequency converter, and every frequency-converter setting in its manual is listed with bypass disabled; the factory default is auto-sensing 50 or 60 Hz with bypass enabled. A unit someone reconfigured years ago may therefore have no bypass even though the hardware has one. Check the setting before assuming.

Normal or critical: a decision method

Figure 2 turns the display message into a decision. The question is never “is the load on?” because on bypass it almost always is. The questions are whether the UPS is really on bypass, who put it there, why, and what the next utility event will do.

Two refinements. An overload that clears on its own still has a cause: in GDF’s field experience, a UPS that bounces between inverter and bypass during the working day is usually sized too close to its rating, and the repeated transfers are the symptom, not the problem. And “unexplained” includes the case where nobody on site can read the log. A bypass nobody can explain is treated as critical until someone can.

Operating a maintenance bypass without dropping the load

A maintenance bypass is safe only if it is operated in the right order. The manufacturers’ procedures agree on the principle: the UPS must already be on its internal static bypass before the manual bypass is closed, so both paths are fed from the same source and in phase at the moment the mechanical contact makes.

• Vertiv GXT5, rear distribution box (UPS replacement, section 6.5). Transfer the load to bypass and confirm bypass mode, then turn the maintenance bypass breaker on, wait one minute if the unit was on battery, confirm the UPS is off, and only then open the input and output breakers. The manual also warns that the input breaker on the distribution box does not disconnect power from the manual bypass breaker, which is fed directly from the input terminal block.
• APC SRT with an external make-before-break panel (SRT5KRMXLT and SRT6KRMXLT). Schneider Electric’s FAQ FA305727 requires the UPS to be placed in internal bypass and confirmed in bypass before the panel is moved from normal to test, and again before returning to normal. Skipping it can produce fault messages including P.11, DC bus overvoltage and a bypass failure.

Two design points follow. First, make-before-break matters: the maintenance path must close before the UPS output opens, or the load sees a break. Second, an integrated bypass has limits. The 15 and 20 kVA GXT5 has no internal maintenance bypass, and on the 8 and 10 kVA models the manual requires an external maintenance bypass (or a shutdown) before the distribution box itself is added, changed or removed. An external bypass panel with its own isolation breakers is what makes a zero-downtime replacement of the whole assembly possible; the planning is described in our note on installing an external maintenance bypass for a zero-downtime migration, and a three-phase example is in the maintenance bypass, not the UPS, was the real problem.

For small rack and tower APC systems, Schneider sells service bypass panels as accessories; one example is the APC SBP3000RM service bypass panel, listed at 120 V, 30 A. Panels are model-specific: check Schneider’s compatibility list for your exact UPS model and plug or hardwire configuration before buying, and do not assume an APC panel fits another brand.

Static bypass switch design in large systems

On larger systems the rating of the static switch matters, because the load may sit on bypass through an overload or during maintenance. Schneider Electric’s White Paper 96 separates two designs:

• Fully rated static switches use SCRs on fan-cooled heat sinks and are generally designed to carry 125% of rated load continuously. The paper adds that a fully rated switch is self-protecting against a short circuit in the distribution system.
• Partially rated switches use smaller SCRs with a contactor or electrically operated breaker in parallel. The SCRs carry the load for approximately one second while the contactor or breaker closes, and the switch is typically designed for 100% of rated load. A continuous overload on bypass then either trips the breaker or drops the contactor, and the load is lost.

The white paper compares the designs using field failure rates derived from Mil Spec 217 data (it notes that field rates are normally 3 to 5 times better than the Mil Spec calculation) and a mean time to repair of 6 hours: 4 hours of service response plus 2 hours of repair. Table 4 shows its published figures.

Reading the table: availability is MTBF divided by MTBF plus MTTR. For the fully rated switch, 513,699 / (513,699 + 6) = 0.99998832. A partially rated switch with a contactor and redundant power supplies only matches a fully rated switch without redundancy, and only the fully rated switch with built-in redundancy reaches “five nines” (99.999%). The paper adds that its figures assume well-maintained contactors and breakers, and that a contactor can weld after a downstream short circuit and drop the load on the next bypass-to-normal transition. When comparing large-system proposals, ask whether the static switch is fully rated and what overload it carries continuously on bypass. For the tests to request before a large system returns to normal after a bypass event, see our note on APC Galaxy VS bypass errors and tests before restoring the critical load.

Canadian conditions that affect bypass

Utility voltage. CSA CAN3-C235-83, Table 3, sets the voltage variation limits that many Canadian distributors commit to at the service entrance (Newmarket-Tay Power’s conditions of service are one example). For a 120 V single-phase service the normal range is 110 to 125 V and the extreme range 106 to 127 V; for a 120/208 V three-phase service, the extremes are 110 and 127 V line to neutral, 190 and 220 V line to line. Compare them with the bypass limits of a real UPS. On the GXT5 the bypass voltage upper limit is +10% by default (settable to +15% or +20%) and the lower limit is -15% by default (settable to -10% or -20%).

The calculation: 120 x 0.85 = 102 V, 120 x 0.90 = 108 V, 120 x 1.10 = 132 V, 208 x 0.85 = 176.8 V, 208 x 0.90 = 187.2 V, 208 x 1.10 = 228.8 V. Three practical conclusions follow. At factory defaults, a supply within CSA limits keeps bypass available. Someone who tightens the lower limit to -10% on a 120 V unit makes bypass unavailable whenever the supply sits between 106 and 108 V. And because the GXT5 re-enables bypass only when the input voltage returns within plus or minus 10%, a bypass that was disabled during a deep voltage dip stays unavailable until the voltage recovers above 108 V. The CSA limits also apply at the service entrance; voltage drop in the building’s own wiring lowers what the UPS actually sees. Record the input voltage when a “bypass not available” alarm appears before anyone replaces parts. Schneider suggests reviewing the upper and lower bypass thresholds in FAQ000275794; change them only with the load owner’s agreement, because a wider window lets poorer power reach the load on bypass.

Generators. Standby generators drift in frequency when load steps on or off. A UPS that cannot synchronize keeps the load on its inverter and declares bypass unavailable, as the GXT5 specification describes. That is the correct behaviour, but it means an overload during a generator run has no bypass to fall back on, so generator runs belong in commissioning tests.

Who may operate the switchgear. Installing a hardwired external bypass panel is electrical installation work governed by the Canadian Electrical Code, Part I (CSA C22.1) and provincial rules, and operating it is work on energized equipment under CSA Z462. Put the switching sequence in writing, keep it at the panel, and train the people who are allowed to use it.

Decision checklist and records to keep

Use this list after every bypass event, planned or not. It is short enough to keep beside the UPS.

1. Which bypass: static (automatic), static (commanded), maintenance, or ECO mode?
2. Time of transfer and time of return to normal, from the event log, not from memory.
3. Load percentage before and after, and any equipment added in the previous month.
4. Room temperature and the state of UPS fans and vents.
5. Input voltage and frequency at the time, and whether a generator was running.
6. Any “bypass not available” events in the same period.
7. Who operated anything, in what order, and whether the written sequence was followed.
8. The service report: root cause, parts replaced, firmware version, and the test that proved the return to normal.

Include the bypass path in the maintenance scope, not just the UPS. Our UPS preventive maintenance service includes the event log review, and our overview of online double-conversion UPS systems shows where the bypass sits in the design.

Sources

1. Schneider Electric, White Paper 96, A Hidden Reliability Threat in UPS Static Bypass Switches, Rev 1, A. Kulkarni. SCR construction, fully and partially rated switches, 125% rating, approximately one second contactor bridging, availability table, 10 to 600 kVA and 1 to 4 MVA placement.
2. Vertiv, Liebert GXT5 UPS Installer/User Guide, 120/208 V models. Operating modes, bypass mode behaviour and indication, bypass voltage limit settings and protection limits, frequency converter and ECO modes, distribution box and maintenance bypass procedures.
3. Schneider Electric, FA335449, What does it mean when a UPS is on bypass? Which Smart-UPS families have bypass.
4. Schneider Electric, FAQ000275794, “Bypass Not in Range” alarms on Smart-UPS On-Line SRT models.
5. Schneider Electric, FA305727, proper use of a make-before-break bypass panel with an SURT or SRT UPS.
6. Schneider Electric, FAQ000263263, function of the bypass switch in the Easy UPS 3S system.
7. CSA Group, CAN3-C235-83, Preferred voltage levels for AC systems, 0 to 50 000 V, Table 3, as reproduced in Newmarket-Tay Power Distribution, Conditions of Service COS-230-05, Voltage Guidelines.
8. CSA Group, CSA Z462:24, Workplace electrical safety.
9. CSA Group, CSA C22.1, Canadian Electrical Code, Part I (current edition as adopted by each province).

APC, Smart-UPS, Easy UPS, Galaxy and Galaxy VS are trademarks of Schneider Electric. Vertiv, Liebert and GXT5 are trademarks of Vertiv. Authored by Christian Barkley, Director, GDF Technologies. LinkedIn: https://www.linkedin.com/in/christian-b-a22619a1/

Next step by symptom

In 1,291 work threads with our support team since December 2024, the main subject was battery replacement or supply in about 27%, UPS replacement in about 18%, repair or troubleshooting in about 17% and preventive maintenance in about 11% (approximate shares).

In Canada, send us the model, the serial number and the alarm or code, or call 514-252-8324. If the unit is under manufacturer warranty or a service contract, contact that provider first. Outside Canada, the self-help column applies as written; for a repair, contact the manufacturer or a local service provider.

UPS stuck on bypass? Get it back on protection

GDF Technologies diagnoses and repairs UPS systems of every major brand across Canada, from single-phase SRT and GXT5 units to three-phase systems with external bypass switchgear, with 24/7 emergency response for loads left on bypass. Send the model, the serial number and a photo of the event log, and we will tell you whether it is a load problem, a site problem or a repair. Contact GDF Technologies or call 514-252-8324. For after-hours incidents, see how our 24/7 emergency response for UPS bypass and critical alarms works. If a failed unit has to be replaced, our partner store UPSPLUSBATTERY carries the APC Smart-UPS SRT online range.