Static bypass switch of a UPS: the thyristor transfer path to the bypass mains, its overload and short-circuit ratings, synchronisation window and backfeed protection
Inside double-conversion UPSs · Automatic transfer on inverter fault or overload · Bypass overload such as 1600 % for 100 ms · ±1, ±3 or ±10 Hz synchronisation window · Backfeed isolation options
In normal operation an online UPS feeds its load from the inverter. The static bypass switch is the second path: a solid-state switch, with its protective device, that connects the load directly to the bypass mains when the inverter fails, is overloaded or is switched off, and hands it back once the inverter is ready. Because the switch carries the full load and any fault current while it is closed, and because it can only transfer cleanly when inverter and mains are in step, a buyer has to look at its overload and short-circuit ratings, the frequency window within which the UPS will synchronise and transfer, how it is isolated for service, and how backfeed into the bypass supply is prevented.
Static bypass switches built into on-line UPSs: their transfer function and control signals, bypass overload and short-circuit ratings, synchronisation window and slew rate, service disconnect and maintenance bypass, and backfeed protection of the bypass input. Thyristor stacks and cooled switching assemblies for bypass duty are covered on the Thyristor power assembly for UPS static bypass page; complete UPSs on the Uninterruptible power supply (UPS) page; automatic transfer switches between utility and generator on the Automatic transfer switch page. Stand-alone static transfer switches selecting between two independent sources are outside this page.
CNFX editorial research · Sources checked October 19, 2026 · 3 sources
What the static bypass does, and the signals a UPS gives about it
The US Department of Veterans Affairs master specification for static UPS systems (Section 26 33 53) requires a static bypass transfer switch as an integral part of the UPS, consisting of a static switch and a bypass protective device or bypass switch. Its control logic senses the inverter logic signals and alarm conditions and makes an uninterrupted transfer of the load to the bypass AC source, within the specified transient limits, when the UPS malfunctions or an external overload occurs. [1]
The same specification asks for a disconnect that isolates the static bypass transfer switch assembly so it can be removed for servicing, with auxiliary contacts and padlocking in the on or off position, and for a maintenance bypass that can take the load from the static bypass and return it with no interruption. On the module panel it lists a static bypass enable/disable selector and alarms for "Load on static bypass", "Static bypass transfer switch disabled", bypass source over- and undervoltage and frequency, and "Bypass source to inverter out of synchronization". For the inverter it states: "The inverter shall normally operate in phase-lock and synchronism with the bypass source." [1]
For the UPS output, the VA specification sets a phase displacement of ±1 degree of the bypass input on balanced load, an overload capability (excluding battery) of 125 % for 10 minutes and 150 % for 1 minute, and a transient response of ±5 % for a 100 % load step. [1]
Ask which conditions trigger transfer to bypass and back (inverter fault, overload level and duration, manual command), whether transfer is blocked when the bypass is out of tolerance or out of synchronisation, and which of these states are signalled remotely.
02
Bypass overload, short-circuit and synchronisation figures in two UPS ranges
Schneider Electric publishes the bypass ratings of its UPSs separately from the inverter ratings. The figures below are from the IEC technical specifications of the Galaxy VL 200–500 kW and the Easy UPS 3-Phase Modular 50–250 kW. [2][3]
The Galaxy VL page lists both 110 % and 125 % continuous for bypass operation in two separate rows without saying which model or condition each applies to; confirm the continuous bypass rating for the frame size ordered. [2]
Editorial calculation: if the 1600 % figure is taken on the Galaxy VL nominal bypass current of 312 A at 380 V, the bypass would carry about 16 × 312 ≈ 5,000 A for 100 ms; the page does not state the base current for the percentage. At the ±3 Hz default window on a 50 Hz system, the UPS synchronises to bypass frequencies from 47 to 53 Hz; at the Easy UPS 1 Hz/s slew rate, following a 3 Hz shift takes about 3 s. [2][3]
Bypass figure
Galaxy VL 200–500 kW (IEC)
Easy UPS 3-Phase Modular 50–250 kW
Overload on bypass
110 % continuous, 1600 % for 100 ms; another row lists 125 % continuous, 1600 % for 100 ms [2]
≤110 % continuous; ≤125 % for 10 min; ≤150 % for 1 min [3]
Ask for the bypass overload curve, the thyristor I²t and recommended bypass fuse or breaker, the synchronisation window and slew rate settings, and what the UPS does if the bypass is outside the window when transfer is needed.
03
Stopping backfeed into the bypass supply
Schneider lists three ways to provide bypass backfeed protection for the Galaxy VL: an upstream circuit breaker with shunt trip connected to the UPS, installation with the maintenance bypass cabinet, or a backfeed disconnect device kit fitted in the UPS; with that kit the short-circuit rating is given as 65 kA Icc. The Galaxy VL input is also listed with built-in backfeed protection and fuses. [2]
The Easy UPS 3-Phase Modular specification lists "Built-in backfeed protection and fuses" in one input table and a "Dry contact signal for backfeed protection" for the bypass in another, so whether the isolating device is inside the UPS or must be provided upstream, driven by that contact, depends on the configuration; confirm it before designing the bypass feeder. [3] The VA specification adds that, for multiple-module systems, the system control cabinet communicates with the static bypass transfer switch and its bypass disconnect. [1]
Show the bypass feeder, its protective device and any shunt-trip or backfeed contactor on the single-line diagram sent with the enquiry.
04
Bypass information to send with a UPS enquiry
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Bypass supply: [voltage; frequency; single or dual mains]
Load: [kVA; inrush and fault-clearing needs]
Overload: [required % and duration on bypass]
Short circuit: [prospective fault current at the bypass input]
Synchronisation: [window; slew rate]
Backfeed: [built-in or upstream device; shunt trip contact]
Service: [static bypass disconnect; maintenance bypass]
Signals: [load on bypass; bypass out of sync; bypass disabled]
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 19 October 2026
Buyer questions
When does a UPS transfer the load to the static bypass?
The VA specification requires automatic, uninterrupted transfer when the UPS malfunctions or an external overload occurs. [1]
How much overload can the static bypass carry?
Galaxy VL lists 1600 % for 100 ms with 110 % or 125 % continuous; Easy UPS 3-Phase lists up to 125 % for 10 minutes and 150 % for 1 minute. [2][3]
Is a static bypass the same as a maintenance bypass?
No; the VA specification has the maintenance bypass take the load from the static bypass switch with no interruption so the UPS can be isolated. [1]
Sources and what still needs a direct check
Sources checked 2026-10-19. The US Department of Veterans Affairs master specification for static UPS systems and Schneider Electric technical specifications for the Galaxy VL and Easy UPS 3-Phase Modular.
