Three switches get called “bypass” and people mix them up constantly. The static bypass inside a UPS, the manual maintenance bypass around it, and a standalone static transfer switch are three different devices doing three different jobs. Here is how to keep them straight.
Every online double-conversion UPS has a static bypass built in. It is an electronic switch, a set of SCRs (thyristors), that can carry the load straight from the raw utility input, around the rectifier and inverter, in a fraction of a cycle. It is automatic, it is internal, and most of the time you never see it act.
The static bypass exists for the moments the inverter cannot hold the load on its own. A large inrush or a downstream short draws more current than the inverter can supply, so the UPS transfers to bypass to let the utility deliver the fault current and clear the breaker. A serious internal fault or an over-temperature condition does the same thing. And in eco-mode the unit deliberately parks the load on the static bypass to save energy, with the inverter held ready to take back over in a couple of milliseconds.
The key point: the static bypass is sub-cycle and unconditioned. The transfer happens fast enough that the load rides through it, but while the load is on bypass it sees raw utility, no regulation, no isolation. That is fine for a fault or an inrush; it is the trade-off you accept in eco-mode.
The static bypass keeps the load fed when the inverter trips. It does not let an engineer work on the UPS safely. For that you need a second, completely separate path: the maintenance bypass, also called the wrap-around bypass.
This is a manual switch, usually a make-before-break rotary or a set of interlocked breakers, that routes the load directly from utility to the output, wrapping around the UPS entirely. Once the load is on maintenance bypass, the UPS can be powered down, opened, have modules or capacitors replaced, or be physically removed, all without dropping the critical load.
The maintenance bypass can be inside the cabinet on smaller units, but on larger three-phase systems it is normally an external panel or a separate wrap-around cabinet. The make-before-break sequence matters: the new path closes before the old one opens, so the load never sees an open circuit during the transfer. Get the interlocking and the procedure right and the swap is invisible to the load; get it wrong and you drop the load you were trying to protect. This is service work, not a switch to flip casually.
The first two switches live with one UPS. A static transfer switch is a different animal: a separate device that sits downstream of two independent sources and feeds a single load from whichever source is healthy.
The reason it exists is the single-corded load. A dual-corded server with two power supplies can take a feed from each of two UPS systems and ride through the loss of either one on its own. But plenty of critical equipment, older gear, network hardware, mechanical controls, anything with one power inlet, cannot. An STS gives that single-corded load two paths anyway. It watches both sources and, if the primary degrades or fails, transfers the load to the alternate in a fraction of a cycle, fast enough that the equipment rides through.
An STS pairs naturally with a 2N redundant design: two complete UPS strings, two distribution paths, and an STS in front of each single-corded load to bridge them. You can also feed an STS from two separate utility services, or one UPS plus one utility feed. The platform you choose, an Eaton 93PM with its internal bypass on each string, for instance, sets the source side; the STS handles the choice between strings.
How an STS transfers between sources is the detail that decides whether it is safe for your site. There are two behaviours.
Break-before-make opens the failed source before closing the alternate. There is a tiny dead gap, but the two sources are never tied together. This is the normal mode for an STS when the sources are not synchronised, because closing both at once would cross-connect two out-of-phase supplies, a large fault.
Make-before-break closes the alternate before opening the original, so the load never sees an open circuit. It is only safe when the two sources are in sync and in phase, which is why an STS monitors both sources continuously and only does a make-before-break transfer when they are aligned. If they drift out of phase, it falls back to break-before-make. A good STS makes that decision automatically; you should know which mode yours uses and under what conditions.
The internal static bypass and the maintenance bypass are not optional, every serious three-phase UPS has the first and should be installed with the second. A standalone STS is a deliberate add-on, and it earns its place in one specific situation: single-corded critical loads that need dual-path resilience.
If your load is dual-corded and you already run two UPS systems, you may not need an STS at all, the load handles the redundancy itself. If your load is single-corded and a single UPS feeds it, an STS only adds a second source if a second source actually exists. The STS is the bridge between two sources for equipment that can only accept one. Specify it where that gap is real, not by default.
One more thing to get right at design time: coordination and selectivity. An STS and the UPS bypasses ahead of it have to be coordinated with the upstream and downstream breakers, so a downstream fault is cleared by the nearest device and not by tripping the whole path. Selectivity is easy to overlook and expensive to discover during an outage. Tell us the load, whether it is single- or dual-corded, and the sources available on your data-centre floor, and we will spec the bypass and transfer scheme to match, from any major brand, and coordinate it properly.
A static bypass is built into a single UPS and carries that UPS’s load on raw utility when the inverter cannot, an overload, a fault, or eco-mode. A static transfer switch (STS) is a separate device that sits between two independent sources and feeds one load from whichever source is healthy. One is internal to a UPS; the other ties two sources together.
No. The static bypass is automatic and electronic (SCRs) and acts sub-cycle to keep the load fed when the inverter trips. The maintenance, or wrap-around, bypass is a manual switch that routes the load from utility around the UPS so the unit can be serviced or removed without dropping the load. A UPS needs both.
Often not. A dual-corded load with two power supplies can take a feed from each of two UPS systems and ride through the loss of either one on its own, so it provides its own path redundancy. An STS earns its place with single-corded equipment that has one power inlet and still needs dual-source resilience.
Make-before-break means the alternate source closes before the original opens, so the load never sees an open circuit during a transfer. It is only safe when the two sources are synchronised and in phase; if they are not, the STS uses break-before-make instead to avoid cross-connecting two out-of-phase supplies.
Tell us the application and we will come back within one business day, sizing, the right system, install and a price. Three-phase installs usually need a licensed electrician, so let us know if you have one.