What is an STS, and why does it matter for emergency power in production?

A Static Transfer Switch, STS, is the electronic switch that monitors two power sources and moves a selected critical load from one to the other if the active source falls outside acceptable limits for voltage or frequency. It is the technical link that actually determines whether a battery system can become more than "we have a battery": whether specific processes and controls can continue when the primary supply fails.

This article assumes the basic difference between an STS and an ordinary automatic transfer switch (ATS), which is covered elsewhere. Here we go one level deeper: where in the installation an STS makes sense to place, how it is combined in practice with a BESS, and what analysis should underpin the decision before anyone invests in the solution.

What an STS does: a short refresher

An STS has two inputs and one output. It continuously monitors both power sources and automatically switches the connected load to the source that is within normal limits if the active source fails, experiences a voltage dip, or otherwise becomes unstable. Because the transfer happens electronically with no moving parts, the transition can be fast enough that sensitive equipment such as PLC controls, servers, and automated production lines do not register the incident as a real outage, unlike an ordinary mechanical switch, where the transfer itself takes a noticeable fraction of a second.

That difference is exactly why speed matters. Some industrial equipment, including control systems and process equipment with built-in logic, can react to even a short voltage deviation by stopping, entering a fault state, or losing its state in an ongoing process, which can cost a batch or a restart procedure even if the interruption lasted only a fraction of a second. The decisive point is not whether the equipment "can tolerate being without power for a moment," but whether it interprets the brief fluctuation as a fault it must respond to.

Where an STS is actually placed in the installation

The most widespread misunderstanding about an STS is that it sits as a kind of main switch for the entire factory, a central divider between grid and backup for all electricity use on the site. That is rarely how an STS is used in practice. An STS only protects against faults between its own installation point and the load, not against faults further out in the installation. That is why peer-reviewed research on STS applications points out that an STS is most effective when placed as close as possible to the critical load it is meant to protect: an STS at the main switchboard only protects against faults outside the building and remains exposed to any single point of failure in the distribution between the main switchboard and the load. Research on large data processing centres in North America also shows that most interruptions to sensitive loads stem from faults in the distribution system between the source and the load, not from the source itself. The consequence is that several smaller STS units close to each critical load typically protect better than one large STS near the top of the installation.

That fits how Danish electrical installations are typically built: with a main switchboard and one or more sub-switchboards supplied from the main switchboard, as described in guidance from Sikkerhedsstyrelsen (the Danish Safety Technology Authority) on the correct construction of switchboards. An STS is normally placed at or close to the sub-switchboard that supplies the selected critical loads, not at the main switchboard and certainly not between the grid connection and the production's entire consumption. That gives a more bounded and less costly design, because only the power the critical loads actually draw needs to pass through two independent sources.

Independence between the two sources is a precondition, not a detail. They must have a very low probability of failing at the same time, and neither may depend on the other. If both sources share the same transformer or physical routing path, a single incident, such as an excavator hitting a cable, can take both sources out at once and cancel the entire point of having two sources. The same research review also notes that the two sources' grounding must be of the same type and connected to the same grounding system, to avoid dangerous voltage differences at the STS itself: a requirement that sets concrete expectations for switchboards and earth connections before an STS can make sense.

How an STS is combined with BESS

When an STS is combined with an industrial BESS, it is typically because the battery supplies one of the two sources the STS monitors, while the grid supplies the other. That changes the battery's role for that specific critical load: from being solely a commercial flexibility resource to also being a technically connected backup source for what has been defined as critical.

That is not the same as saying the battery automatically "becomes emergency power" because it is connected to the installation. The combination requires that the system is sized and controlled so it can deliver the necessary power to the prioritised loads when the grid fails, while the rest of the capacity continues to be used commercially when the grid is stable. The STS makes the separation possible: it ensures that only the selected critical load switches to the battery, while the rest of the factory's consumption stays on the grid instead of suddenly placing an unknown load on the battery at the same moment.

The same principle of independent sources and clearly bounded loads that applies to STS placement also applies to prioritisation itself: the point is not that more sources automatically give more security, but that a clear definition of which loads actually need protection makes it possible to hit the right balance between investment and risk, rather than oversizing to cover everything.

In practice, an STS combined with BESS therefore typically forms part of an architecture with three elements for the critical load. The grid is the primary source. The battery is the secondary source the STS switches to. And the STS itself is the monitoring link between them. If there is also a generator in the installation, it can function as a third, longer-duration source if the outage lasts longer than the reserved battery capacity is sized for, but that requires a separate design and is not a precondition for STS and BESS to work together on shorter incidents.

What needs to be analysed before an STS is chosen

Because an STS is always designed for a specific, bounded part of the installation, it cannot be chosen or sized without a concrete technical analysis of that part. In practice the analysis falls into six areas.

Critical loads. The first and most important step is to identify which equipment and processes genuinely must not be interrupted, as opposed to what is merely inconvenient to lose. Energistyrelsen (the Danish Energy Agency) guidance on emergency power for organisations points out that some functions, such as ventilation, cooling, and pumps, can often tolerate a short interruption, while others cannot. The list of genuinely critical loads is rarely as long as a first, intuitive assessment suggests.

Power demand. Once the critical loads are identified, their combined and simultaneous power demand must be mapped, not only their nameplate ratings individually. That determines how large a share of the battery's capacity must actually be reserved, and how large an STS solution must be sized to handle.

Inrush currents. Motors, compressors, and other equipment with moving parts draw markedly more current at the moment they start or restart than during normal operation, as covered in general technical literature on transfer-switch equipment. That is a well-known issue for any switch between two sources, because the unit must be able to handle the transient load without failing. If several motor loads restart at the same time after a transfer, that must be included in the sizing of both the STS and the source behind it; otherwise the restart itself can trigger a new fault.

Phases. Is the critical load single-phase or three-phase, and is the load reasonably evenly distributed across the phases? The IEC standard for STS performance, IEC 62310-3, covers single-phase, phase-to-phase, and three-phase static transfers up to 1000 volts, including whether the two sources share a common neutral or each have their own. That is not theoretical: incorrect handling of neutral and grounding between two sources can create dangerous conditions, not only operating disturbances.

Switchboard structure. How is the installation built up (main switchboard, sub-switchboards, which circuits supply which machines), and where in the structure can an STS naturally be inserted without a thorough rebuild? The answer is typically found in the installation's single-line diagram, which normally documents incoming supply, main breakers, transformers, distribution paths, and critical equipment such as UPS, batteries, generators, and transfer switches with their connection points. Without an up-to-date electrical diagram, it is in practice not possible to assess where an STS is most appropriately placed.

Requirements for transfer time and backup runtime. Finally, it must be clarified how sensitive the equipment is to short interruptions, and how long supply must be maintained if the grid is gone for more than a few seconds. The first determines whether an STS is the right solution category rather than a slower mechanical switch. The second determines how much capacity must be reserved, and whether a supplementary, longer-duration source such as a generator is needed. Both are answered through concrete analysis of the company's equipment and risk picture, not a general rule of thumb.

A concrete example of this kind of split is the model where an installation is divided into two supplies: one fed exclusively from the grid and not covered by backup, and one fed by both the grid and a backup source, where only the prioritised functions are connected to the latter. That is precisely the type of structural separation an STS analysis should establish for the individual company, regardless of whether the backup source is a generator, a UPS, or a battery.

Why STS is an optional add-on in ZynexGroup's model

Because an STS solution always requires this specific analysis, it does not make sense to include it as a fixed part of a standard system. Our standard model is about commercially optimising an industrial BESS in the market, against a fixed contractual availability payment. STS, emergency power, and reserved capacity are instead optional add-ons that we offer case by case, precisely because the solution must be designed for the individual installation, not delivered as a finished package.

When a customer wants to explore an add-on such as STS, we coordinate the process and work with partners on the concrete installation on site. We do not ourselves recommend a particular generator or UPS architecture as part of what we specify, but we help establish which loads are genuinely critical, and how an STS solution best fits the existing switchboard structure.

How reserved capacity affects the economics

When part of a BESS's capacity is reserved to supply an STS-protected critical load during a grid outage, that capacity is no longer fully available for commercial optimisation on the same terms as the rest of the system. The more capacity that is reserved, the less we can optimise. In practice that can mean that the calculation basis and the fixed availability payment are adjusted proportionally in the concrete agreement, without any fixed percentage or guaranteed model. It is an option assessed after the analysis this article has covered, not a fixed surcharge or deduction that can be looked up in advance. For companies where the consequence of a short outage is large, the lower commercial utilisation of the reserved capacity can still be a reasonable trade-off, but it is a trade-off made concretely, not assumed in general.

Overall perspective

An STS is not where "emergency power" starts in the abstract. It is the concrete technical link that determines whether a decision to prioritise certain loads can be carried out without a noticeable interruption for equipment that cannot tolerate one. The solution is not found by placing a switch somewhere in the installation and hoping it fits, but by mapping which loads are actually critical, how large their power and inrush demand is, how switchboards and phases are built up, and how long supply must be maintained if the grid is gone. When the battery and the STS are combined on that basis, the difference between "we have a battery" and "our critical load can continue" becomes a concrete property of the system, not an assumption.

Let us assess the potential for your facility

If you are considering whether an add-on such as STS or reserved capacity makes sense for your production, it starts with a mapping of your critical loads, switchboard structure, and consumption profile. We use that type of data for an initial assessment of whether there is a match between your situation and our model, and whether there is a concrete reason to proceed with a technical analysis of an add-on. You are welcome to contact us here.