Critical Power Systems: How Generators, UPS and ATS Work Together

When the mains supply fails, different loads fail in different ways. A server or process controller drops out within milliseconds. Emergency lighting must come back within a short, defined time. A chiller or a production line can usually wait a few seconds, but not hours. No single piece of equipment covers all of these needs economically. A UPS reacts instantly but stores limited energy. A generator can run for days but needs time to start. An automatic transfer switch moves the load between sources but generates no power itself. A critical power system combines these elements so that each covers the part of the outage it is suited to.

This article answers the question: what role does each element of a critical power system play, how do they interact during a mains failure and restoration, and what must be decided to make them work together?

Why no single device is enough

Each source of backup power has strengths and limits:

Element	Strength	Limit
Mains (normal supply)	Large capacity, continuous	Outages, voltage dips and disturbances outside the user’s control
UPS with batteries	Supplies the load with no break or a very short one; conditions the output voltage	Limited stored energy (autonomy, typically minutes); power rating sized for the critical load only
Generator set	Long-duration supply limited only by fuel; can supply large loads including motors	Needs time to start and accept load; voltage and frequency deviate during load steps
ATS	Automatically connects the load to whichever source is available	Generates no power; open-transition transfer always involves a short break

Combining them gives what none provides alone: no-break supply for the most sensitive loads, and long-duration supply for all essential loads.

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The elements and what each does

Generator set

A generator set (genset) is a reciprocating engine, usually diesel or gas, driving an alternator, with a controller, a starting system (typically batteries and a starter motor), cooling, fuel supply and exhaust. Generating sets of this type are covered by the ISO 8528 series:

• ISO 8528-1 defines applications, ratings and performance.
• ISO 8528-5 covers the performance of the complete set, including how far voltage and frequency may deviate when load is applied or removed. This is expressed through performance classes.
• ISO 8528-12 covers sets supplying safety services.

Two properties matter most here:

• Rating category. ISO 8528-1 defines power ratings for different duties, for example emergency standby power (ESP) for limited annual running during outages, and prime power (PRP) for unlimited hours at variable load. A standby set in a critical power system is usually ESP-rated, but long or frequent outages may call for a prime rating. See Sizing a Diesel Generator.
• Start and load acceptance. From a start signal, the set must crank, run up to rated speed and build up voltage before it can accept load. It then accepts load in steps, each causing a transient dip in voltage and frequency. The time to readiness and the acceptable step size come from the manufacturer’s data and the set’s performance class.

UPS (uninterruptible power system)

A static UPS converts and stores energy so that its output continues when its input fails. Its performance is specified under IEC 62040-3 (performance and test requirements), and its safety under IEC 62040-1.

IEC 62040-3 classifies a UPS by a performance code:

• Input dependency: VFD (output depends on input voltage and frequency), VI (output voltage independent of input voltage variations, still frequency dependent) or VFI (output independent of both). Online double-conversion UPS designs are typically VFI.
• Output waveform, in normal mode and in stored-energy mode.
• Dynamic output performance: classes describing how much interruption and voltage deviation the output shows when the UPS changes operating mode and when load steps are applied. Class 1 means no interruption.

For critical loads, the classification tells the buyer what will actually happen at the load terminals when the mains fails. It is more useful than topology names alone. UPS topologies are explained in a separate Engineering Insights article.

Other key UPS elements:

• Energy storage, typically a battery. Autonomy is the time the UPS can supply its actual load from the battery. It depends on load, battery capacity, battery age and temperature.
• Bypass: a path, often a static switch plus a maintenance bypass, that connects the load directly to the input supply if the UPS inverter is overloaded or faulty, or for maintenance. The bypass only works if a source is present and, for a no-break transfer, synchronised.
• Rectifier and charger: after the outage, it recharges the battery while supplying the load. This adds to the input demand on the generator.

ATS (automatic transfer switching equipment)

Transfer switching equipment (TSE) disconnects a load from one supply and connects it to another. IEC 60947-6-1 covers TSE for systems up to 1 000 V AC or 1 500 V DC. The current edition is the fourth (2026), which added annexes for bypass/isolation TSE, ATSE with closed-transition capability, stand-alone ATS controllers and TSE for fire-pump controllers. During the transition, projects may still refer to the previous (2021) edition where national standards based on it remain valid, so a specification should state which edition applies. Under the standard:

• ATSE is self-acting transfer switching equipment and includes its controller. Remotely operated (RTSE) and manually operated (MTSE) types also exist.
• The ATS controller monitors the supplies for voltage and frequency deviations and runs the operating sequence: detect, confirm, start the generator, transfer, retransfer.
• TSE is grouped into classes. PC class equipment can make and withstand short-circuit currents but is not intended to break them. CB class equipment has overcurrent releases (circuit-breaker based). CC class equipment is contactor based.
• Transfer can be open transition (break-before-make, with a short break), delayed transition (a deliberate pause in the OFF position, for example to let motor voltages decay), or closed transition (make-before-break, briefly paralleling the sources). Closed transition must be agreed with the utility and the protection design.

Important ATS ratings include rated operational current, utilization category for the load type, short-circuit ratings (withstand, or conditional with a named protective device), number of poles, and transfer times.

Distribution and other elements

• Essential and non-essential boards. Loads are separated so that only essential loads are transferred to the generator. Non-essential loads are shed or left off. Where these boards sit in the overall LV hierarchy is shown in LV Distribution Explained.
• Critical load distribution downstream of the UPS, with selective protection. The UPS can supply only limited fault current, so downstream protection must be selected to clear faults within the UPS’s capability.
• DC systems. Switchgear trip and control circuits, and some process controls, rely on station batteries and chargers. These must ride through the outage and are part of the critical power picture.
• Battery energy storage systems (BESS) can complement this arrangement, for example by supporting the load during generator start or reducing generator running. They are a separate design topic.

AdepaTrade sources generators, UPS systems, batteries, BESS and industrial DC systems, as well as the transfer switching and LV/MV distribution equipment around them. See Critical & Backup Power.

How long can each load wait? Interruption tolerance

The design starts with the loads, not the equipment. Each load, or group of loads, has a maximum tolerable break:

IEC 60364-5-56 (safety services) classifies automatic supplies for safety services by maximum changeover time. The classes run from no-break (continuous supply during changeover), through very short break (within 0.15 s), short break (within 0.5 s), average break (within 5 s) and medium break (within 15 s), to long break (more than 15 s). Safety services, such as supplies required for life safety, are often governed by national rules and the local authority. Systems kept for business continuity (standby systems) follow the owner’s requirements. The changeover class needed for each load must be confirmed against the regulations that apply at the destination.

The operating sequence: from mains failure to restoration

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0. Normal operation. The mains supplies all loads through the ATS (normal position). The UPS supplies the critical loads, charges its battery, and keeps its bypass synchronised with the mains. The generator is on standby: engine heater on, starting batteries charged, controller in automatic.

1. Mains fails. The UPS switches to stored-energy mode. For a UPS with dynamic performance class 1, the critical load sees no interruption. Loads on the essential board that are not on the UPS lose supply, as do non-essential loads.

2. Outage confirmed. The ATS controller detects the voltage or frequency deviation and waits for a short confirmation delay. This avoids starting the generator for a momentary dip that the mains recovers from by itself. It then sends the start signal.

3. Generator starts and stabilises. The engine cranks, starts, runs up to speed and builds up voltage. If a start attempt fails, the controller makes further attempts within a defined crank cycle and raises an alarm if all fail. The ATS waits until the generator voltage and frequency are within its acceptance limits.

4. Transfer. The ATS transfers the essential board to the generator, normally by open transition. The essential loads, apart from the UPS-backed ones, see a break equal to the whole sequence from step 1 to step 4.

5. Load steps. Loads are connected in planned steps rather than all at once, so each transient stays within what the generator can accept. Large motors are started in their own step, often with reduced-voltage starting or drives. The UPS rectifier walks in gradually, ramping up its input current, then supplies the critical load and recharges the battery. From this point the generator supplies everything on the essential board.

6. Mains returns. The controller waits until the mains has been stable for a retransfer delay. This avoids returning to a supply that fails again. It then transfers back. Open transition causes a second short break for the non-UPS essential loads; closed transition avoids this where permitted.

7. Cool-down and standby. The generator runs unloaded for a cool-down period, then stops and returns to standby. The UPS returns to normal mode and completes recharging its battery.

Every delay in this sequence is either a controller setting or a property of the equipment (generator start time, UPS walk-in, ATS operating time). These values come from the project design and the manufacturers’ data. They are not universal constants.

Illustrative scenario: checking that the pieces fit

Scenario (illustrative, not a design): a small facility with a UPS-backed control and IT room, an essential board with pumps, ventilation and lighting, and one standby generator with an ATS.

Assumptions (example settings, stated only to show the method)

• Outage confirmation delay: 3 s (ATS setting).
• Generator time from start signal to ready-to-accept-load: taken from the selected set’s data. Here the symbol T_start is used.
• Crank cycle: up to n start attempts with rest periods (generator controller setting).
• ATS operating transfer time: from the ATS data, T_transfer.
• UPS autonomy at the actual critical load, end-of-life battery capacity and lowest expected room temperature: T_aut (from the UPS and battery data).

Check 1: UPS autonomy against the generator window. The UPS must hold the critical load until the generator is supplying its input, with margin, including the time lost in failed start attempts:

T_aut > 3 s + T_start(including up to n attempts) + T_transfer + UPS walk-in time + margin

If the generator might not start at all, a separate decision is needed. The autonomy can be set to allow an orderly shutdown of the critical equipment, or redundancy can be added (a second generator, or a longer battery). The owner decides, based on how critical the load is.

Check 2: Essential-load break against tolerance. Non-UPS essential loads see a break of about 3 s + T_start + T_transfer. Compare this with each load’s tolerance and with any regulatory changeover class. If a load cannot tolerate it, move it to the UPS or a battery-backed supply. Do not rely on the generator starting “fast enough”.

Check 3: Generator capacity for the load steps. Each step, including UPS walk-in and battery recharge and any motor starting, must stay within the generator’s load-acceptance and transient-performance capability. This check is the subject of Sizing a Diesel Generator.

Check 4: Protection on generator supply. The generator’s fault current is much lower than the transformer’s. Confirm that breakers on the essential board still trip within the required times when supplied by the generator (see Choosing LV Circuit Breakers).

This scenario contains no product data. It shows which figures must be obtained from manufacturers and how they relate.

Typical arrangements (conceptual)

The following are common patterns, presented to explain the principles. Each real installation needs its own engineered design.

• Single generator, single ATS, UPS for critical loads. The simplest standby arrangement, shown in the figure above. It is common in commercial buildings, small industrial sites and telecom facilities.
• Several ATSs at different boards. Each essential board, or each priority group, has its own ATS. Life-safety and critical boards can then be transferred first, and a fault on one board does not affect the others. Any ATS that sees a mains failure can start the generator.
• Paralleled generators. Two or more sets synchronise onto a common generator switchboard. This gives capacity and redundancy (for example N+1) and allows load to be added as each set comes on line, with load shedding if a set fails. It requires synchronising and load-sharing controls and careful protection design.
• Redundant UPS and dual-path distribution. UPS modules in N+1 or 2N configurations, dual-corded loads, or static transfer switches (covered by the IEC 62310 series) for loads that must survive a UPS or distribution fault. Common in data centres and other high-availability facilities.
• MV-connected generation. At larger sites, generators may be connected at medium voltage through a step-up transformer and MV switchgear, with transfer performed at MV. The principles are the same, but the switching and protection equipment changes (see Medium Voltage Distribution & Protection).

Making the generator and UPS compatible

A generator supplying a UPS is a common source of commissioning problems. Points to agree early:

• Generator sizing for the UPS input, not the UPS output rating. Include rectifier losses, battery recharge current and the UPS’s input power factor and harmonic current. Generator manufacturers typically provide guidance on supplying non-linear loads. Some UPS input stages also present a leading power factor at light load, which can affect generator voltage control.
• Walk-in and load steps. The UPS rectifier should ramp up its input current gradually after transfer, so it does not add a sudden step on top of the other loads.
• Frequency and voltage windows. On generator supply, frequency varies more than on mains supply. If the UPS bypass must remain synchronised, its input frequency window and the generator’s frequency stability, set by its ISO 8528-5 performance class and governor, must be compatible.
• Transient performance class. Sensitive loads that are not on the UPS may need a generator with tighter transient limits, a higher performance class under ISO 8528-5.
• Neutral and earthing. Whether the ATS switches the neutral (3-pole or 4-pole) depends on the earthing arrangement of the mains and generator supplies. It affects earth-fault detection and residual current protection, so it must be decided in the design, not left to the supplier.

Specification and procurement considerations

When preparing an enquiry for a critical power package, provide:

• a load schedule grouped by interruption tolerance, with kW, power factor and type (linear, motor with starting method, non-linear) for each load, and any regulatory changeover class;
• the required autonomy for each UPS, at end-of-life battery capacity, and the outage scenario it is meant to cover;
• the generator duty: expected outage frequency and duration, so the correct ISO 8528-1 rating category can be selected; site conditions; required performance class;
• the ATS requirements: rated current, number of poles, utilization category, short-circuit ratings at its location, transfer mode (open, delayed or closed transition), bypass/isolation needs, and controller functions such as load-shed outputs, test modes and communication;
• the earthing system and how neutrals are arranged between sources;
• the required redundancy (N, N+1, 2N) and maintainability;
• the test and documentation evidence expected, such as factory tests and type-test references to IEC 62040-3, IEC 60947-6-1 and ISO 8528.

AdepaTrade sources generator sets, UPS systems, batteries, BESS, industrial DC systems and associated transfer and distribution equipment to project specifications. It does not carry out installation or system integration on site. On request, AdepaTrade can arrange on-site engineering supervision and commissioning support under a separate scope.

Common mistakes

• Putting all essential loads on the UPS (oversized and expensive), or putting no-break loads on the generator only (which assumes the start time is zero).
• Sizing UPS autonomy for a “normal” generator start with no allowance for a failed start attempt, battery ageing or low temperature.
• Sizing the generator for the UPS output rating instead of its input demand, including recharge and harmonics.
• Connecting all loads to the generator in a single step, so the voltage and frequency dip is too deep and the generator trips or the UPS rejects the supply.
• Leaving the ATS neutral-pole arrangement undecided, leading to nuisance residual current tripping or undetected earth faults.
• Forgetting that protection settings chosen for the transformer supply may not operate on the much lower generator fault current.
• Treating a typical arrangement diagram as an engineered design, and skipping the load analysis.
• Ignoring local regulations for safety services, which may set changeover times, autonomy and equipment requirements independently of the owner’s preferences.

Conclusion

A critical power system works because each element covers the part of an outage it is best suited to. The UPS covers the first instants with no break, the generator covers the long run, and the ATS connects the right source at the right time and returns the load safely when the mains is restored. The design starts from the loads and the interruption each can tolerate. It then checks that UPS autonomy covers the generator start window with margin, that the generator can accept the planned load steps including the UPS, and that protection still works on generator supply. The relevant IEC and ISO standards (IEC 62040 for UPS, IEC 60947-6-1 for transfer switching equipment, ISO 8528 for generating sets) define the terms used to specify each element. How the elements fit together is a matter of system design for each project.

Related AdepaTrade solutions

AdepaTrade sources generators, UPS systems, batteries, BESS and industrial DC systems, together with LV and MV distribution equipment. Critical & Backup Power · Low Voltage Distribution & Control

Relevant standards

IEC standards are the primary reference for the electrical equipment, with the ISO 8528 series for generating sets. Confirm the applicable edition for each project. National rules for safety services may add requirements.

Standard	What it covers	Why it matters
ISO 8528-1: RIC engine driven AC generating sets, Part 1: Application, ratings and performance	Rating categories (ESP, PRP, LTP, COP, DCP)	The rating category must match the duty; offers rated in different categories cannot be compared directly
ISO 8528-5: Part 5: Generating sets	Transient performance and performance classes	Classifies how the set responds to load steps; state the performance class the loads need
ISO 8528-12: Part 12: Emergency power supply to safety services	Additional requirements for generating sets supplying safety services	Applies when the generator supplies safety services rather than standby loads only
IEC 62040-3: UPS, Part 3: Method of specifying the performance and test requirements	VFD/VI/VFI classification and dynamic performance classes	Gives a common basis for comparing UPS offers by how the output behaves when the input is disturbed or lost
IEC 62040-1: UPS, Part 1: Safety requirements	UPS safety	The safety reference for the UPS equipment itself
IEC 60947-6-1:2026: LV switchgear and controlgear, Part 6-1: Transfer switching equipment	Transfer switching equipment, including ATSE with its controller, and open- and closed-transition transfer	The standard to specify an ATS against. Some national standards may still be based on the previous (2021) edition during the transition, so state the applicable edition
IEC 60364-5-56: LV installations, Part 5-56: Safety services	Changeover-time classes for safety-service supplies	The maximum changeover time helps decide whether a load needs a UPS, a battery system or a generator
IEC 62310 series: Static transfer systems	Static transfer systems	Relevant where static transfer switches are used in redundant distribution
IEC 60947-2 / IEC 61439 series	LV circuit-breakers and LV assemblies	Govern the breakers and boards around the transfer point; see Choosing LV Circuit Breakers: MCB, MCCB and ACB