Backup power for buildings: generator, UPS and ATS

In short

How much backup power a building needs depends on what a power cut would cost. Azerbaijani and Russian rules sort electrical loads into three reliability categories. Category I, which includes fire safety systems, lifts and emergency lighting, needs two independent sources and may lose power only for the time an automatic switch-over takes. A UPS covers that gap with no break or almost none. A generator takes over within seconds (up to 10 seconds for a US "Type 10" system) and runs for hours on its fuel. An automatic transfer switch (ATS) moves the load between the sources. Fire safety systems also need their own batteries and cables that keep working in a fire.

Why it matters

According to the International Energy Agency, "on 3-4 July 2018, Azerbaijan was hit by two blackouts that left most of the country without electricity, and in some cases without water, for several hours. Thirty-nine cities and districts across the country were affected." A building whose fire pumps, lifts and emergency lighting depend on the grid alone is only as reliable as the grid.

Reliability categories

In Azerbaijan, the categories are set by Cabinet of Ministers Decision No. 329 of July 29, 2019, for residential and public buildings and utility facilities. Its definitions match those of the Russian electrical installation rules (PUE, 7th edition):

• Category I. Two independent, mutually redundant sources. If one fails, power may be interrupted only for the time of automatic restoration. A special group of Category I loads, those that must keep working to prevent threats to life, explosions and fires, gets a third independent source. UPS units and batteries may serve as the second source, or as the third for the special group.
• Category II. Two sources, but the interruption may last as long as duty staff or a mobile crew need to switch to the reserve.
• Category III. One source, provided that outages for repairing or replacing a damaged element do not exceed one day.

Decision 329 puts the following in Category I, among others:

• in residential buildings: fire protection equipment (fire pumps, air pressurization, smoke extraction, fire alarm and warning systems), lifts, emergency lighting and warning lights on tall structures;
• the fire protection systems and emergency lighting of offices, schools, hotels, shops with more than 500 m² of sales area and restaurants with more than 50 seats;
• all loads of office buildings with more than 1,000 employees or more than 10 storeys.

The other loads of a residential building are Category II or III. Buildings with electric stoves are Category II, except those with 1 to 8 apartments, which are Category III. Buildings with gas or solid-fuel stoves are Category II above 5 storeys and Category III up to 5 storeys.

For high-rise buildings, the Azerbaijani code AzDTN 2.7-9 goes further. Passenger and fire lifts, fire protection systems, emergency and evacuation lighting, communication and monitoring systems and the building control center form the special group of Category I. A third independent source must keep them running for 3 hours.

UPS: three types

The international standard IEC 62040-3 classifies static UPS units by how much their output depends on the mains:

• VFD (standby, or offline). Protects only against a complete loss of power; otherwise the output follows the mains voltage and frequency.
• VI (line-interactive). Keeps the output voltage within declared limits using correcting circuits.
• VFI (online, double conversion). The output does not depend on mains voltage and frequency variations, and the unit corrects them without using the battery.

According to Schneider Electric, standby and line-interactive units take "almost 10 ms" to switch to battery. A double-conversion unit has a transfer time of 0 ms, because no switch operates. In its high-efficiency ("eco") mode, a double-conversion UPS behaves like a standby unit, with a transfer time under 10 ms. Line-interactive units dominate from 0.5 to 5 kVA; double conversion is the most popular type above 10 kVA.

Generator: how fast and how long

NFPA 110, the US standard for emergency and standby power systems (current edition 2025), classifies generator systems by:

• Type, the maximum time from loss of power to restoration. A Type 10 system restores power within 10 seconds.
• Class, the minimum time the system runs at its rated load without refueling: 5 minutes, 15 minutes, 2, 6 or 48 hours. The fuel must be sized for 133% of what the class requires.
• Level. Level 1 is for systems whose failure could cost lives or cause serious injury; Level 2 is for less critical ones. Level 1 typically supplies life safety lighting, fire alarm systems, lifts, fire pumps, emergency communications and smoke removal.

An NFPA engineer notes that "battery failure is the most common cause of generator failure": if the starter batteries fail, the whole system does not start.

Automatic transfer switch

An ATS moves the load from the normal source to the emergency one and back. The international standard for transfer switching equipment up to 1,000 V AC is IEC 60947-6-1; its fourth edition was published in April 2026. It covers manual, remotely operated and automatic devices, but not static transfer switches, which have their own standard (IEC 62310). The 2021 edition defined automatic transfer switching equipment as self-acting equipment "including all necessary sensing inputs, monitoring, and control logic."

Power for fire safety systems

Russia. A new edition of the code SP 6.13130 was approved by order No. 1263 of December 29, 2025. The order takes effect six months after it was issued, and it replaces SP 6.13130.2021. Its main rules:

• All loads of fire protection systems are Category I. For hospitals with round-the-clock inpatient care and for data centers, they belong to the special group.
• The systems are fed from a dedicated fire protection panel. It connects to the building's input panel that has an ATS, or to a separate board with its own ATS.
• During the switch-over, the fire alarm and control panels are bridged by a UPS or batteries.
• Batteries are sized for 24 hours on standby plus 1 hour in the "Fire" mode.
• The wiring must keep working in a fire:
  • 1 hour for evacuation lighting;
  • 1 hour for the warning system (less is allowed, but not less than the evacuation time);
  • 2 hours for firefighter lifts.
• Cables follow GOST 31565. For example, ng(A)-FRLS is used in apartment buildings. ng(A)-FRHF is used in residential buildings taller than 75 m, public buildings taller than 50 m, theaters and shops. ng(A)-FRLSLTx is used in kindergartens, hospitals and hotels.
• Residual current devices, arc fault devices and fuses are not allowed in fire protection circuits. An emergency lighting circuit may have at most 20 luminaires.

USA. According to NFPA staff, the standby supply of a fire alarm system under NFPA 72 must run it for 24 hours on standby and then 5 minutes in alarm, or 15 minutes for voice evacuation systems. If a generator is the standby source, batteries are still required, sized for 4 hours.

UK. Under BS 5839-1 (2013 text, as quoted by the Fire Industry Association), the batteries of Category M and L systems last 24 hours on standby, then 30 minutes of alarm. With an automatically started generator, 6 hours plus 30 minutes.

Azerbaijan, high-rise buildings. AzDTN 2.7-9 requires:

• a separate 0.4 kV feed to every switchboard in a fire compartment;
• at least 1 m between mutually redundant cables;
• cables from the substation and the independent source to the input panels, and from there to fire lifts and fire safety systems, laid separately in fire-resistant ducts or made fire-resistant.

Fire-resistant cables

• IEC 60331 tests whether a cable keeps working in a fire with mechanical shock, at 830 °C or more, for cables rated up to 0.6/1 kV.
• BS 8519:2020, the UK code for fire-resistant cable systems in life safety and firefighting applications, uses categories for 30, 60 and 120 minutes of survival.
• European cable classes from Aca to Fca, set by EU Delegated Regulation 2016/364, describe how a cable reacts to fire, not whether it keeps working in one. According to Europacable, the fire resistance of fire-resistant cables cannot currently be classified under the EU Construction Products Regulation.

Testing

Under NFPA 110 (2022 text; the 2025 revision reports show no change to these clauses):

• The system is run under load at least once a month, for at least 30 minutes. A diesel generator must either reach the exhaust temperature the manufacturer recommends or run at no less than 30% of its nameplate rating, to avoid "wet stacking": unburned fuel building up in the exhaust.
• Transfer switches are operated monthly.
• Level 1 systems are inspected weekly. Every 36 months they are tested for the duration of their class, up to 4 hours.

Common mistakes

• Treating a generator as instant. Even a Type 10 system may take 10 seconds; systems that cannot stop need a UPS or batteries in between.
• Running a critical UPS in eco mode. It then behaves like a standby unit, with a transfer time under 10 ms instead of 0.
• Neglecting the generator's starter batteries. NFPA staff name them as the most common cause of generator failure.
• Choosing fire circuit cables by their EU class. That class describes reaction to fire, not circuit integrity.
• A residual current device in a fire pump circuit. The Russian code forbids it in fire protection circuits.
• Mixing battery rules from different countries. 24 hours plus 1 hour, 24 hours plus 5 minutes and 24 hours plus 30 minutes all exist; use the rule of the code the project follows.