UPS and Backup Power

What is a backup generator for data centers and why you need one even if CFE gives you stability

5 min read By Noxtel
Illustration: What is a backup generator for data centers and why you need one even if CFE gives you stability

Mexico’s electrical grid has reasonable availability. For a DC operating 24×7, that is not enough. Redundancy is built with a backup generator that comes online when the commercial grid fails.

A UPS lasts between 5 and 30 minutes depending on the load. A properly sized generator lasts as long as the fuel tank holds, with periodic maintenance and refueling. That is the difference between a controllable outage and an operational shutdown.

Why CFE is not enough even when it gives you stability

CFE reports aggregate availability indicators at the national level. The metric does not count the frequency or duration of outages at a specific site. A DC in an industrial zone can experience between 6 and 20 interruptions per year, some of a few seconds, others of several minutes.

  • Scheduled maintenance: CFE announces maintenance windows with prior notice, but in 24×7 operation there is no acceptable window.
  • Distribution network faults: storms, accidents, wildlife on overhead lines, local transformer failures.
  • Peak-hour overload: in some zones voltage drops or micro-outages occur before protections trip.
  • Extraordinary events: earthquakes, hurricanes, floods affecting regional electrical infrastructure.

    • The UPS covers the immediate transition. The generator covers sustained operation. Without a generator, any interruption longer than 30 minutes leaves the DC without service until CFE returns or the UPS runs out.

    What a backup generator does for a DC

    It is an internal combustion engine (diesel or natural gas) coupled to an alternator that produces three-phase electrical power. When commercial grid drop is detected, the Automatic Transfer Switch (ATS) starts the generator and transfers the DC load to the new source within seconds.

  • Diesel engine: the most common option for power density and fuel availability in Mexico.
  • Natural gas engine: option for DCs with access to a natural gas network; lower emissions and lower fuel cost, but requires a firm gas network. Pipeline gas does not qualify for strict mission-critical certifications (Uptime Tier III/IV) because of the lack of on-site fuel storage under direct control and its weaker transient response to sudden step-loads.
  • Containerized generator: prefabricated solution for DCs without a dedicated machine room or that need additional capacity.

    • Typical capacity and sizing

    A small DC (10 to 30 kW of IT load range) needs a 30 to 80 kVA generator. A mid-sized DC (50 to 200 kW of IT load) needs between 150 and 500 kVA. A large DC (over 500 kW) requires parallel generators of 1 MVA or more.

    Generator capacity must account for IT load plus cooling, because the DC cannot operate without air conditioning. A frequent mistake is sizing the generator only for IT load and discovering that without the chiller the DC shuts down from temperature within 15 minutes.

    Growth factor also applies. It is wise to oversize between 25 and 40 percent over current maximum load to leave margin for natural DC growth over the next 3 to 5 years.

    Critical generator subsystems

  • Starting system: dedicated starting batteries, with permanent charger and periodic automatic test.
  • Fuel tank: sized for at least 12 hours of operation at full load (aligned with Uptime Institute Tier III/IV and TIA-942 standards), with condensate drainage and adequate ventilation.
  • ATS (Automatic Transfer Switch): transfer switch that shifts load between CFE and generator. Must be independent of the generator, not an accessory.
  • Exhaust system: silencer, piping with condensate drainage, outlet to the exterior away from DC air intakes.
  • Control and synchronization panel: for operating in parallel with the grid or with another generator, depending on design.

    • Mandatory testing and maintenance

    A generator that is never tested is a generator that will not start when needed. Tests must be part of the monthly routine.

  • Monthly test under load (minimum 30% of nominal capacity or via Load Bank): start the generator and operate it at a minimum 30% load for 15 to 30 minutes. Operating diesel engines without load or with insufficient load causes wet stacking (carbon buildup and unburned fuel accumulation in the exhaust), which voids the engine warranty and compromises operational reliability.
  • Quarterly test under load: start and transfer real load (at least 30% of nominal capacity) for 30 to 60 minutes.
  • Oil and filter change: every 250 to 500 operating hours or per manufacturer.
  • Starting battery test: quarterly, with replacement every 2 to 3 years.
  • Fuel refilling: per consumption, with stock rotation to avoid aged diesel.
  • ATS inspection: annual, with simulated transfer test.

    • Common mistakes when installing a generator

    The most expensive installation mistakes are the ones you do not see until the first real failure.

  • Undersized fuel tank: 2 hours of autonomy is not enough for emergency refueling.
  • Generator installed in an unventilated room: overheating reduces engine life.
  • Mismatched exhaust: high back pressure degrades the engine and raises consumption.
  • ATS shared between two incompatible generators: transfer fails or takes longer than declared.
  • Forgetting cooling load when sizing: the generator starts but the DC still shuts down from temperature.
  • Not testing under real load: no-load tests do not detect problems that only appear under applied load.

    • Applicable regulations

    Electrical installation in Mexico is governed by NOM-001-SEDE (electrical installations) and by local municipal and CFE provisions. NFPA 110 is the most widely used international standard for emergency and standby systems in DCs and specifies the performance, installation, and maintenance requirements for the emergency generator system, including the Type, Class, and Level classification tiers.

    Before installing a generator, it is advisable to review local regulations on noise (municipal ordinances), emissions (SEMARNAT for engines above certain power), and fuel storage (Civil Protection for diesel storage).

    A DC operating 24×7 without a generator is one storm away from an operational shutdown. The UPS buys you time; the generator buys you continuity.

    Sources

    [1] Caterpillar — Diesel Generator Sets (manufacturer catalog for data center backup) — https://web.archive.org/web/20240419162311/https://www.cat.com/en_US/products/new/power-systems/electric-power-generation/diesel-generator-sets/

    [2] NFPA 110 — Standard for Emergency and Standby Power Systems — https://www.nfpa.org/codes-and-standards/nfpa-110-standard-development/110

    [3] IEEE 141 — Recommended Practice for Electric Power Distribution for Industrial Plants (Red Book) — https://standards.ieee.org/ieee/141/2053/

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