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A Generator Installation Guide for UPS Systems

For data centres and server rooms, generators can be installed upstream of their uninterruptible power supplies (UPS) to provide the UPS with a source of long runtime power. This type of power protection device combination reduces the size of the UPS battery set, which will typically be sized for 10-30minutes runtime; long enough to automatically start-up the generator (under 1minute) and/or find out the reason the generator has not started.

With the UK media also reporting the potential for planned power outages, to cope with high demand in cold periods, it is not surprising that demand and interest in generators and generator/UPS combination systems is on the rise. Whilst there will be specific exemptions such as hospitals, medical facilities, military bases, blue light services, and air traffic control centres, it is up to businesses and public sector organisations, especially those reliant on their own server rooms and data centres to put in place their own power protection plans.

More information on planned power outages and grid blackouts:

Diesel Generating Set Installation Tips

When deciding to install generator power it is important to undertake a site survey to ensure the project runs smoothly and deliveres the expected runtime and protection. Aspects to consider include the following:

  • Generator Sizing: generators have two ratings or mode of operation. One for the Primary and one for the Standby mode. Primary refers to continuous running, and standby to the mode when where the generator provides power only when the mains power supply fails. For UPS applications, the generator is sized for standby mode and should be oversized when compared to the UPS system it will power and the rule of thumb is 1.5 x the UPS size at full load. Critical loads will include IT within the server room or data centre, and these will be directly powered from the uninterruptible power supply. Any business or organisation will look to install a generator sized to power more loads, essential to business continuity and general operations. Essential loads outside the server room could include other building power sockets, lighting, heating, ventilation, and air conditioning (HVAC) systems, lifts, CCTV, and security systems. Suppose a server room only requires a single phase rackmount 3kVA UPS system but the building requires a three-phase 40kVA solution, then 40kVA three-phase dictates the size of generator required.
  • Location: generators are typically installed outside of the main building and exhaust into the open air. The route to the generator site will walked during a site survey to identify logistics issues and ensure that the delivery vehicle (typically a HIAB) has sufficient reach from its crane arm. A generator will have an acoustic weatherproof canopy to reduce noise output when running and provide protection from extreme weather. The generator needs a flat surface, strong enough to support the generating set when loaded with fuel. Where a suitable surface cannot be provided, then a concrete plinth may be required.
  • Automatic Transfer Switches: an ATS or Automatic Mains Failure (AMF) panel is part of a generator installation. The panel has a sensing circuit that monitors the building incomer electrical supply. As soon as this is seen to fail (power outage) or fluctuate, the generator will be automatically started, and will provide power to building and UPS circuits. When mains power returns and stabilises, the ATS/AMF will monitor until there is stability, run the generator into a cool down mode and then power down, automatically. An ATS/AMF can also be switched manually when required for testing.
  • Electrical Works and Cabling: generator location will determine the length of cable required into the building to where the ATS/AMF panel is installed. The installation may require ducting to house the cables. The overall distance will influence cable size choices, as does the maximum cables size that the generator terminals can take. The ducting used for the electrical cables will also be used to house the generator remote monitoring cable. Care must be taken when designing the circuit for the generator, especially if there is other onsite electrical power generation including solar PV panels.
  • Remote Monitoring and Alerts: a basic generator control panel will provide a relay contact signal to indicate a generator fault or low fuel level. This can signal can be picked up by an environmental monitoring system to initiate email and/or SMS text alerts. A more sophisticated control panel will provide more detailed information and communications protocols but will cost more.
  • Fuel Type and Refuelling: the most common fuel type is diesel or liquid petroleum gas (LPG). Diesel generators may also be approved to use with HVO (Hydrotreated Vegetable Oil) according to EN15940, an eco-friendly alternative. It is important to monitor fuel levels. When a generator is delivered, the set typically delivered with a full fuel tank (first fill fuel service). A typical fuel tank is referred to as a ‘day-tank’ providing 8-12hours of power. Arrangements should be made with a the generator maintainer to supply top-ups as required to avoid running dry. Emergency supplies for long duration power outages.
  • Regular Testing: most UPS systems are left to run in automatic mode and will test their battery sets every 24hours. A generator in standby mode, requires monthly testing to ensure that the set will automatically start. Running monthly, for 5-10minutes ensures that oil and lubrications are distributed, the starter battery is charged, the fuel flow is constant, the fuel is not contaminated (with air or water) and there are no circuit breakers in the wrong place following server work or maintenance of the electrical circuits.
  • Service and Maintenance: generators do require regular inspection and maintenance. The common service internal is every 6months and there are consumables that require inspection and replacement including filters and the lead acid starter battery. Generator preventative maintenance visits include alternating ‘A’ and ‘B’ schedules, with ‘B’ schedules covering more detail during the PM visit.

A professionally installed and maintained generator should provide 10-15years of service without a major overhaul. Like any motor, running hours are also important as an indicator of ‘wear and tear.’ Problems with poorly maintained generators do occur and the worst case is that the generator fails to start when required to do so, leading to system downtime.

The Six Reasons Why Standby Generators Fail

Generating sets, whether used in primary or standby mode are by design, extremely dependable and robust. Emergency callout visits can be made to identify the reason(s) for a non-start incident, but the corrective action or repair may take several hours, especially if off-site parts are required.

The 6 most common causes of a generator failing to start are:

  1. Fuel Level Issues: a low or empty fuel tank is one of the most common reasons why a generator fails to start or cannot run for the expected period. Fuel can also become contaminated and require ‘polishing’ and this is one of the reasons that it is important to carryout regular testing and fuel inspections.
  2. Starter Battery: the battery powering the starter motor is comparable to that in an ICE car or vehicle. The battery will be a lead acid with a design life of say 5 or 10years and require replacement earlier than this. Temperature extremes do affect battery efficiency and life expectancy. Lead acid batteries lose charge even with no load connected. If the generator starter circuit fails to turn and start the generator, only those loads on the local UPS system will be protected, and only for as long as the UPS battery can provide power.
  3. Clogged Filters: a diesel or HVO powered generator is similar in design and function to a car engine. Filters requiring regular replacement at set service intervals dependent on usage (miles or run hours) and the filters and connecting air, fuel and oil pipes required for lubrication and combustion can become clogged. A poorly maintained generator could start and then fail to run for time required to cover a power outage.
  4. Oil and Coolant Leaks: the pipes running oil and fuel to and around a generating set will age and could suffer a leak. Patching leaks is sometimes possible and leaking pipes can also be replaced. Both require a suitably trained generator engineer. Any leaks will also require cleaning up in the most efficient way to protect the local environment.
  5. Broken Block Heater: placed outside, exposes a generator to weather extremes. An electrical heat circuit provides heating to the engine block and helps to prevent oil lubrications from thickening. Block heaters require regular inspection and if necessary, replacement, if a generator is to automatically start-up even in extreme winter conditions.
  6. Electrical Works/Circuit Breakers: circuit breakers can be left in the wrong position when works are undertaken on site. A generator, for example, could also be left in manual start-mode, instead of automatic. Any work to the electrical supply circuitry, especially when in the plant room or place where the generator ATS/AMF panel, should be inspected with a checklist for the work undertaken and other critical circuits in the room including generation.

Regular generator testing, service inspection and preventative maintenance can help to prevent generating set issues and ensure the generator is ready to protect the connected loads.


The threat of planned power outages adds a level of uncertainty for those responsible for service continuity. Businesses and organisations need to ensure that their critical IT, server rooms and data centres are protected from power outages. Now their power continuity plans must consider a larger building load including critical and essential loads. The recommended solution is a UPS and generator installation, regularly serviced and maintained. Installing a UPS and local onsite power generation provides the most robust of power protection plans. This ensures that both critical IT loads and other essential building systems, are supported during a power outage to prevent data loss and ensure uptime.

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