Standby Generators


    Emergency and standby power systems are designed to provide an alternate source of power if the normal source of power, most often the serving utility, should fail. As such, reliability of these types of systems is critical and good design practices are essential.


    The classification of emergency and standby power systems is as follows:

    • Emergency Power System: Defined in Institute of Electrical and Electronics Engineers (IEEE) Std. 446-1995 as “an independent reserve source of electric energy that, upon failure or outage of the normal source, automatically provides reliable electric power within a specified time to critical devices and equipment whose failure to operate satisfactorily would jeopardize the health and safety of personnel or result in damage to property.

      The National Electrical Code (NEC) gives a slightly different definition for Emergency Systems as “those systems legally required and classed as emergency by municipal, state, federal, or other codes, or by any governmental agency having jurisdiction. These systems are intended to automatically supply illumination, power, or both, to designated areas and equipment in the event of failure of the normal supply or in the event of accident to elements of a system intended to supply, distribute, and control power and illumination essential for safety to human life.”

    • Standby Power System: Defined in IEEE Std. 446-1995 as “an independent reserve source of electric energy that, upon failure or outage of the normal source, provides electric power of acceptable quality so that the user’s facilities may continue in satisfactory operation.

      The NEC divides standby power systems into two categories, as follows:

      • Legally Required Standby Systems: Those systems required and so classed as legally required standby by municipal, state, federal, and other codes or by any governmental agency having jurisdiction. These systems are intended to automatically supply power to selected load (other than those classed as emergency systems) in the event of failure of the normal source.

        Legally required standby systems are typically installed to serve loads, such as heating and refrigeration systems, communications systems, ventilation and smoke removal systems, sewage disposal, lighting systems, and industrial processes that, when stopped during any interruption of the normal electrical supply, could create hazards or hamper rescue and fire-fighting operations.

      • Optional Standby Systems: Those systems intended to supply power to public or private facilities or property where life safety does not depend on the performance of the system. Optional standby systems are intended to supply on-site generated power to selected loads either automatically or manually.

        Optional standby systems are typically installed to provide an alternate source of electric power for such facilities as industrial and commercial buildings, farms, and residences and to serve loads such as heating and refrigeration systems, data processing and communications systems, and industrial processes that, when stopped during any power outage, could cause discomfort, serious interruption of the process, damage to the product or process, and the like.


    • IEEE Standard 446-1995: IEEE Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications, is a general engineering reference for the design of these systems.

    • The National Electrical Code: The NEC contains requirements for emergency systems in Article 700, Legally-Required Standby Systems in Article 701, and Optional Standby Systems in Article 702. In addition, Article 445 (Generators), 517 (Health Care Facilities), 665 (Integrated Electrical Facilities), and 705 (Interconnected Electrical Power Production Sources) are all of particular interest for emergency and standby power systems.

    • National Fire Protection Association (NFPA) 110: Standard for Emergency and Standby Power Systems, defines how emergency and standby power systems are to be installed and tested. It contains requirements for energy sources, transfer equipment, and installation and environmental considerations. It divides emergency power systems into Types, Classes, and Levels.

      • The Type refers to the maximum time that an emergency power system can remain unpowered after a failure of the normal source.

      • The Class of an emergency power system refers to the minimum time, in hours, for which the system is designed to operate at its rated load without being refueled or recharged.

      • The Level of an emergency power system refers to the level of equipment installation, performance, and maintenance requirements.

    • NFPA 101: Life Safety Code, addresses those construction, protection, and occupancy features necessary to minimize danger to life from fire, including smoke, fumes, or panic. It defines the requirements for what systems the Emergency Power System will supply.

    • NFPA 99: defines establishes criteria to minimize the hazards of fire, explosion, and electricity in health care facilities. It defines several specific features of electric power systems for these facilities.

    Emergency and standby power systems are generally designed into the over-all electrical system for one of the following two reasons:

    • Legal Requirements: As required by the NEC, NFPA 101, NFPA 99, and other local, state, and federal codes and requirements. These are concerned with the safety of human life, protection of the environment.

    • Economic Considerations: Continuous process applications often require a continuous source of electrical power to avoid significant economic loss. In some cases even a momentary loss of power can be disastrous.


    Generators are by far the most prevalent source of power for emergency and standby power systems. For most commercial and industrial power systems these will be engine-generator sets, with the prime-mover and the generator built into a single unit. There are two main options for engine-generator sets:

    • Reciprocating Engines: For reciprocating engines, diesel engines are the most popular choice of prime- mover for generators, due to the cost of the diesel engines as compared to other forms of power and the relative ease of application. Gasoline engine generator sets are also available and are generally less expensive than diesel generator sets, but suffer from the disadvantages of higher operating costs, greater fuel storage hazards, and shorter fuel storage life as compared to diesel. Diesel engines can also run on natural gas, although for maximum efficiency specially-tuned engines for natural gas use are available.

    • Turbine Generators: The other alternative for generators is the turbine generator, typically powered by natural gas. Gas-turbine generator sets are generally lighter in weight than diesel engine-generator sets, run more quietly, and generally require less cooling and combustion air, leading to lower installation costs. However, gas-turbine generator sets are more expensive than diesel engine-generator sets, and require more starting time (normally around 30 s compared to the 10-15 seconds for diesels). The long starting-time requirement and lack of available small sizes (< 500 kW) makes the gas-turbine generators infeasible in many applications.

    Generator installations must consider the combustion and cooling air required by the generator and prime mover, as well as the provisions for the removal of exhaust gasses. Noise abatement must also be considered.

    These considerations increase the installation costs, especially for reciprocating-engine units such as diesel or gasoline engines. Further, the fuel supply must be considered; building code and insurance considerations may force the fuel storage tank to be well removed from the generator(s), usually forcing the addition of a fuel transfer tank near the generator(s).

    Care must be taken when sizing engine-generator sets for a given application since several ratings exist for the output capability of a given machine. The continuous rating is typically the output rating of the engine-generator set on a continuous basis with a non-varying load. The prime power rating is typically the continuous output rating with varying load. The standby rating is typically the output rating for a limited period of time with varying load. The manufacturer must be consulted to define the capabilities of a given unit.


    Solid-state converters that invert DC voltage from a battery system are another alternative, although they can be difficult to apply and generally are not available in the larger sizes that may be needed for a medium to large emergency or standby system.

    Because motor starting and block loading can have a big effect on the output voltage and frequency of a small generator such as the engine-generator sets described above, and also because power is not available during the engine starting period, a buffer between the generators and sensitive load equipment is generally required. The Uninterruptible Power Supply (UPS) is usually the buffer of choice for these applications. UPS’s are available in several different topologies, but the operational goals are the same regardless of topology: The supply of uninterrupted power to sensitive, critical loads. The most popular topology for a UPS is the double conversion topology.

    So long as the batteries are properly maintained, the AC output should not be affected by change in frequency or voltage, or even a complete loss, at the input, so long as backup time of the UPS is not exceeded. Other topologies exist, including the line interactive, double-conversion rotary, hybrid rotary, and line-interactive rotary topologies, each with advantages and disadvantages of application. UPS systems do not alleviate the need for a generator or second utility service power source, but they do serve to buffer critical loads from the effects of generator starting time and voltage and frequency variations.

    A means must be provided to switch the critical loads from the normal utility source to the standby power source. Several types of device are available for this.

    An automatic transfer switch is defined as “self-acting equipment for transferring one or more load conductor connections from one power source to another”. The automatic transfer switch is the most common means of transferring critical loads to the emergency/standby power supply.

    An automatic transfer switch consists of a switching means and a control system capable of sensing the normal supply voltage and switching over to the alternate source should the normal source fail. Automatic transfer switches are available in ratings from 30-50 A, and up to 600V. Because automatic transfer switches are designed to continuously carry the loads they serve, even under normal conditions, care must be used in sizing these so that the potential for failure is minimized.

    Automatic test switches with adjustable pickup and dropout setpoints and integral testing capability are generally preferred. An automatic transfer switch is generally an open-transition device that will not allow paralleling of the two sources. Manual versions of transfer switches are also available.

    Other options for transferring devices include electrically-operated circuit breakers. For medium voltage transfers, medium voltage circuit breakers are generally used. Manual versions of circuit-breaker transferring schemes are, of course, also available.

    Bypass/isolation switches, as their name suggests, are used to bypass an automatic transfer switch (or other switching means) and connect the source directly to the load and allow isolation of the transfer switch for maintenance.

    Bypass/isolation switches are available with a test position in which only the ATS-to-load isolation contact is open, allowing the transfer switch to be operated without disconnecting the load.

    Static Transfer Switches are typically used when high-speed (~4ms) operation is required. The most common application is to bypass a UPS so that a UPS failure will not result in interruption of service to the load.