Included in this category are several major power sources, such as engine-generators in various fuel offerings, along with power and control transformers and storage batteries

Modern power sources include engine-driven generators (diesel, gaseous, or gasoline), gas-turbine generator sets, and battery powered equipment. Depending on the power failure time tolerance of the system and the required length of time of engine-generator performance, a specific type of power source can be chosen. Also, the type of power transformer must be considered along the lines of installation requirements, operating and maintenance costs, and insulation system.


Diesel-engine generator sets range in size from about 100 to 2000kW and are used as standby or emergency power supplies. They are also selected to supply alternate power for critical applications such as computer installations, life safety systems in large facilities, and required emergency power in hospitals or other legally required installations in public institutions.

Gas-driven units are available in the same size range as diesel units, and offer simpler, less costly installations along with better starting capabilities in cold weather. However, the possibility of a fuel cutoff does exist.

Diesel-fueled engines provide greater reliability since diesel fuel can be easily stored on-site. A disadvantage of the diesel against the gas engine is that, over a period of time, leaks can occur as tanks, piping, or valves corrode. This can result in a fuel spill and an inability of the engine to operate when needed.

Gasoline-driven engine-generator sets are a practical choice in smaller sizes (from a few kW to SO or 75kW). They are frequently used in residential or small commercial applications, mobile homes, recreational vehicles, or other portable applications.

Features to consider when selecting an engine-generator set include installation cost and requirements, maintenance costs, reliability, and starting ability.

Gas-turbine generators feature small size, light weight, and minimum vibration. They also are used in standby/emergency-power applications; however, they are slow starting and may require auxiliary equipment to achieve fast starting where required. These units are particularly suited for use as alternate power supplies.

Large generators for cogeneration and their affiliated equipment, instruments, and controls are also included in this category.

For more detailed information, see the "Special Report on Engine Generators" in the March, 1991 issue of EC&M.

Power and control transformers

Power transformers range in size from dry types of a few kVA to multiple MVA oil-filled units. Dry types can vary from unventilated models to fully encapsulated units. Besides mineral oil, transformers can use silicone or refrigerant-type liquid as an insulation medium.

Transformers can be padmounted, combined with other equipment in a substation, or pole mounted as smaller sized distribution types.

Control transformers find wide application in reducing line voltage to the operating voltage of the control system, usually 24V or 120V. These transformers are in VA sizes and can have provisions for primary and secondary fusing.

K-rated transformers are specifically designed as well as constructed to handle nonlinear loads and the resultant harmonic currents.

For more detailed information on transformers, see the series "Choosing the Right Transformer" in the February, May, and November 1992 issues of EC&M.

Storage batteries

Storage batteries provide an independent source of DC power for switchgear control, emergency lighting, engine-generator set starting, signal power, and UPS backup power. The most commonly used is lead-acid type, either lead-calcium or lead-antimony. Nickel-cadmium batteries offer special features that, in many instances, offset their higher cost.