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 (b) An available reliable power source to which critical loads are rapidly switched
automatically when the prime source of power fails.
Emergency systems are frequently characterized by a continuous or rapid availability of electric
power. This electric power operates for a limited time and is supplied by a separate wiring
system. The emergency power system may in turn be backed by a standby power system if
interruptions of longer duration are expected.
1.6.2.2 Standby Power Systems. Standby power systems are made up of the following
main components:
(a) An alternate reliable source of electric power separate from the prime source.
(b) Starting and regulating controls when on-site standby generation is selected as the
source.
(c) Controls which transfer loads from the prime or emergency power source to the
standby source.
1.6.3 Engine-Driven Generators. These units are work horses which fulfill the need for
emergency and standby power. They are available from fractional kW units to units of several
thousand kW. When properly maintained and kept warm, the engine driven generators reliably
come on line within 8 to 15 seconds. In addition to providing emergency power, engine-driven
generators are also used for handling peak loads and are sometimes used as the preferred source
of power. They fill the need of backup power for uninterruptible power systems.
1.6.3.1 Generator Voltage. The output of engine-driven generators used for emergency or
standby power service is normally at distribution or utilization voltages. Generators rated at 500
kW or less operate at utilization voltages of 480Y/277 V, 208Y/120 V, or 240Y/120 V. Higher
rated generators usually operate at nominal distribution system voltages of 2400 V, 4160 V, or
13,800 V.
1.6.3.2 Diesel Engine Generators. The ratings of diesel engine generators vary from about
2.5 kW to 6500 kW. Typical ratings for emergency or standby power service are 100 kW, 200
kW, 500 kW, 750 kW, 1000 kW, 1500 kW, 2000 kW, and 2500 kW. Two typical operating
speeds of diesel engine generators in emergency and standby service are 1800 rpm and 1200 rpm.
Lower speed units are heavier and costlier, but are more suitable for continuous power while
nearly all higher speed (1800 rpm) sets are smaller.
1.6.3.3 Gasoline Engine Generators. Gasoline engines are satisfactory for installations up
to approximately 100 kW output. They start rapidly and are low in initial cost as compared to
diesel engines. Disadvantages include: higher operating costs, a great hazard due to the storing
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