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There are two common types of generator systems available to supply emergency power to essential electrical systems. One common type of generator set utilizes a combustion engine fueled by natural gas. This type of engine is easy to maintain; however, codes do not recognize the utility company gas supply as an emergency service back-up fuel source unless it can be proven to be uninterruptible. Natural gas systems normally require a propane system back-up (or must operate entirely on propane). Either arrangement requires large above-ground tanks, extra maintenance and great care in selecting a storage location.

A second common type of generator set utilizes a diesel oil fired combustion engine with a fuel tank sized for a minimum of eight hours of full load operations. Depending upon the utility company's reliability and the number of primary services feeding the site, hospitals may have up to twenty four hours of fuel supply to get them through a utility outage.

Buried fuel tanks for diesel systems require double-walls and sophisticated systems to monitor for fuel leaks. Above ground tanks avoid the need for leak detection and are used quite commonly when they can be concealed effectively.

The location of the standby generator is very important. If possible, never locate the emergency generator system at the lowest building level where it may be subject to flooding, as a result of water cascading down the building interior due to a natural disaster, a water main break, or water from fire hoses or sprinklers. Above grade installations would also simplify the cooling and discharge air systems. If the building has a penthouse designed to accept the weight of the generator, this would also be a preferred location.

Installing the generator set in a penthouse makes it easier to handle the exhaust system. The surface temperature of a superheated generator exhaust pipe is over 1000 degrees Fahrenheit. This requires insulation and the piping must be threaded through the structure if installed on the lower levels of the hospital. The location of the exhaust system discharge is a major concern. Prevailing winds can blow exhaust fumes back into a building through HVAC air intakes if it is not properly located.

Generators must be installed in sound attenuated rooms to reduce airborne noise and vibration. They are generally installed on spring isolators and are equipped with critical type mufflers. If the generator system is installed outdoors, sound attenuation systems are also available.

There is a natural tendency to oversize the generator set for future expansion. However, generator sets operating under lightly loaded conditions experience incomplete fuel combustion. The unburned fuel accumulates on the engine injectors or in the exhaust muffler and/or exhaust stack. This phenomenon causes choking, which leads to premature failure and limits the overall system capacity. Latest NFPA 99 guidelines require monthly testing at 30% of the generator nameplate rating or 50% of the connected load (whichever is greater). Oversizing the units could lead to a requirement for load banks to make up for a shortfall in actual load available to be applied to the unit during testing.

Often it is better to consider installing two smaller generators operating in parallel instead of one large one. Both generators start and come on line. If the load does not warrant both units, one will drop off line, leaving the remaining unit running at optimum load.

In addition to governing national codes, states and municipalities may impose local regulations. All of these codes change from time to time and the engineer must remain familiar with the latest requirements.