Conclusiones y comentarios finales

The interface details between the HVAC and electrical designers in the design of the generator plant for a building were discussed in chapter 8. This chapter is concerned with the technical means of providing power to the life safety system as required by the model building codes and NFPA. The NFPA requirements are provided in the National Electric Code, which is detailed in NFPA 70.

The distinction between an “emergency” and a “standby” generator is not a func- tion of the equipment used to provide secondary electrical capacity but rather is deter- mined by the loads that are being operated by the generator; specifically, the National Electric Code recognizes three classifications of load that will need to be or can be con- nected to a secondary power system:

• “Emergency loads,” which are addressed in the National Electric Code in Article

700. These are the loads that are considered essential to the life safety of people and are required to permit safe evacuation of the building. Included are load items such as the fire command station (including its lighting), the fire alarm system, communication systems, egress lighting (including the main lobby in the building), exit signs, fire pumps, elevators, and elevator cab lighting, and other equipment that, if not in receipt of backup power, could result in a hazard to building evacuees.

• “Legally required standby loads,” which are addressed in the National Electric

Code in Article 701. These are loads that are required to assist fire fighting and res- cue operations. Included are load items such as smoke exhaust or pressurization fans, sump pumps, sewage ejector pumps, and other mechanical equipment that could be used by fire fighters or rescuers as well as loads that, if not in receipt of backup power, could result in a hazard to rescuers.

• “Optional standby loads,” which are addressed in the National Electric Code in

Article 702. These are loads that the building operation may choose to provide with backup power for business continuity purposes. As noted in chapter 8, this could involve telecommunications or data processing equipment and the HVAC systems that allow this equipment to operate by maintaining the critical environment for the spaces within which the equipment is installed.

The National Electric Code requires that the generator plant supplying the emer- gency and legally required standby loads be automatically started with full power in a specified period of time. The emergency loads must receive backup power 10 seconds after a loss of power. The legally required standby system must be capable of accepting electrical loads within 60 seconds of the failure of the normal electric service. The source of power may be separate for each category, but usually a single generator plant will supply both the emergency and legally required standby loads so the 10-second delivery of power for the emergency loads will govern if the secondary source is the generator plant.

The secondary source need not be a generator plant. For example, the fire alarm system must be installed so any data collected by it will not be lost when a failure of the primary electric source occurs. The data collected by the system can only be protected by the inclusion of a battery system to protect the volatile memory of the fire alarm sys- tem during the short period of time for the transition of the system electric power to the

generator plant. While it is possible to use a battery backup as the sole secondary source, this becomes complicated by the time frame for which the batteries must pro- vide the power. As a result, the generator plant will be used and the batteries will only be employed for the secondary source of power until the generator is on line.

It is also possible to use battery pack units as the secondary source of power to egress lighting fixtures and exit signs, but the available power from the generator is usu- ally the source of power for these fixtures in a new building. In recent projects, some owners, in a reaction to possible external threats, have also placed the egress lighting on both emergency power and battery packs as well.

The approved secondary source of power beyond battery systems is oil-fired diesel generator sets or oil-fired gas turbines. The use of oil-fired gas turbines is not common. In part this is a result of the 20 to 40 seconds that are required by a gas turbine to pro- vide beneficial power after being started. This exceeds the time limit for power to the emergency loads, which would require battery packs at all lighting fixtures and exit signs and UPS power to the other emergency loads if a gas turbine were to be used. In addition to this limitation, an oil-fired gas turbine generator set is more expensive than a diesel generator set of equal size and gas turbines are only available in a limited number of units whose capacity could be utilized to meet the secondary power requirements of tall commercial buildings. Oil-fired gas turbine generators have found application in existing buildings where the generator is to be installed on an upper floor when the structure has limited ability to handle additional structural loads. In these cases, the lighter weight of the gas turbine as compared to a diesel engine may well lead to the use of the gas turbine as a more cost-effective solution.

Spark plug-energized diesel engines or gas turbines using natural gas as a fuel rather than oil are not permitted under the model building codes, as the code require- ment is to have the fuel source for the generator captive to the building. This require- ment is in the building codes so that in the event of a disaster that interrupted both the electric and natural gas service to the building, the performance of the life safety system would not be compromised.

The model codes and NFPA also state that the capacity of the emergency/standby power system must be sufficient to be capable of supplying power to all of the equip- ment that must be operational by the life safety system at a given point of time. Where a backup power supply system is required to serve emergency loads, it is permitted to also serve legally required standby loads, as well as optional standby loads, as long as there is a load-shedding capability to ensure that life safety loads are given priority.

With the inclusion of permitted load shedding, this does not mean that all connected equipment must be simultaneously operational. For example, not all elevators in a building would be required to operate at the same time. The elevator needs would be met by conforming to the building code, which, in the event of a power outage, would usually permit the operation of any one elevator at a time in each bank, with the other elevators in the bank being operated sequentially after the first elevator is brought to its agreed terminal floor. The elevator design would then permit a second and then the additional elevators to be operated in the bank until all elevators and their passengers are at the terminal level in the building, but the emergency/standby generator would only be sized to handle the largest electric motor in a given bank of elevators.

What is therefore required to determine the capacity of the emergency/standby power system is to determine what equipment will need to operate at any given point of time and select the generator to provide that required capacity. As was noted in chapter 8, this necessitates that the electrical design engineer obtain from the HVAC, fire pro- tection, and elevator designers their secondary electrical needs and review these require- ments along with the needs of the equipment on the electrical drawings. This will allow the electrical designer to determine the size of the generator plant that must be provided for the project.

Chapter 10—Life Safety Systems | 101

It is possible on very large projects with high optional standby loads beyond the life safety system to install a separate generator to meet the needs of the life safety system and a separate plant of one or more generators for the special optional loads. This could maintain a higher degree of protection and isolation for the life safety generator while avoiding the complication of the interface with the conventional building loads.