For the most part, the supply of electricity is very reliable — 99.9 percent reliable on the average. But 99.9 percent reliability still leaves a little more than eight hours per year that the average facility will be without power. Some facilities, particularly those that are subjected to extreme weather, have the potential for even longer electrical outages.
The problem is that even short interruptions can be disruptive and dangerous. Power outages can leave occupants in the dark, with no safe means of exiting the building. Computer systems crash. Telecommunications systems stop working. And if the outage is a lengthy one, losses to business operations can be very significant.
Facility executives have a range of options when it comes to providing emergency power. Battery-powered lights can be installed to provide illumination for a limited time. An alternate power feed from a separate utility substation can be installed, complete with an automatic transfer switch. And while these options are used, by far the most common emergency power system is a standby generator.
While code requirements are still an issue for emergency power, much of the emphasis has shifted to providing power to certain loads. To determine which of these loads needs to be connected to an emergency power system, it will be necessary to understand the impact that a power outage would have on the facility and the operations that take place in that facility. Therefore, the first step in the design of an emergency power system is to identify risks.
Facilities located in areas exposed to extreme weather are more at risk of power outages than areas that are not. Similarly, facilities served by overhead distribution systems tend to have more power outages than those supplied by an underground system.
In assessing risk, it is also important to identify critical loads — those that must be supplied with power in order to minimize safety risks or to prevent major losses to the facility or its systems. These loads must be met even if the facility is going to close down for the duration of the power outage. Examples of critical loads include emergency lighting, exit lights, fire protection systems, security systems and select mechanical systems such as sump pumps.
Essential loads must be powered if a portion of the facility is to continue to operate. Some of these loads are facility-related, such as the lighting and ventilation systems. Others are related to the activities that take place within the facility, the disruption of which would result in major business losses.
To determine which occupant loads are essential, it is necessary to meet with occupants to discuss their requirements. In may be possible for some building occupants to transfer certain essential loads to other facilities during an outage. Those that cannot be transferred must be powered by the emergency power system.
Once all critical and essential loads have been identified, the emergency power system can be sized. All connected loads should be divided into two categories: those that can survive a momentary interruption of power and those requiring continuous power.
When the power goes out, it may take a generator-based emergency power system up to a minute to come online. For loads such as lighting and building mechanical systems, this interruption is an inconvenience. For computer-based systems, it can result in lengthy restart times and the loss of data. To provide power to these systems during the generator startup, a UPS system will have to be added to the emergency power system. UPS systems also offer protection from momentary outages and voltage sags.
The sizes of the generator and UPS system, if required, are determined in part by the sum of the connected loads. But determining the connected kW isn’t sufficient. Sizing must also take into consideration additional loads that occur during motor starting. Many loads that will be connected to a system will generate harmonics, also impacting systems sizing. And new loads are added regularly, some of them essential. Therefore, the sizing of the emergency system will have to take into consideration any possible growth in connected loads.
There are several options when it comes to selecting the type of engine used to drive the generator in an emergency power system. The most common of these are gasoline, natural gas and diesel. The type selected for a particular application depends on a number of factors including the connected load, anticipated run time and the project budget.
Gasoline-driven generators have the lowest first costs. Units are available in sizes up to around 100 kW. These generators are easy to start, particularly in cold climates. Their biggest drawback is their fuel. Gasoline has a relatively short tank life and requires special handling.
Natural-gas driven generators are only slightly more expensive than gasoline-driven units. They are available in sizes up to 100 kW. Natural gas eliminates problems associated with gasoline, but places the facility at the mercy of the natural gas supplier.
Diesel-driven generators offer greater reliability than gasoline- or natural-gas-driven generators. They are available in sizes ranging from 10 to 2,000 kW. In capacities under 70 kW, diesel-driven generators are more expensive than gasoline and natural gas units. For larger units, diesel units are less expensive. Diesel units are more difficult to start than gasoline or natural gas units, particularly during cold weather. In cold climates, most units include an electric engine heater. Diesel fuel has a long tank life and must be periodically tested and treated for water absorption.
Emergency generators require careful siting. The generators tend to be rather large and heavy, making it difficult to locate within or next to existing facilities. Space must be found for the automatic transfer switches that will transfer the loads from the utility supply to the generator. Suitable spaces must be found to locate the fuel storage tank. And with the need to tie into existing building wiring, the system will have to be located close to the electrical feed into the building.
Noise concerns must also be addressed. Generator sets create noise that can interfere with the operation of the facility. Vibrations from the generator can be transmitted through the building structure.
What’s more, engine exhaust must be directed away from any opening into the building. And adequate ventilation must be supplied to the generator for both cooling and for supporting combustion.
Many of these siting concerns can be dealt with through good design practices. Noise levels can be controlled by installing a sound enclosure around the generator or by installing soundproofing on the interior walls of the generator room. A well-designed silencer will reduce system exhaust noise. Flexible links for the fuel system, exhaust and all electrical connections will help to reduce noise and vibration transmission to the building structure. Isolation springs will further reduce the transmission of vibrations. Carefully locating the generator exhaust will prevent drawing exhaust fumes into the building through doors, windows or HVAC intakes.
Facility executives will also have to put in place a number of procedures to direct the response of personnel during an outage. Personnel will need an up-to-date one-line diagram of the facility’s electrical distribution system, telling them what loads are connected to what circuits. Not only is this required in laying out the emergency generator installation, but it also will help in troubleshooting problems that may be encountered during an outage.
One of the most important procedures to be established is the outage response plan. This plan will spell out exactly who is responsible for what actions in the event of a power outage. Someone will have to confirm that the generator started and that the loads transferred as designed. Since the length of time that power will be off is unknown, someone will have to confirm and monitor the quantity of fuel stored on site. A contract will already have to be in place for fuel delivery in the event that the outage is a lengthy one, and someone during the outage will have to call to arrange for delivery of fuel.
During the outage, personnel should walk the facilities to determine that critical and essential loads are adequately powered. Personnel should also check other loads, ones not designated to be critical or essential, to make certain that they have not been connected to the system. The longer a system is in place and the more power outages that the facility experiences, the more new loads that tend to be added to the system. This load creep can result in overloading of circuits and the generator system.
No matter how well the system is planned, designed and installed, its performance will only be as reliable as the level of maintenance that is performed on the entire system. It is surprising how often tens of thousands of dollars are invested in an emergency power system, only to have it fail to start during an outage due to a dead battery. The system manufacturer will provide a recommended maintenance program for all components of an emergency power system. In general they will include the following items:
Reliable emergency power requires facility executives to understand the needs of their facility, plan carefully and make an ongoing investment. Emergency power systems may not be needed very often, but when they are, it generally is under the worst of conditions. And emergency conditions are not when one wants to test the emergency system and procedures.
James Piper, PE, PhD, is a consultant and writer with more than 25 years of experience in the facilities field.
