Public and/or large buildings have emergency power systems installed to allow certain electrical components to operate during a power outage. Homeowners are also installing smaller standby generators on their property, especially if they are in rural areas where power restoration after a storm could take days or weeks. Emergency and standby power systems can be fairly simple (for homes) or very complex (for large institutional buildings like hospitals).
Emergency vs Standby Power
The difference between the 3 types of backup power is often confusing. We will describe the systems here, but it is important to note that code officials and other authorities having jurisdiction can require that certain elements and devices be on a system other than what we call for here. In the United States, backup power systems are governed by NFPA 110, Standard for Emergency and Standby Power Systems.
Emergency Power Systems provide automatic backup power in the event of normal power loss. They are required by code and shall provide power within 10 seconds to all life safety systems such as egress lighting, smoke evacuation, fire alarm systems, elevators, etc. Simply put, anything that will protect the lives of the building occupants should be on Emergency Power. Another important thing to remember is that emergency power systems must be completely separate; this means that they shall have their own conduit runs, their own panels, their own transfer stations, etc.
Legally Required Standby Power Systems also provide automatic backup power in the event of normal power loss, but they have 60 seconds to engage. They are required by code, but they can share system components - they are not required to be fully separate systems like Emergency Power Systems. These can be thought of as systems that enhance the act of egress and improve firefighter operations, but are not critical to life safety. Systems such as heating, ventilation, communications, building automation, and hospital equipment may be part of the Legally Required Standby System.
Optional Standby Power Systems are not required by code, but will provide backup power to operations that the building owner deems important to keep electrified during normal power outages. These systems can be manually or automatically engaged and can share the same components and wiring as normal power or legally required standby power. In general, Optional Standby Systems are used to prevent financial or data loss, but they can also extend to enabling human comfort during normal power outages.
What Should be on Backup Power?
A backup power system should be designed to provide electricity to only the most important pieces of equipment in a building. It is not cost effective to have backup power available for every electrical component in a building. Most facilities, even the most critical, can be ramped down during an outage so that fuel or battery power can be conserved.
As stated earlier, life safety systems are always required to be on an Emergency Power System. This includes lighting of egress paths, power for sprinkler pumps, and power to fire alarm systems. Hospitals will put life-saving equipment, like respirators, on standby power. Fire and Police Stations will make sure that their radio systems are on standby power so they can manage operations during emergencies.
Homeowners are free to size their standby generators to meet their needs. Refrigerators, freezers, and sump pumps are normally on circuits tied to the backup system, as are lights throughout the home. A few convenience outlets are also put on the system to allow phones to be charged and to keep a television or radio operational during major outages. Fuel storage capacity tends to be the limiting factor for the size of a home generator - you want to have enough fuel to keep the system operational through the outage; therefore, many of life's conveniences are turned off to conserve fuel.
Backup power is supplied by a generator, which is essentially an engine that burns fuel to create electricity. The generator can be a reciprocating or a turbine engine, but reciprocating are usually preferred because they start up quicker and are more economical.
Generator testing and maintenance are critical to the success of backup power systems. Generators and all components of the system should be tested regularly to ensure that they will be operational when needed. As with any engine, routine maintenance will prolong the life and increase the efficiency of the generator.
There are a variety of fuels that can be used, including diesel, gasoline, natural gas and liquid petroleum. Diesel is the most common due to its cost and the fact that it is safer to store than gasoline. The fuel is usually stored on site in a series of tanks. A day-tank (not necessarily a full day of fuel) is located near the generator and provides an immediate and constant amount of fuel. Large installations will also have a bulk storage tank that may be located away from the generator. The bulk storage tank holds enough fuel for a long outage; this fuel is pumped to the day-tank as needed. Fuel in any storage tank must be constantly used or mixed to prevent degradation.
Uninterruptible Power Supply
An uninterruptible power supply (UPS) is an electrical device that provides instantaneous backup power to a system when the normal power source goes down. The power from the UPS lasts only a short time, but long enough to engage other backup power sources or to safely shut down a system. UPS devices are commonly found connected to computer systems where even the slightest blip in the electrical source could cause data loss. UPSs are also used for critical systems (healthcare, communications, etc) to provide enough operational time to let the emergency generator ramp up to full capacity.
There are two main options for storing electricity in a uninterruptible power supply: batteries or a fly wheel. The battery system is fairly common for smaller loads and is comprised of one or a number of rechargeable batteries. A battery UPS requires routine maintenance and replacement since the lifespan of a battery is fairly short.
A fly wheel UPS system, also known as a rotary UPS system, uses a spinning mass to generate electricity. Fly wheel systems are generally used for larger loads and currents. In addition, fly wheels are preferred for their lifespan; due to the mechanical nature of the system, it will last up to 30 years. Maintenance is required, during which long downtimes can be expected.
An added benefit of UPS devices, beyond providing backup power, is that they can protect the systems they are connected to from voltage surges, voltage drops, noise, or distortion. They are essentially able to clean the power, which further protects the sensitive systems connected to them. The dual nature of the UPS devices means that the building owner doesn't have to purchase and maintain a separate power conditioner.
The level of redundancy is important to analyze when designing a backup power system because the designer must factor in failures within the emergency system. Rather than provide one large generator or battery backup, the backup system will be spread over multiple generators or batteries. For instance, A data center will never rely on one backup generator - instead a calculation will be run that ensures that power is available even if one (or more) generators are unavailable.
N + 1 redundancy refers to a backup system that is broken down into N components, and then an additional component is added. If the critical systems in a building can be run with 3 generators, the designer would provide 3 + 1, or 4, generators. The fourth generator would not be run during typical emergency operations, but would be engaged if generator 1, 2, or 3 had a problem. There may be a period of time where power is lost while the +1 power source starts up.
1 + 1 redundancy describes a system where there are two separate power sources that can each supply the full critical power needs of a building. In addition, both sources are always active. Should one of the backup systems fail, the second system is already active and operational so there will not be any interruption in power. 1 + 1 redundancy is less efficient than N + 1, but it provides a much more robust and transparent backup system.
Generator Safety - Prevent Back-feeding
***If you are a homeowner considering a generator for long power outages, be sure that you hire an electrician to configure your setup and properly install the system. This is not a task for the novice. Serious injury or death (to you or others) can be caused by an improper installation, not to mention significant property damage is possible. Be sure to have all safety features installed and NEVER override a safety feature.***
A proper transfer switch is required for a generator setup. A transfer switch will allow either the main / normal power from the utility, or power from the generator, to be fed into the building's electrical system. It will never allow both normal power and emergency power to be on at the same time; the transfer switch disconnects the building from the utility feed while the generator is active, which is also referred to as islanding. This ensures that the generator cannot back-feed the electrical grid, which could cause injury or death to technicians working to repair the power lines. In addition, it prevents serious damage to the generator and other building equipment when normal power is restored by preventing the two out-of-phase power feeds from "fighting" with each other and overloading equipment that is intended for a lower amperage.