The use of emergency power systems in aviation can be on the aircraft or on the ground.

On commercial and military aircraft, it is critical to maintain power to essential systems during an emergency. This can be done through Ram air turbines or battery emergency power supplies allowing pilots to maintain radio contact and continue to navigate using MFD, GPS, VOR receiver or directional gyro for over an hour.

Localizer, glide slope") and other instrument landing aids (such as microwave transmitters) are power-hungry and mission-critical, and cannot be operated reliably on a battery, even for short periods. Therefore, where absolute reliability is required (such as when Category 3 operations are in effect at the airport), it is common to run the system from a diesel generator with automatic switchover to the mains in the event of generator failure. This avoids any transmission interruption while a generator reaches speed of operation and opposes the typical view of emergency power systems, where backup generators are considered secondary to the main electrical supply.

Computers, communication networks, and other modern electronic devices not only need power, but also a constant flow of it to continue functioning. If the source voltage drops significantly or drops completely, these devices will fail, even if the power loss is only for a fraction of a second. Because of this, even a backup generator does not provide protection due to the startup time involved.

To achieve more complete loss protection, additional equipment such as surge protectors, inverters "Inverter (electronics)") or sometimes a complete uninterruptible power supply (UPS) are used. UPS systems can be local (to one device or one outlet) or can extend to the entire building. A local UPS is a small box that fits under a desk or telecommunications rack and powers a small number of devices. A whole-building UPS can take several different forms, depending on the application. It directly powers a system of outlets designated as UPS power and can power a large number of devices.

Since telephone exchanges use DC, the building's battery room is usually wired directly to the consumer equipment and floats continuously on the output of rectifiers that typically supply rectified DC from utility power. When utility power fails, the battery carries the load without needing to switch. With this simple if somewhat expensive system, some exchanges have never lost power for a moment since the 1920s.

In recent years, large units of a utility power plant are usually designed on the basis of a system of units in which the necessary devices including the boiler, turbine generator unit and its power transformer (boost) and unit (auxiliary) are solidly connected as one unit. A less common configuration consists of two units grouped together with a common auxiliary station. As each turbine generator unit has its own auxiliary unit transformer attached, it connects to the circuit automatically. For unit starting, the auxiliaries are supplied by another unit (auxiliary) transformer or station auxiliary transformer. The changeover period from the first transformer from the unit to the next unit is designed for automatic and instantaneous operation at times when the emergency power system needs to activate. It is imperative that power to the unit's auxiliaries does not fail during a station shutdown (an event known as a blackout when all regular units temporarily fail). Instead, during shutdowns, the network is expected to remain operational. When problems occur, they are usually due to reverse power relays and frequency operated relays on the network lines due to serious disturbances in the network. Under these circumstances, the emergency station supply must be activated to avoid damage to equipment and dangerous situations, such as the release of hydrogen gas from generators into the local environment.

For a 208 VAC emergency supply system, a central battery system with automatic controls, located in the power plant building, is used to avoid long power supply cables. This core battery system consists of units of lead-acid battery cells to form a 12 or 24 V DC system, as well as reserve cells, each with its own battery charging unit. Also needed is a voltage sensing unit capable of receiving 208 VAC and an automatic system that can send a signal and activate the emergency supply circuit in case of failure of the 208 VAC station supply.

The use of emergency power systems in aviation can be on the aircraft or on the ground.

On commercial and military aircraft, it is critical to maintain power to essential systems during an emergency. This can be done through Ram air turbines or battery emergency power supplies allowing pilots to maintain radio contact and continue to navigate using MFD, GPS, VOR receiver or directional gyro for over an hour.

Localizer, glide slope") and other instrument landing aids (such as microwave transmitters) are power-hungry and mission-critical, and cannot be operated reliably on a battery, even for short periods. Therefore, where absolute reliability is required (such as when Category 3 operations are in effect at the airport), it is common to run the system from a diesel generator with automatic switchover to the mains in the event of generator failure. This avoids any transmission interruption while a generator reaches speed of operation and opposes the typical view of emergency power systems, where backup generators are considered secondary to the main electrical supply.

Computers, communication networks, and other modern electronic devices not only need power, but also a constant flow of it to continue functioning. If the source voltage drops significantly or drops completely, these devices will fail, even if the power loss is only for a fraction of a second. Because of this, even a backup generator does not provide protection due to the startup time involved.

To achieve more complete loss protection, additional equipment such as surge protectors, inverters "Inverter (electronics)") or sometimes a complete uninterruptible power supply (UPS) are used. UPS systems can be local (to one device or one outlet) or can extend to the entire building. A local UPS is a small box that fits under a desk or telecommunications rack and powers a small number of devices. A whole-building UPS can take several different forms, depending on the application. It directly powers a system of outlets designated as UPS power and can power a large number of devices.

Since telephone exchanges use DC, the building's battery room is usually wired directly to the consumer equipment and floats continuously on the output of rectifiers that typically supply rectified DC from utility power. When utility power fails, the battery carries the load without needing to switch. With this simple if somewhat expensive system, some exchanges have never lost power for a moment since the 1920s.

In recent years, large units of a utility power plant are usually designed on the basis of a system of units in which the necessary devices including the boiler, turbine generator unit and its power transformer (boost) and unit (auxiliary) are solidly connected as one unit. A less common configuration consists of two units grouped together with a common auxiliary station. As each turbine generator unit has its own auxiliary unit transformer attached, it connects to the circuit automatically. For unit starting, the auxiliaries are supplied by another unit (auxiliary) transformer or station auxiliary transformer. The changeover period from the first transformer from the unit to the next unit is designed for automatic and instantaneous operation at times when the emergency power system needs to activate. It is imperative that power to the unit's auxiliaries does not fail during a station shutdown (an event known as a blackout when all regular units temporarily fail). Instead, during shutdowns, the network is expected to remain operational. When problems occur, they are usually due to reverse power relays and frequency operated relays on the network lines due to serious disturbances in the network. Under these circumstances, the emergency station supply must be activated to avoid damage to equipment and dangerous situations, such as the release of hydrogen gas from generators into the local environment.

For a 208 VAC emergency supply system, a central battery system with automatic controls, located in the power plant building, is used to avoid long power supply cables. This core battery system consists of units of lead-acid battery cells to form a 12 or 24 V DC system, as well as reserve cells, each with its own battery charging unit. Also needed is a voltage sensing unit capable of receiving 208 VAC and an automatic system that can send a signal and activate the emergency supply circuit in case of failure of the 208 VAC station supply.