D      [A B C D E F G H I J K L M N O P Q R S T U V W X Y Z]

DA - Department Of the Army

dB - Decibel. A unit used to express relative difference in power or intensity, usually between two acoustic or electric signals, equal to ten times the common logarithm of the ratio of the two levels. Decibels indicate the ratio of power output to power input dB = 10 log10 (P1/P2). 

DC - Direct Current. Electrical current that flows in one direction only.

DC-DC Converter - A circuit or device that converts a DC input voltage (usually unregulated) to a regulated DC output voltage. The output voltage may be lower, higher, or the same as the input voltage. Switching regulator DC-DC circuits most often require an inductor or transformer to achieve the regulated output voltage. Switching regulator circuits can achieve a higher level of power efficiency when compared to non-switching techniques.

Delta Connection - A standard three phase connection with the ends of each phase winding connection in series to form a closed loop with each phase 120 electrical degrees from the other. In this system, three power carrying conductors are used, sometimes with a fourth safety ground wire.

Delta-Delta - The connection between a delta source and a delta load.

Delta-WYE - The connection between a delta source and a WYE load.

Derating - Utilizing a component or piece of equipment intentionally below its specified rating to allow for variations in operating parameters such as temperature.

DHS - Department of Homeland Security

DOD - Department Of Defense

DOE - Department Of Energy

DON - Department Of the Navy

Double Conversion - A UPS design in which the primary power path consists of a rectifier and inverter. Double conversion isolates the output power from all input anomalies such as low voltage, surges and frequency variations by converting AC to DC to AC. See Online UPS.

Dropout Voltage - The voltage at which a device fails to operate properly or safely.  Computer systems will reboot, reset, or lose data when line voltage falls below approximately 95-100 Volts AC.  See Brownout.

Dual Source Input - In this case, the UPS has both a primary and secondary source feeding it. If either source fails, the other takes over with no break. If they both fail, the batteries take over with no break.

Dry Contact - Isolated contacts through which the end user supplies an external circuit.  UPS dry contacts provide basic communication capabilities such as monitoring and shutdown.

Dynamic regulation - The ability to hold the controlled level to specification while other parameters vary, i.e. dynamic voltage regulation while the load is changing.

E      [A B C D E F G H I J K L M N O P Q R S T U V W X Y Z]

Earth ground - A low impedance path to earth for the purpose of discharging lightning, static, and radiated energy, and to maintain the main service entrance at earth potential.

Efficiency - The ratio of the output power from the UPS to the input power from the utility. The mathematical formula is: Efficiency = Po/ Pi Where "Po" equals power output, "Pi" equals power input, and power is represented by watts. This shows the percentage of the input power that is available as useful output power. For example, a UPS that is 85% efficient delivers 85% of the utility power it receives to the load. The remaining power takes the form of dissipated heat.

EIA - Electronic Industries Alliance. An organization that helps set standards in the electronics industry.

Electrician - Refers to an installation electrician qualified to install heavy-duty electrical components in accordance with local codes and regulations. May not qualified to maintain or repair electrical equipment; compare to Technician.

Electromagnetic - A magnetic field caused by an electric current. Power lines cause electromagnetic fields that can interfere with nearby data cables.

Electromechanical - A mechanical device which is controlled by an electric device. Solenoids and shunt trip circuit breakers are examples of electromechanical devices.

Electrostatic - A potential difference (electric charge) measurable between two points which is caused by the distribution if dissimilar static charge along the points. The voltage level is usually in kilovolts.

Electrostatic Discharge (ESD) - Current produced by two objects having a static charge when they are brought close enough to produce an arc or discharge (static electricity). The effects of static discharge can range from simple skin irritation for an individual to degraded or destroyed semiconductor junctions for an electronic device.

Electrostatic Shield - A metallic barrier or shield between the primary and secondary windings of a transformer which reduces the capacitive coupling and thereby increases the transformers ability to reduce high frequency noise. See Faraday Shield.

EMF - Electromotive force or voltage. See Voltage.

EMC - Electro Magnetic Compatibility. The requirement for both electromagnetic emissions and susceptibility of a piece of equipment to comply with a governing set of standards.

Emergency Shutdown - Used to instantly or quickly shutdown all of the electrical power available to the UPS, the power supply and/or the load. An emergency shutdown device is usually used during a crisis to prevent damage to the UPS and the load. The local control switch or switches are commonly referred to as the EPO or Emergency Power Off. Some installations require a Remote Emergency Power Off (REPO) capability as part of their security/safety system.

EMI/RFI - Electro Magnetic Interference/Radio Frequency Interference. Electrical energy emitted by one electrical device that may disturb the normal operation of another device. The interference is either radiated emissions (RE) from the surface of a device or conducted emissions (CE) via its electrical connections.

EMI/RFI Filter - A device that suppresses or reduces the level of EMI/RFI emitted by electrical or electronic equipment.

EPO - Emergency Power Off. See Emergency Shutdown.

Ethernet - A baseband LAN specification invented by Xerox Corporation and developed jointly by Xerox, Intel, and Digital Equipment Corporation. Often referred to as a contention-based topology because workstations are in competition for channel bandwidth. For this reason, Ethernet uses a cable-sharing process called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). Ethernet has become a series of standards produced by IEEE referred to as IEEE 802.3.

ETL - Electrical Testing Laboratories. A US independent safety agency that sets standards for product safety. See UL, CSA, CE, VDE and TÜV.

ETM - Elapsed Time Meter. A time measurement device that is counting time only when a piece of equipment is energized. Typically used to log total operational hours of the equipment for maintenance and event logging.

F     [A B C D E F G H I J K L M N O P Q R S T U V W X Y Z]

Fahrenheit (°F) - An English measure of temperature level, degrees Fahrenheit.

Farad (F) - Unit of measurement for capacitance.

Faraday Shield - A grounded electrostatic shield that reduces coupling capacitance in transformers. The shield, which effectively reduces output common mode noise, is placed between the primary and secondary windings of a transformer.

Fault Current - The level of current that can flow if a short circuit is applied to a voltage source. 

Fault Tolerance - The ability of a system to continue operating in the event of a fault.

FCC - Federal Communications Commission. The FCC specifies the maximum amounts of EMI and RFI that electrical equipment is permitted to generate in Part 15 of the FCC Rules and Regulations. 

Ferroresonant Transformer - Transformer in which part of the core is driven into saturation by a resonant tank circuit. The output of the transformer, taken from the saturated portion, is relatively immune to variations in input voltage.

Firmware - Software that is stored in semiconductor memory such as ROM, EPROM or EEPROM. Firmware is typically an embedded instruction set to control the operation of logical hardware found in monitoring and/or control systems.

Flame retardant - The materials will not support combustion on their own and will in fact help suppress combustion. This applies to the battery case materials, wire insulation, switches, circuit breakers, fuse holders, and connector bodies.

Float Charging - The battery charging mode for which sealed lead acid batteries are designed. The float voltage is the ideal maintenance voltage for the battery which maximizes battery life. When the float voltage is applied to a battery, a current known as the float current flows into the battery, exactly canceling the battery's own internal self-discharge current. Sealed lead acid batteries require float charging at least occasionally or they will become permanently degraded by a process called sulfation. Maximum lifetime is obtained when a sealed lead acid battery is permanently float charged.

Flyback Converter - Also called a buck-boost converter, this topology typically uses a single transistor switch and eliminates the need for an output inductor. Energy is stored in the transformer primary during the first half of the switching period when the transistor switch is on. During the second half or flyback period when the transistor is off, this energy is transferred from the transformer secondary. Also see Boost Regulator, Buck Regulator, Bridge Converter, Forward Converter, Push-Pull Converter and Resonant Converter.

Flux - The lines of force of a magnetic field.

Forward Converter - Also called a Buck-Derived converter, this topology, like the flyback converter, typically uses a single transistor switch. Unlike the flyback converter, energy is transferred to the transformer secondary while the transistor switch is on, and stored in a output inductor. See Boost Regulator, Buck Regulator, Bridge Converter, Flyback Converter, Push Pull Converter and Resonant Converter.

Forward Transfer Impedance - The amount of impedance placed between the source and load with installation of a power conditioner. With no power conditioner, the full utility power is delivered to the load; even a transformer adds some opposition to the transfer of power. On transformer based power conditioners, a high forward transfer impedance limits the amount of inrush current available to the load.

Frequency - The number of cycles (oscillation positive and negative) completed in one second. Defined as Hertz (Hz). In North America, utility power is generally 60 cycles per second or 60 Hertz. Many countries based on European standards use 50 Hertz. Aircraft typically use 400 Hertz.

Frequency Converter - A unit that changes the power from one frequency to another frequency (i.e. from 120 VAC, 400 Hz input to 120 VAC, 60 Hz output). Input and output voltages may also be converted as well.

Frequency Variations - A change in frequency from the normally stable utility frequency of 50 Hz, 60 Hz or 400 Hz. This may be caused by erratic operation of emergency generators or unstable frequency power sources. For sensitive electronic equipment, the result can be data corruption, hard drive crash, keyboard lockup and program failure.

Full Bridge Converter - Converter topology that typically operates as forward converter but uses a bridge circuit, consisting of four switching transistors, to drive the transformer primary. Also see Bridge Converter.

Full Height (FH) - Refers to a computer device that requires a full size drive bay or space, typically 5.25” wide

Fuse - A one-time overcurrent protection device employing a fusible link that melts (blows) after a certain current level is exceeded for a certain length of time.

Full Load - The greatest load that a circuit is designed to carry under specific conditions; any additional load is considered an overload.



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