HP DL360 Power basics for IT professionals - Page 37

Downflow cooling: A downflow cooling system delivers supply air under a raised floor where it is distributed to the heat source through perforated tiles. Alternatively, supply air is commonly discharged through plenum grilles. Dropout: A decrease in voltage (volts) or current (amperes) anywhere, but usually describing a Backup loss of electric utility power. Earth: See Ground. Electrical noise: Electromagnetic interference or crossover from other circuits or signals, or the selfsame circuit, when one lacks the proper cable shielding, power supply, or power conditioning to maintain phase integrity. Electrostatic charge: An electric current (often brief) caused by induced voltage and stored charge. Energy: The amount of work that has been done over time, expressed in watt-hours (Wh), kilowatthours (kWh) or megawatt-hours (MWh). Farad (F): A measurement of electrical charge, named after Michael Faraday. One farad is the storage capacity of a capacitor having a charge of 1 coulomb on each plate and a potential difference of 1 volt between the plates. First Law of Thermodynamics: Every watt of power consumed is converted to heat or mechanical work. Frequency: The number of cycles that are completed in a one-second period. The unit of frequency is the Hertz (Hz). One Hertz equals one cycle per second. Full-wave rectifier: A device that changes the positive phase and the negative phase of alternating current (AC) to direct current (DC). Fuse: An electrical or electromechanical device that interrupts current to a circuit when a fault condition occurs. A fuse is usually an enclosed wire that melts when current exceeds its rated capacity. Electronic fuses perform the same function, using transistors to interrupt the flow of current. Ground (earth) contact (conductor, core, line, pin, pole, prong, terminal): A power plug element that conducts an electric flow (via a direct or a secondary connection) into the ground (earth) during an insulation fault that would otherwise cause harm or danger. Grounded: A device is grounded by making a connection between its electric circuit and ground (or to a component that is connected to ground). Ground leakage current: Residual current flow through the grounding conductor, which is always undesirable. With data processing occurring at ever-increasing speeds, most IT equipment includes capacitors in the power circuits to filter radio frequency (RF) signals to ground. While effective at filtering RF, these components tend to allow a small amount of AC current to pass to the ground. Leakage current is additive so that as more equipment is connected to the AC mains, the amount of leakage can increase. Ground potential: Electrical grounding provides this reference voltage level (also called zero potential). Henry (H): A unit of measurement for inductance. One henry permits the inductance of one volt when the current through a coil changes at the rate of one ampere per second. Hertz (Hz): A unit of measurement for frequency. One hertz equals one cycle per second. 37