Glossary of Power and Energy-Related Terms

This extensive glossary aims to provide clear and concise descriptions of commonly used terms in the fields of power protection, renewable power, and energy management. Understanding these terms is essential for anyone involved in the power industry or seeking to optimise their power supply systems.

Active Power: The actual power (measured in Watts) dissipated by a load.

Alternating Current (ac): An electrical current in which electrons flow periodically in one direction to a maximum level, then drop back to zero before flowing in the opposite direction. The waveform resembles a sinewave.

Amp or Amperes (A): A unit of measurement for electrical current flow.

Ampere-hour (Ah): A measure of the number of Amps a battery set can deliver per hour.

Apparent Power: The current drawn by a load at a given supply voltage, measured in VA.

Availability: A ratio of system uptime compared to its downtime, expressed as a percentage.

Automatic Bypass: A circuit within a UPS or bypass panel used to transfer the load from and to a bypass supply, which may be relay or static switch-based.

Automatic Voltage Stabiliser (AVS): A device that stabilises the mains power supply voltage to a load. Also known as an Automatic Voltage Regulator (AVR) or Voltage Regulator (VR).

Autonomy (Runtime or Backup Time): The duration (in minutes or hours) that a battery set or power source can support the load.

Battery Block: A self-contained battery consisting of several connected battery cells.

Battery Cell: A simple electrical circuit within a battery block comprising positive and negative electrodes or plates, an electrolyte, and a separator.

Battery Set: A collection of battery strings or multiple battery strings.

Battery String: A series arrangement of battery blocks to achieve a specific Vdc and Ah rating.

Blackouts: A term used to describe a complete mains power supply failure, also referred to as an outage.

Boost charge: A high charge voltage applied to a battery set.

Booster-Converter: An assembly used within transformerless UPS to step up the dc supply from a rectifier or battery set to the level required by an inverter.

Branch Circuits: Circuits run on individual breakers from the main distribution board.

Break-Before-Make (BBM) Bypass: A bypass that introduces a break when transferring a load from the output of a UPS to the bypass supply, and vice versa.

Brownouts: Extended periods of voltage decreases below the nominal mains power supply voltage, lasting for many cycles or more.

Building Management System (BMS): A centralised alarm monitoring system, usually providing visual status indication boards or computer monitoring screens.

Bunding: A method of containing liquid spillage from a system, such as oil or diesel from a standby generator.

Bypass: A power path arrangement normally providing an alternative power supply path in case the primary one fails.

Capacitance: The ability of a circuit to store electrical energy as a charge. The circuit is known as a capacitive circuit.

Central Power Supply System (CPSS): A type of standby power system used within emergency lighting, security, and medical applications, operating similarly to a UPS.

Circuit-Breaker: A device that inhibits high surge currents over a set stated figure, isolating the circuit under such conditions.

Clamping level: The level at which a spike or transient protection device clamps the voltage down to a lower level.

Common Mode Noise: A form of high-frequency electrical noise resulting from disturbances between the supply lines and the earth (phase-to-earth or neutral-to-earth).

Constant Voltage Transformer (CVT): A type of ferroresonant transformer.

Crash Kits: Spare kits held on-site to enable a fast emergency response to a system failure.

Crest Factor: The mathematical ratio of the peak to RMS value of an ac waveform.

Critical Loads: Systems that directly affect an organisation's ability to operate and must be kept running during a mains power supply failure.

Current (A): The 'volume' of electricity flowing in a circuit, expressed in Amps.

Current Limit: The restriction of the amount of current that can be drawn from any point within an electrical circuit or UPS output.

DCIM Software: Data Centre Infrastructure Management software used to monitor and manage physical and asset-level components.

Deep Discharge: A battery charge state where the battery voltage (Vdc) has dropped below a safe operating level from which it cannot recover.

Depth of Discharge (DOD): A measure of battery capacity, e.g., a 5kWh battery pack will give 4kWh at a DOD of 80%. Higher DOD increases battery life expectancy.

Direct Current (dc): Electric current in which electrons flow in one direction only.

Discrimination: The protection around a device within a Power Continuity Plan that disconnects it if a short-circuit or overload occurs, preventing damage to other devices without interrupting their operation.

Displacement Power Factor: The ratio of Real Power (W) to Apparent Power (VA) at the fundamental frequency.

District Network Operator (DNO): The organisation responsible for local electrical supply cables and infrastructure.

Distortion: A variation in waveform from a true wave shape.

Distortion Power Factor: The power factor produced by the harmonics generated from non-linear loads.

Dry-Contact: Same as a Volt-Free Contact.

Dynamic Stability: The ability of a device to respond to a load-step change and deliver a stable output voltage waveform.

Earth Fault (Ground): A connection from the live component of a mains power supply to an earth connection.

Electrical Noise: High-frequency noise on a sinewave, either Common Mode or Normal Mode.

Electro-Magnetic Compatibility (EMC): The extent to which an electronic or electrical device tolerates and generates Electro-Magnetic Interference (EMI).

Electro-Magnetic Induction: The production of an electrical potential difference (voltage) across a conductor situated in a changing magnetic flux.

Electro-Magnetic Interference (EMI): A type of electrical noise that causes an electromagnetic disturbance.

Emergency Power Off (EPO) / Emergency Shutdown (ESD): A signal contact on a UPS that initiates a total UPS shutdown.

Emergency Response Time: The speed of response specified within a maintenance plan for an engineer to attend the site.

Extra Low Voltage (ELV): A safety measure to prevent electric shock, divided into Functional Extra Low Voltage (FELV), Protected Extra Low Voltage (PELV), and Safety Extra Low Voltage (SELV), with progressively more restrictive safety properties.

Ferroresonance or Ferroresonant: A term applied to resonant interactions between capac

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