Uninterruptible Power Supplies (UPS), Power Quality, and IT Earthing

Comprehensive vocabulary flashcards covering Uninterruptible Power Supply (UPS) topologies (rotary, offline, online), power line disturbances and noise suppression, IT equipment earthing, NSAI IS 10101:2020 rules, and electrical grid resilience.

Uninterruptible Power Supply (UPS)

A device or system that provides near instantaneous protection from power interruption using a battery (which can be a supercapacitor) or a flywheel, maintaining power to sensitive electrical equipment such as computers and medical systems during a mains supply failure.

Multiple Redundancy

A technique of dividing total generating requirements across a number of smaller generating sets while keeping at least one set in reserve, guaranteeing that essential services remain maintained under all but the most improbable circumstances.

Diesel Rotary UPS (DRUPS)

A system where the load is supplied by an alternator driven by an electric motor; during normal operation, kinetic energy is stored in a flywheel, and upon mains failure, the flywheel maintains alternator rotation while a diesel engine starts and engages via an electromagnetic clutch.

Rotary UPS System using a DC Motor

A rotary UPS configuration consisting of a rectifier supplying a shunt-excited DC motor coupled to a synchronous AC generator and flywheel; the flywheel provides bridging energy for supply breaks up to $2\text{ seconds}$, with a secondary battery bank included for longer bridging times.

Static (Solid State) UPS

An all-electronic power protection system with no moving parts except cooling fans, comprising a rectifier, battery bank, inverter, and static bypass switch to convert and condition electrical power.

Offline (Standby) UPS System

A UPS configuration where the load is normally supplied directly by filtered AC mains power through a transfer switch; when a power failure occurs, the transfer switch changes state to supply power from the battery via an inverter within a transfer time of $2\text{ to }10\text{ ms}$.

Online (Double-Conversion) UPS System

A UPS configuration where incoming mains AC is continuously converted to DC by a rectifier to feed the DC bus and charge batteries, and an inverter continuously converts the DC back to AC to supply the load with zero transfer time and complete electrical isolation.

Static Bypass Switch

A solid-state switch in a UPS that provides an alternate path directly from the mains to the load, allowing maintenance on the UPS components or automatically bypassing the system in the event of an internal fault or overload.

Line-Interactive UPS

A UPS system that sits between offline and online designs by incorporating automatic voltage regulation (AVR) to correct voltage sags and surges without switching over to battery power.

Transfer Time

The duration taken by a standby or offline UPS to switch the load from utility mains to battery/inverter power during an outage, typically $5\text{ to }10\text{ ms}$ in high-quality systems.

Voltage Sag (Brownout)

A short-term reduction in the AC voltage level to $90\%$ or less of nominal voltage (often between $80\%$ and $90\%$) lasting from half a supply cycle to a few seconds, commonly caused by the starting inrush current of heavy electrical loads like motors or compressors.

Blackout

A total loss of mains electrical power lasting from seconds to days, caused by severe weather, power line damage, grid overload, or equipment failure, leading to loss of volatile computer memory and unsaved data.

Micro Cut (Micro-Break)

A complete loss of mains power for less than one full supply cycle (less than a few milliseconds to $1\text{ second}$), which can cause systems to lock up and cause computer hard disk read/write heads to crash onto disk platters.

Voltage Spike (Impulse)

An instantaneous, dramatic increase in supply voltage typically caused by nearby lightning strikes or the restoration of power after an outage, capable of causing catastrophic semiconductor and component destruction.

Voltage Surge

A short-term increase in supply voltage exceeding $110\%$ of nominal voltage with a duration of at least half of a supply cycle, commonly caused by the rapid switching off of heavy inductive loads in the immediate vicinity.

Transient Over Voltage

A sudden, fast-dissipating rise in voltage or current lasting from microseconds to milliseconds, produced between lines, neutral, or earth by lightning strikes and the switching of large inductive loads.

Electromagnetic Interference (EMI)

Unwanted electrical noise induced in cables or circuits by electromagnetic fields created by changes in current levels in neighboring cables, magnetic flux leakage from transformers, or industrial switching equipment.

Radio Frequency Interference (RFI)

High-frequency electromagnetic disturbance that is radiated through the air or conducted along supply lines, generated by sparking contacts, arcing commutators, fluorescent starter switches, and unscreened dielectric heating equipment.

RF Choke

An inductive coil comprising insulated wire wound on a ferrite core that offers near-zero impedance at mains frequency ($50\text{ Hz}$) but an impedance of approximately $1\text{ M}\Omega$ at radio frequencies, blocking conducted RFI.

Normal Mode Noise (Transverse Noise)

AC or DC electrical interference that appears across the power feed conductors (between the live and neutral lines), often caused by EMI coupling directly with the power source.

Normal Mode Rejection Ratio (NMRR)

A quantitative measure of the ability of an electrical device or circuit to reject or attenuate normal mode noise across the live and neutral conductors.

Common Mode Noise (Longitudinal Noise)

AC or DC electrical interference occurring between the common earth (ground) conductor and the power feed conductors (live and neutral).

Common Mode Rejection Ratio (CMRR)

A quantitative specification describing the effectiveness of a circuit or device in rejecting common mode noise present between the power lines and earth.

Harmonics

Sinusoidal voltages or currents at frequencies that are integer multiples of the fundamental power system frequency ($50\text{ Hz}$), generated by non-linear loads such as rectifiers, switched-mode power supplies, variable speed drives, and discharge lighting.

Triplen Harmonics

Odd multiples of the third harmonic frequency (e.g., $150\text{ Hz}$, $450\text{ Hz}$) that add together in the neutral conductor of a three-phase system as zero-sequence currents, causing excessive neutral current and conductor overheating.

Power Line Conditioner

A power quality improvement unit typically containing an isolating transformer, high-impedance choke coil ($X_L = 2\pi f L$), and surge suppressors that eliminates spikes, filters harmonic noise, and stabilizes the voltage band without battery backup.

Pulse Width Modulation (PWM)

A power control technique that rapidly switches an inverter's semiconductor devices on and off at high frequency, varying the pulse widths (duty cycle) to synthesize an output voltage waveform that closely approximates a pure sine wave without high heat dissipation.

Metal Oxide Varistor (MOV)

A voltage-dependent non-linear resistor that exhibits very high electrical resistance under normal nominal voltage but drops dramatically in resistance when voltage exceeds a preset clamping threshold, diverting surge energy to earth.

Surge Protection Device (SPD)

An electrical protective unit incorporating voltage-limiting components (such as MOVs and TVS diodes) or voltage-switching components (such as gas discharge tubes and spark gaps) designed to clamp high-voltage transients and divert surge currents safely to earth.

No-Break Supply (NSAI Classification)

An automatic supply system classified under NSAI IS 10101:2020 that ensures a continuous supply without any interruption or break during the change-over period within specified voltage and frequency tolerances.

Very Short Break Supply (NSAI Classification)

An automatic supply classified under NSAI IS 10101:2020 that restores power to the system within a change-over time of $0.15\text{ s}$.

Very Clean Wiring Category

The cabling classification reserved for sensitive data and signal communication circuits (e.g., Ethernet, optical fiber, RS-232, analog signals), requiring dedicated segregation and often enclosed within grounded metallic containment.

Dirty Wiring Category

The cabling classification containing high-noise and heavy-current circuits, including raw mains supplies, motor circuits, lift feeders, fluorescent lighting, and electric welders.

Cable Segregation Rule ($\ge 300\text{ mm}$)

The installation requirement mandating that dirty wiring circuits must be separated from clean or very clean data cables by a minimum distance of $300\text{ mm}$, or physically isolated using a continuous earthed metallic partition.

Technical Earth Conductor (TEC) / Clean Earth

A dedicated, low-noise functional earth conductor routed directly from the main earthing terminal to serve exclusively as a signal reference ground for sensitive IT and communications equipment, separate from the safety CPC.

Circuit Protective Conductor (CPC)

The standard safety earthing conductor designed to carry high prospective fault currents to disconnect protective devices during an insulation failure, but subject to noise and unsuitable for use as an IT signal reference.

Earth Loop (Ground Loop)

An unwanted conductive loop created when equipment interconnected by data cables is earthed at multiple separate points with a potential difference between them, driving a circulating $50\text{ Hz}$ current through cable screens and causing signal corruption.

Screen Reinforcement By-Pass Conductor

An equipotential bonding conductor installed in close physical proximity along a screened signal cable route to divert prospective power fault currents away from the delicate cable screen and minimize inductive loop area.

Signal Reference Grid (SRG)

A low-impedance mesh network of copper conductors or bonded raised-floor pedestals installed beneath server room floors to provide a uniform high-frequency equipotential reference ground for IT racks and cabinets.

Mesh Common Bonding Network (Mesh-BN)

An equipotential bonding topology in data centers where all equipment enclosures, racks, cable trays, and structural metalwork are interconnected in a dense grid to provide multiple parallel low-impedance paths to ground.

All-Island Grid Interconnections

The synchronized electricity transmission network combining the Republic of Ireland (operated by EirGrid) and Northern Ireland (operated by SONI), connected to Great Britain's National Grid via the Moyle Interconnector and the East-West Interconnector.

Battery Energy Storage System (BESS)

An electrochemical installation that stores surplus power (e.g., from solar panels or off-peak grid power) via an inverter and delivers it during outages, peak-demand periods, or grid frequency stabilization events.

Flow Battery

An energy storage technology utilizing liquid electrolytes stored in external tanks and pumped through an electrochemical cell stack, offering independent scaling of power and energy capacity with a long cycle life for grid-scale applications.

Sodium-Sulphur Battery

A high-temperature molten-salt secondary battery technology utilized for utility- and industrial-scale grid energy storage and backup applications.

Standby Diesel Generator

An emergency backup unit comprising an internal combustion diesel engine coupled to an AC alternator that generates high electrical power for hours or days during prolonged grid blackouts, but requires fuel storage, regular maintenance, and exhaust ventilation.

Storm Ophelia (October 2017)

A major severe weather event in Ireland that caused widespread power infrastructure damage and left $360{,}000$ homes without grid electricity for multiple days.

Microgrid

A localized grouping of electricity generation sources, energy storage systems, and loads that normally operates connected to the centralized grid but can disconnect ('island') to operate autonomously during grid disruptions.