MEngC PE Electrical Building Services key terms

The ratio of the maximum demand of a building or group of loads to the sum of the individual maximum demands of its parts, reflecting that not all loads peak at once.

The highest average load over a defined period, such as 15 or 30 minutes, used to size incoming supplies, switchgear and protective devices for a building installation.

A tabulated inventory of every connected load in an installation with its rating, quantity, demand factor and calculated current, forming the basis for supply and distribution sizing.

The ratio of the maximum demand of a single load or appliance category to its connected load, accounting for intermittent or partial use such as cycling of air-conditioning plant.

The reduction in voltage along a conductor caused by its impedance carrying current, calculated for cable sizing so that equipment at the far end receives adequate voltage under load.

Reduction of a cable's tabulated current-carrying capacity to account for installation conditions such as ambient temperature, grouping, thermal insulation, soil conditions or protective enclosures.

The total impedance of the path through which fault current flows from the source, through the fault, back via the protective conductor and earth to the supply neutral point.

The current that would flow at a given point in an installation if a short circuit of negligible impedance occurred there, used to select switchgear and verify protective device ratings.

The maximum fault current a protective device or switchgear can safely interrupt without damage or danger, expressed as a rated short-circuit value for the installation point where it is applied.

Selection and setting of protective devices so that the device nearest a fault operates first, limiting outage extent while ensuring adequate speed and sensitivity for safety.

The deliberate time and current grading of protective devices so that a fault is cleared by the device closest to it, leaving upstream devices closed and unaffected.

The maximum permitted time for a protective device to clear a fault and disconnect the circuit, set by standards as a function of circuit voltage and whether socket outlets are involved.

A device that continuously compares currents in live conductors and trips when their imbalance exceeds a set residual current, providing additional protection against electric shock and earth leakage.

Deliberate electrical connection of exposed conductive parts and extraneous conductive parts so they sit at substantially the same potential, reducing touch voltage during a fault.

The designated bar or block in an installation where earthing conductors, circuit protective conductors and bonding conductors are terminated and connected to the earth electrode or supply earth.

The resistance between an earth electrode and the general mass of earth, dependent on soil resistivity and electrode geometry, measured to confirm it is low enough for protective and functional purposes.

A designed arrangement of air terminations, down conductors and earth terminations that intercept and safely conduct lightning current to earth, with a defined level of protection.

A component that limits transient overvoltages from lightning or switching by diverting surge current to earth, installed at appropriate points in the distribution system.

A short-duration, high-amplitude voltage spike caused by lightning, switching operations or fault clearance, capable of damaging insulation and sensitive electronic equipment.

The SI unit of illuminance, one lumen per square metre, used to express the amount of light arriving on a surface and to set maintained illuminance targets for tasks.

The ratio of luminous flux emitted by a lamp or luminaire to the electrical power it consumes, expressed in lumens per watt and central to lighting energy comparisons.

A calculation technique that estimates the average illuminance on a working plane from total lamp lumens, room geometry, utilisation factor and maintenance factor.

The ratio of minimum to average, or minimum to maximum, illuminance over a task area, indicating how evenly light is spread and whether shadows or dark patches are acceptable.

A multiplier applied in lighting calculations to allow for the gradual loss of light output from lamp ageing and dirt accumulation on luminaires and room surfaces.

A complete lighting unit comprising one or more lamps together with the parts that distribute light, position and protect the lamps, and connect them to the supply.

An engine-driven alternator held in reserve to supply essential or whole-building loads when the normal supply fails, typically started automatically by a transfer signal.

A system that maintains supply to critical loads without interruption during a mains failure, typically using a battery or flywheel energy store with an inverter.

A switch that detects loss of the normal supply and automatically transfers load to a standby source, then restores it when the normal supply returns.

Building systems required to remain operational during a mains failure for life safety or statutory reasons, such as fire pumps, smoke control, lifts and emergency lighting.

Lighting provided to operate automatically when normal lighting fails, covering escape routes, open areas and high-risk tasks for a defined minimum duration.

An interconnected arrangement of detection, manual call points, control equipment and sounders or visual devices that detects fire and warns occupants to evacuate.

A fire detection arrangement in which the control panel identifies each device individually by address, allowing precise location of alarms and faults.

A networked control system that monitors and manages building services such as HVAC, lighting and energy use, using controllers, sensors and a supervisory interface.

Software and metering that collects, displays and analyses building energy consumption, supporting benchmarking, targeting and identification of efficiency opportunities.

Deliberate reduction or shifting of electrical demand in response to a signal or price incentive, helping balance the grid and reduce peak charges.

Addition of capacitors or other reactive compensation to reduce the phase angle between voltage and current, lowering reactive power drawn and avoiding utility penalties.

Distortion of the current or voltage waveform caused by non-linear loads such as electronic power supplies, producing integer multiples of the fundamental frequency.

A conductor that connects exposed conductive parts of equipment to the earthing system, providing a low-impedance fault path and equalising potential.

Insurance covering a professional for claims arising from negligent design, specification or advice, protecting against financial loss from errors in the services provided.

The legal and ethical obligation of a professional to act with reasonable skill and care towards clients, the public and others affected by the work.