MEngC PE Electrical Building Services flashcards

34 cards. Read the term and the definition here. The deck at the top of this page is optional.

Voltage drop formula

For three-phase circuits, ΔV = √3·I·L·(R cosφ + X sinφ); for single-phase, ΔV = 2·I·L·(R cosφ + X sinφ), with R and X expressed in ohms per unit length.

Diversity factor

The ratio of the maximum demand of an installation or part of it to the total connected load of that installation or part. Applied at each distribution level to size cables and switchgear.

TN-S earthing system

A system in which the neutral and protective conductors are separate throughout the installation, from the source to the final circuits, with the protective conductor connected to the source earth.

Prospective short-circuit current

The current that would flow at a given point if a zero-impedance fault occurred there. Calculated from source and cable impedances and used to select protective devices and switchgear ratings.

Load factor

Average demand divided by peak demand over a defined period. A low load factor indicates infrastructure sized for brief peaks and suggests demand management or load shifting opportunities.

Power factor correction kVAr

Capacitor rating required equals P·(tanφ₁ − tanφ₂), where P is real power and φ₁ and φ₂ are the existing and target power factor angles.

Motor starting current

Direct-on-line induction motors draw roughly five to eight times full-load current during starting. Star-delta starting reduces this to about one third of the DOL value.

Energy use intensity

Annual energy consumption in kWh divided by conditioned floor area in square metres. Used to benchmark a building's energy performance against similar building types.

Lumen method formula

N = (E × A) / (Φ × CU × MF), where N is the number of luminaires, E the maintained illuminance, A the area, Φ the luminaire flux, CU the utilisation factor and MF the maintenance factor.

Unified Glare Rating

A numerical scale for evaluating discomfort glare from interior lighting. Lower values indicate less glare; offices commonly specify UGR of 19 or lower.

Maintenance factor

The product of lamp lumen depreciation, luminaire dirt depreciation and room surface dirt depreciation. It reduces initial illuminance to the maintained value used for design compliance.

Emergency escape lighting

Lighting provided to illuminate escape routes and safety signs. Typically at least 1 lux along the centre line of escape routes, maintained for a defined duration such as 90 minutes.

Earth-fault loop impedance

The impedance of the path from the point of fault back to the source via the protective conductor. It must be low enough for the protective device to disconnect within the required time.

TT system RCD rule

In a TT system, the earth electrode resistance RA multiplied by the RCD residual operating current IΔn must not exceed 50 V, ensuring touch voltage stays within safe limits.

Rolling sphere method

A lightning protection design method in which a sphere of specified radius is rolled over the structure; protected volumes are those not touched by the sphere. Radius depends on protection level.

Main equipotential bonding

Connection of extraneous conductive parts to the main earthing terminal to prevent dangerous potential differences. Conductor size is typically half the main protective conductor, minimum 6 mm² copper.

Ib ≤ In ≤ Iz rule

The design current must not exceed the protective device rating, which must not exceed the cable current-carrying capacity. This is the fundamental cable and protection coordination check.

Cable correction factors

Factors applied to tabulated current-carrying capacity for ambient temperature, grouping, thermal insulation and soil conditions. The product of all factors gives the installed capacity.

MCB trip types

Type B trips at three to five times rated current, type C at five to ten times, and type D at ten to twenty times. Selection depends on the inrush current of the load.

Short-circuit withstand

The cable and its protection must satisfy k²S² ≥ I²t, where S is the conductor cross-section, I the fault current, t the disconnection time and k a material-dependent constant.

Generator step-load capability

The fraction of rated capacity a generator can accept in a single load step, typically 25 to 50 percent. Motor starting and large chiller loads must be checked against this value.

Online double-conversion UPS

A UPS topology in which the load is continuously supplied through a rectifier and inverter, providing voltage and frequency regulation and isolation from mains disturbances.

Generator altitude derating

Generator output reduces with altitude because air density falls and cooling and combustion efficiency drop. A common rule of thumb is about one percent per 100 m above 1000 m.

Separately derived system

A generator whose neutral is not solidly connected to the utility neutral. It requires its own neutral-earth bond and earth electrode, and a four-pole transfer switch may be needed.

BACnet protocol

An open building automation protocol operating over IP, MS/TP and other data links. It enables interoperability between controllers and building management systems from different manufacturers.

4–20 mA analogue loop

A current-based sensor signal that is immune to voltage drop and noise over long cable runs. A 250 ohm resistor converts it to 1–5 V at the controller input.

Cat 6A cabling

A twisted-pair copper cabling category supporting 10GBASE-T transmission to 100 m. It is tested for insertion loss, return loss, near-end crosstalk and length.

DDC controller

A direct digital controller that executes control logic and PID loops for building services plant, communicating with sensors, actuators and the building management system.

Addressable fire alarm system

A system in which each detector and call point has a unique address, allowing the panel to identify the exact device in alarm and supporting cause-and-effect control logic.

Fire alarm sound pressure

Alarm signals must reach at least 65 dB(A) at the sleeping position, or 5 dB above ambient noise, whichever is greater, so occupants are reliably alerted.

Cause-and-effect matrix

A table defining which outputs are triggered by each input or combination of inputs in a fire alarm system. It is the primary commissioning and verification document.

Fire-resistant cable

Cable designed to maintain circuit integrity for a specified period under fire conditions. Used for circuits that must operate during a fire, such as sounders and pump supplies.

Lump sum contract

A contract in which the contractor agrees to complete a defined scope for a fixed total price. Variations are instructed separately and priced under the contract terms.

Practical completion

The stage at which the works are substantially complete and fit for use. It triggers the defects liability period and usually the release of part of the retention.