MEngC PE Electrical Building Services cheat sheet

Voltage drop (three-phase): ΔV = √3·I·L·(R·cosφ + X·sinφ); for single-phase use 2·I·L·(R·cosφ + X·sinφ), with R and X in Ω per unit length.

Percent voltage drop = 100·ΔV/V_rated; typical design limits: 3% at final circuit, 5% total from origin to load (verify against the governing local standard).

Transformer full-load current: I = kVA·1000/(√3·V_LL). Short-circuit current at LV terminals ≈ I_FLC/Z_pu, where Z_pu includes transformer impedance plus source impedance.

Diversity (demand) factor = maximum demand / connected load; apply per final circuit and again at each distribution board to size upstream cables and switchgear.

TN-S uses separate neutral and protective conductors throughout; TN-C-S combines PEN upstream and separates at the origin; TT relies on local earth electrodes and requires RCD protection.

Radial circuits are simplest and cheapest but have no supply redundancy; ring circuits allow two paths but require continuity verification of the loop conductors.

Busbar/switchboard short-circuit withstand (Icw) must exceed the calculated prospective fault current at the point of installation, accounting for motor contribution.

Cable sizing sequence: design current → protective device rating (Ib ≤ In ≤ Iz) → voltage drop → short-circuit withstand (k²S² ≥ I²t) → earth-fault loop impedance check.

Load density benchmarks (indicative, confirm per project): office lighting 8–12 W/m², general power 15–25 W/m², HVAC electrical demand 30–60 W/m² depending on climate and system type.

Motor starting current for a DOL induction motor is typically 5–8 times full-load current; star-delta reduces starting current to about one third of DOL.

Power factor correction: capacitor kVAr = P·(tanφ₁ − tanφ₂), where φ₁ is the existing and φ₂ the target power factor angle.

Load factor = average demand / peak demand over the period; a low load factor signals oversized infrastructure or poorly managed peaks.

Maximum demand (MD) is measured over a utility-defined integration window (commonly 15 or 30 minutes); tariff charges usually apply to the highest MD in the billing period.

Energy use intensity (EUI) = annual energy consumption (kWh) / conditioned floor area (m²); benchmarking compares EUI against similar building types.

Harmonic distortion limits: THD voltage typically ≤ 5% and THD current ≤ 8% at the point of common coupling for typical commercial installations (verify local standard).

Elevator and escalator loads are intermittent and regenerative in modern drives; they should not be summed at full nameplate but assessed with a duty/usage factor.

Illuminance is measured in lux (lm/m²); maintained illuminance accounts for lumen depreciation (LLD) and luminaire dirt depreciation (LDD): E_maintained = E_initial × LLD × LDD × RSDD.

Lumen method: N = (E × A) / (Φ_luminaire × CU × MF), where N is the number of luminaires, CU is coefficient of utilisation and MF the maintenance factor.

Luminous efficacy (lm/W) ranks sources: LED typically 100–180 lm/W, fluorescent 60–100 lm/W, halogen 15–25 lm/W, incandescent 10–15 lm/W.

Colour rendering index (CRI/Ra) of 80+ is typical for offices; 90+ is used where colour judgement matters (healthcare, retail, design studios).

Unified Glare Rating (UGR) limit for offices is commonly UGR ≤ 19; higher values indicate greater glare discomfort.

Lighting power density (LPD) limits are set by energy codes; compliance can be shown by the space-by-space method or the whole-building area method.

Daylight-linked dimming and occupancy sensing typically yield 20–40% lighting energy savings; automatic switching must not create nuisance switching in occupied spaces.

Emergency lighting: escape route illuminance typically ≥ 1 lux along the centre line, with 50% of that value within a 1 m band; duration commonly 90 minutes (verify local code).

Earth-fault loop impedance: Zs ≤ U0/Ia, where U0 is nominal line-to-earth voltage and Ia is the current that disconnects the protective device within the required time.

TT systems: RA × IΔn ≤ 50 V, where RA is the earth electrode resistance and IΔn the RCD residual operating current.

Main equipotential bonding connects extraneous conductive parts to the main earthing terminal; conductor size is typically half the main protective conductor, with a minimum of 6 mm² copper.

Touch voltage limit for general locations is conventionally 50 V AC; for wet or special locations it may be 25 V AC.

Earth electrode resistance: a single rod in homogeneous soil has R ≈ ρ/(2πL)·[ln(4L/d) − 1]; multiple rods in parallel reduce resistance but with mutual coupling effects.

Lightning protection: a rolling sphere radius of 45 m corresponds to protection level III, 30 m to level II and 20 m to level I under IEC 62305.

Lightning protection system components: air termination, down conductor (typically at ≤ 10 m spacing) and earth termination with a recommended total resistance ≤ 10 Ω.

Residual current devices: 30 mA is used for supplementary protection against electric shock; 300 mA is used for fire protection of installations.

Coordination rule: Ib ≤ In ≤ Iz, where Ib is design current, In the protective device nominal rating and Iz the cable current-carrying capacity.

Cable current-carrying capacity: Iz = It × Ca × Cg × Ci × Cs, applying correction factors for ambient temperature, grouping, thermal insulation and soil conditions.

Short-circuit withstand: k²S² ≥ I²t, where S is conductor cross-section, I the fault current and t the disconnection time; k depends on conductor and insulation material.

Prospective fault current is calculated from source impedance plus cable impedance; transformer impedance dominates at the LV origin, cable impedance dominates at remote boards.

Discrimination: time-current curves of upstream and downstream devices must not overlap; current-limiting breakers may achieve discrimination by energy limitation rather than time delay.

MCB types: B trips at 3–5×In, C at 5–10×In, D at 10–20×In; selection depends on inrush current of the load.

Cable derating for harmonics: when third harmonic content is high, the neutral conductor may carry more than the phase current and must be sized accordingly.

Breaking capacity (Icu/Icn) of a protective device must be at least equal to the prospective short-circuit current at its installation point.

Generator sizing must cover steady-state load, motor starting kVA (largest motor plus running load), harmonic content and step-load acceptance; typical step-load capability is 25–50% of rated kVA.

Generator derating: approximately 1% per 100 m above 1000 m altitude and additional derating for ambient temperatures above the reference (commonly 40 °C).

UPS types: offline (standby) transfers on failure, line-interactive regulates voltage with an AVR, online double-conversion isolates the load continuously.

UPS autonomy time = battery capacity (Ah) × battery voltage × depth of discharge / load (W); battery capacity falls with age and low temperature.

Emergency generator changeover time for life-safety loads is typically ≤ 10 s; for some healthcare and data applications it is shorter per the applicable standard.

Fuel storage: typical design basis is 8–24 hours of full-load operation, subject to local code and client requirements.

Generator neutral earthing: a separately derived system requires its own neutral-earth bond and earth electrode; a solid neutral transfer switch may not.

Load bank testing verifies generator capacity under load; annual testing at 30–50% load for a defined period is common practice to prevent wet stacking.

BACnet is an open protocol over IP, MS/TP or other data links; Modbus is a master-slave protocol over RS-485 or TCP/IP; KNX is common in European lighting and room control.

DDC controllers execute PID loops; proportional band, integral time and derivative time are tuned per controlled variable, and unstable loops usually indicate excessive gain.

Sensors: 4–20 mA analogue loops are less noise-sensitive than 0–10 V; a 250 Ω resistor converts 4–20 mA to 1–5 V for input to a controller.

Structured cabling: permanent link and channel testing verify insertion loss, return loss, near-end crosstalk and length; Cat 6A supports 10GBASE-T to 100 m.

Fibre optic links are immune to electromagnetic interference and support long runs; single-mode is used for long distances, multimode for in-building backbones.

ICT rooms require dedicated power circuits, UPS-backed outlets, cooling and earthing; rack power density is commonly 3–8 kW per rack in commercial buildings.

Network topology: star topology dominates in building networks; redundancy is provided by dual-homed switches or ring topologies with rapid spanning tree.

Integrated building management systems require a points schedule listing each monitored or controlled point, its address, type, range and alarm limits.

Fire alarm system categories: conventional systems use zones, addressable systems identify individual devices; the category is selected based on building size and risk.

Detector coverage: a smoke detector typically covers up to 80 m² with a maximum spacing of about 10.5 m; heat detectors cover up to 50 m² with spacing around 7.5 m (verify local code).

Sound pressure level for alarm signals: at least 65 dB(A) at the sleeping position or 5 dB above ambient noise, whichever is greater.

Fire alarm cables must maintain circuit integrity; fire-resistant cables are used for circuits required to operate during a fire (sounders, control, pump supplies).

Cause-and-effect matrix defines which input triggers which output; it is the primary commissioning document for an addressable system.

Interface with other systems: fire alarm must release door holders, recall lifts, shut down air handling units and initiate smoke control, per the approved design.

Emergency lighting and fire alarm systems are separate but coordinated; both require regular testing and documented maintenance records.

Battery standby for fire alarm: typically 24 hours standby plus 30 minutes alarm (verify local code), and battery capacity must be derated for ageing and temperature.

Design responsibility: the engineer of record signs and seals drawings; changes during construction require formal revision control and re-approval.

Contract types: lump sum fixes total price for defined scope; remeasured contracts pay for actual quantities; cost-plus reimburses actual cost plus a fee.

Professional liability is generally limited to negligence; strict liability is uncommon, and limitation periods are defined by local statute.

Ethical duties: competence, honesty, confidentiality, disclosure of conflicts of interest, and reporting of unsafe conditions to the responsible authority.

Variations must be instructed in writing; verbal instructions should be confirmed in writing within the contract-defined period.

Practical completion triggers the defects liability period; retention is typically released in two parts (on completion and at the end of the defects period).

Standards hierarchy: national law overrides national standards, which may reference international standards (IEC, ISO); project specifications may be more stringent than codes.

Record documents as-built drawings, test certificates, O&M manuals and commissioning records are contract deliverables, not optional extras.