Iran Nezam Electrical Design Exam cheat sheet

Diversity (demand) factor = maximum demand / connected load; apply per-use factors before summing, never after, to avoid double-counting.

Illuminance, socket, and motor loads are summed as volt-amperes; convert kW to VA by dividing by power factor (kVA = kW / cos φ).

Three-phase power: S = √3 × U_L × I_L; single-phase: S = U_ph × I_ph. For balanced 400/230 V systems, 1 kVA ≈ 1.44 A line current.

Harmonic-rich loads (SMPS, LED drivers, VFDs) make neutral current additive; design neutral for at least 1.45–1.73 × phase current when triplen harmonics dominate.

Continuous loads are sized at 125% of rated current for overcurrent device and conductor ampacity per IEC 60364-5-52 practice.

Motor starting: direct-on-line inrush is 5–8 × FLC for 0.1–0.5 s; star-delta reduces starting current and torque to one third.

Cable grouping derating factors apply multiplicatively with ambient temperature correction: I_z = I_t × k_temp × k_group × k_soil.

Design sequence: load schedule → diversity → transformer/generator sizing → panel schedule → cable sizing → voltage drop → fault level → protection coordination.

Connected load = sum of all nameplate ratings; maximum demand = connected load × diversity factor; use measured or tabulated diversity, not guesswork.

Standard demand factors: residential socket outlets 0.4–0.6; office lighting 1.0; office sockets 0.5–0.7; HVAC 0.8–0.9; elevators 0.5–0.7.

After-diversity maximum demand (ADMD) per dwelling is typically 2–4 kVA in residential towers; use utility-published ADMD where available.

Load schedule columns: circuit ref, description, phase, kW, cos φ, kVA, demand factor, diversified kVA, current per phase, cable size, breaker rating.

Balance single-phase loads across L1/L2/L3; unbalance should stay below 15% to limit neutral current and voltage unbalance.

Future growth allowance: add 20–25% spare capacity at transformer and main switchboard level; spare ways 20–25% in panelboards.

Transformer sizing: kVA ≥ diversified demand / (loading limit × power factor); typical design loading 70–80% for oil-immersed, 60–70% for dry-type.

Generator sizing must check both steady-state kW and transient motor-start kVA; largest motor start typically governs, not total load.

Radial distribution is standard for LV building services; ring circuits are limited to socket outlets in some national practices.

TN-S: separate PE and N from origin; TN-C-S: combined PEN upstream, split at origin; TT: separate earth electrode at installation.

Selectivity: upstream device must not trip for downstream faults; time-current curves must not overlap, or use current-limiting and energy-selective pairs.

Panelboard busbar rating must be ≥ incoming device rating; outgoing ways sized for 1.25 × largest outgoing circuit current where continuous.

Cascading (back-up protection) is permitted only where the manufacturer's tested combination is documented; never assume generic cascading.

Form of separation (Form 1–4b) defines internal barriers between busbars, functional units, and terminals; specify per IEC 61439-2.

IP rating selection: indoor dry areas IP2X/IP4X; wet plant rooms IP54; outdoor IP55–IP65; verify against actual environment.

Label every panel with unique tag, upstream source, voltage, rated current, and short-circuit withstand (Icw) per IEC 61439.

Ampacity rule: I_b ≤ I_n ≤ I_z, and I_2 ≤ 1.45 × I_z, where I_b is design current, I_n device rating, I_z tabulated corrected capacity.

Correction: I_z = I_t × k_temp × k_group × k_soil × k_burial; apply all applicable factors, never add them.

Voltage drop single-phase: ΔU = 2 × I × L × (R cos φ + X sin φ); three-phase: ΔU = √3 × I × L × (R cos φ + X sin φ).

Quick mV/A/m method: ΔU = mV/A/m × I × L / 1000; sum tabulated mV/A/m values for cable and any series elements.

Typical limits: 3% at final circuit and 5% total from origin to load for public LV supplies; verify against local code and utility.

Minimum conductor sizes: 1.5 mm² lighting, 2.5 mm² socket outlets, 4 mm² for fixed appliances; verify with actual load and derating.

Cable routing: separate power and data by at least 300 mm, cross at 90°, and avoid parallel runs longer than 3 m without metallic separation.

Fire-rated cable (e.g., FE180/PH120 or BS 7846 F120) is required for life-safety circuits; route outside fire compartments where possible.

Three-phase fault current: I_k3 = U / (√3 × Z_total); include source, transformer, cable, and busbar impedances.

Transformer fault level: I_sc ≈ S / (√3 × U × Z_pu); typical distribution transformer Z is 4–6% for 1–2 MVA units.

Cable impedance reduces fault current with distance; never size protective devices using only the transformer fault level at origin.

Earth fault loop impedance Z_s must satisfy Z_s ≤ U_0 / I_a, where I_a is the current that trips the device within the required disconnection time.

TT systems require RCD protection for all socket outlets and most final circuits; RCD 30 mA for additional protection, 300 mA for fire protection.

Earthing conductor sizing: half the phase conductor up to 16 mm², equal up to 35 mm², and per table above; verify with adiabatic equation S = √(I²t)/k.

Equipotential bonding: main earthing terminal bonds incoming metallic services; supplementary bonding in bathrooms and special locations.

Coordination study outputs: time-current curves, discrimination tables, and settings for adjustable breakers (Ir, Isd, Ii, tr, tsd).

Lumen method: N = (E × A) / (Φ × UF × MF), where E is maintained illuminance, A area, Φ lamp lumens, UF utilisation factor, MF maintenance factor.

Maintenance factor typically 0.8 for clean office, 0.7 for industrial, 0.6 for dirty environments; apply to design illuminance, not to lamp output.

Room index K = (L × W) / (H_m × (L + W)), where H_m is mounting height above working plane; drives UF selection.

Typical maintained illuminance: offices 300–500 lx, corridors 100 lx, classrooms 300–500 lx, industrial precision 500–750 lx.

UGR limits: ≤19 for offices and classrooms, ≤22 for general industry, ≤25 for corridors; verify with photometric data.

Uniformity U_0 = E_min / E_avg; target ≥0.6 for offices, ≥0.4 for general areas; check with point-by-point calculation.

Emergency lighting: escape routes ≥1 lx along centreline, open areas ≥0.5 lx, high-risk tasks ≥10% of normal; duration 60–180 min per code.

LED retrofits require re-checking driver inrush, harmonic distortion, and neutral loading; LED drivers often produce high third-harmonic current.

Transformer loading: design at 70–80% for oil-immersed, 60–70% for dry-type to allow for harmonics and future growth.

Vector group Dyn11 is common for distribution; verify phase shift and neutral availability before paralleling or feeding mixed loads.

Generator rating: prime vs standby; standby rated for limited hours, prime for continuous; never size on standby rating for continuous duty.

Generator step load: typical acceptance 25–50% of rated kVA in one step; verify with manufacturer's transient data and largest motor start.

ATS/AMF sequence: mains fail → start signal → generator start (10–15 s) → transfer → retransfer with cool-down (5–10 min).

UPS sizing: kVA = load kW / (PF × efficiency); add 20–25% spare; verify battery autonomy at end-of-life (typically 80% capacity).

Neutral switching: 4-pole ATS required where separately derived neutral or RCD protection would otherwise be defeated.

Fuel storage: typical 8–12 h at full load for standby; check local code and refuelling arrangements.

Detection zones should not exceed 2000 m² or one fire compartment; each zone must be independently identifiable at the panel.

Smoke detector spacing: typically ≤10.6 m between detectors and ≤5.3 m from walls for flat ceilings up to 3 m; halve for higher ceilings per code.

Heat detector spacing is smaller than smoke; use for kitchens, plant rooms, and areas where smoke detection is unsuitable.

Sound pressure: alarm must be ≥65 dB(A) at all accessible points, or ≥5 dB above ambient, whichever is greater; ≥75 dB(A) at bedheads.

Voice alarm intelligibility (STI) target ≥0.5 for general areas, ≥0.45 for complex spaces; verify with acoustic modelling.

Fire-rated cables for detection and alarm circuits: PH30/PH60/PH120 or F30/F60/F120 depending on required survival time.

Interface with HVAC, lifts, and access control: cause-and-effect matrix must define every output and interlock.

Battery standby for fire panels: 24 h standby + 30 min alarm (or per local code); verify with panel manufacturer's load data.

IEC 60364 series governs LV installations; national codes (e.g., Iran's مبحث ۱۳) adapt it and take precedence where they differ.

IEC 61439 covers LV switchgear and controlgear assemblies; specify Form, IP, Icw, and Ipk.

IEC 62305 covers lightning protection; risk assessment determines class of LPS (I–IV).

IEC 60598 and IEC 62471 cover luminaires and photobiological safety; verify photometric files (IES/LDT) and CEI compliance.

IEC 60079 series applies to explosive atmospheres; zone classification drives Ex equipment selection.

Fire codes (e.g., NFPA 72 and local equivalents) define detection spacing, audibility, and emergency lighting duration.

Documentation must include single-line diagram, panel schedules, cable schedules, load schedule, and as-built drawings.

Designer's responsibilities: verify discrimination, voltage drop, fault level, and thermal withstand before issuing construction documents.

Design deliverables: basis of design, single-line diagram, load schedule, panel schedules, cable schedule, layout drawings, and calculations.

Specifications must state standards (IEC/ national), ratings, IP/IK, materials, testing, and submittal requirements for each item.

Shop drawing review: compare contractor submittals against design intent, ratings, and coordination results; record all deviations.

Site supervision: verify cable routing, glanding, torque values, labelling, and test results against approved drawings and specifications.

Testing and commissioning: insulation resistance, continuity, earth loop impedance, RCD trip times, polarity, and phase rotation.

Handover documents: as-built drawings, O&M manuals, test certificates, and training records; confirm before practical completion.

Change control: any deviation from design must be reviewed for load, voltage drop, fault level, and coordination impact.

Punch list and snag closeout: track non-conformances to closure; verify retest of any modified circuit.