Iran Nezam Electrical Design Exam flashcards

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

Diversity factor

Ratio of maximum demand to connected load over a defined period; applied per load group before summing to avoid over-sizing transformers and main switchgear.

ADMD

After-diversity maximum demand per dwelling, typically 2–4 kVA in residential towers; used to size building transformers and main incoming feeders.

Lumen method formula

N = (E × A) / (Φ × UF × MF), giving the number of luminaires required for a maintained illuminance E over area A using lamp lumens, utilisation factor, and maintenance factor.

Room index

K = (L × W) / (H_m × (L + W)); relates room dimensions to mounting height above the working plane and drives utilisation factor selection from photometric tables.

Maintenance factor

Multiplier applied to lamp output to account for dirt and lumen depreciation; typically 0.8 for clean offices, 0.7 for industrial, and 0.6 for dirty environments.

UGR limit

Unified Glare Rating cap; ≤19 for offices and classrooms, ≤22 for general industry, and ≤25 for corridors, verified against luminaire photometric data.

Ampacity rule

Coordination condition I_b ≤ I_n ≤ I_z and I_2 ≤ 1.45 × I_z, where I_b is design current, I_n device rating, and I_z corrected cable capacity.

Voltage drop three-phase

ΔU = √3 × I × L × (R cos φ + X sin φ); the mV/A/m tabulated method gives ΔU = mV/A/m × I × L / 1000 for quick checks.

Correction factors

Temperature, grouping, soil, and burial factors multiply the tabulated cable capacity: I_z = I_t × k_temp × k_group × k_soil; they are multiplied, never added.

Earth fault loop impedance

Z_s must satisfy Z_s ≤ U_0 / I_a, where I_a is the current that trips the protective device within the required disconnection time; verified by measurement at commissioning.

Adiabatic equation

S = √(I²t) / k, sizing the protective conductor or earthing conductor for the let-through energy of the protective device and the conductor's k factor.

TT system

Installation has its own earth electrode separate from the supply neutral; RCD protection is required for socket outlets and most final circuits because loop impedance is high.

TN-C-S system

Combined PEN conductor upstream of the origin, split into separate PE and N at the installation origin; common for public LV supplies to buildings.

Selectivity

Coordination in which the upstream protective device does not trip for a fault cleared by the downstream device; verified by non-overlapping time-current curves or tested pairs.

Form of separation

IEC 61439-2 classification (Form 1–4b) defining internal barriers between busbars, functional units, and terminals; specifies maintainability and fault containment.

Icw

Rated short-time withstand current of an assembly for a stated duration (typically 1 s); a key specification for main and distribution switchboards per IEC 61439.

Transformer fault level

I_sc ≈ S / (√3 × U × Z_pu); typical distribution transformer impedance is 4–6%, giving fault levels that drive switchgear and cable thermal ratings.

Dyn11 vector group

Delta primary, star secondary with neutral brought out and 11 o'clock phase shift; common for distribution transformers and allows unbalanced LV loads with a neutral.

Generator step load

Acceptance of a sudden load block, typically 25–50% of rated kVA in one step; verified against manufacturer transient data and the largest motor start.

ATS/AMF sequence

Mains failure triggers generator start (10–15 s), transfer to generator, and retransfer with cool-down (5–10 min) after mains restoration; documented in the cause-and-effect matrix.

UPS autonomy

Battery runtime at rated load, specified at end-of-life capacity (typically 80%); sizing adds 20–25% spare and divides load kW by power factor and efficiency.

Smoke detector spacing

For flat ceilings up to 3 m, typically ≤10.6 m between detectors and ≤5.3 m from walls; spacing reduces for higher ceilings per the applicable fire code.

Alarm audibility

Alarm sound must be ≥65 dB(A) at all accessible points or ≥5 dB above ambient, whichever is greater, and ≥75 dB(A) at bedheads in sleeping areas.

Fire-rated cable

Cable with a rated survival time (e.g., PH30, PH60, PH120 or F30/F60/F120) used for life-safety circuits so they remain functional during a fire.

Cause-and-effect matrix

Document mapping every detection input to every required output (alarm, HVAC shutdown, lift homing, door release); forms the basis for fire system testing.

IEC 60364

Series of standards for low-voltage electrical installations covering protection, earthing, conductor sizing, and verification; national codes adapt it and take precedence where they differ.

IEC 61439

Standard for low-voltage switchgear and controlgear assemblies; specifies Form of separation, IP rating, Icw, and Ipk for panelboards and switchboards.

IEC 62305

Lightning protection standard; a risk assessment determines the class of lightning protection system (I–IV) and the resulting air termination, down conductor, and earthing arrangement.

Load schedule

Tabulation of every circuit with description, phase, kW, power factor, kVA, demand factor, diversified kVA, current, cable size, and breaker rating; the basis for all downstream sizing.

Single-line diagram

Schematic showing sources, transformers, switchgear, panelboards, and feeders with ratings and settings; the primary design document for fault level and coordination studies.

Commissioning tests

Insulation resistance, continuity, earth loop impedance, RCD trip time, polarity, and phase rotation; results recorded in test certificates before practical completion.

Harmonic neutral current

Triplen harmonics (3rd, 9th, 15th) add in the neutral; design neutral for 1.45–1.73 × phase current where SMPS, LED drivers, or VFDs dominate the load.