Iran Nezam Electrical Supervision Exam common mistakes

Adopt one written procedure and apply it every time: (1) use the calculated demand current at the load end, not the breaker rating; (2) for three-phase use Vd = √3·I·(R·cosφ + X·sinφ)·L, with R and X from the cable manufacturer's table at operating temperature; (3) for single-phase two-wire use the 2·L factor, and for three-wire single-phase or two-phase systems use the correct multiplier; (4) express the result as a percentage of nominal voltage and compare against the project limit (commonly 3 % for branch circuits and 5 % total for power, tighter for lighting). Show the L, I, R, X, cosφ and the resulting percentage explicitly in the calculation sheet so the check is auditable.

Build the load schedule by category (lighting, socket outlets, motors, HVAC, elevators, special loads), apply the demand factor for each category from the code table, and sum the results. Then select the protective device so that Ib ≤ In ≤ Iz and I2 ≤ 1.44·Iz, verify the breaking capacity exceeds the prospective fault current from the utility or transformer data, and check time-current discrimination against the upstream device. Document each step with the table reference used.

For every circuit, record four things before touching the table: insulation type and conductor material, installation method reference, ambient temperature, and number of loaded conductors in the group. Look up the base current-carrying capacity for that exact method, multiply by the ambient correction factor and the grouping factor (and the soil thermal resistivity factor for buried runs), and confirm the result still satisfies Ib ≤ Iz. Where the derated value falls below the breaker rating, increase the cross-section rather than reducing the breaker, and note the derated value on the panel schedule.

Prepare two separate schedules. Earthing schedule: earth electrode type and resistance target, earthing conductor cross-section, main earthing terminal location, protective conductor cross-sections per circuit derived from the phase conductor size. Bonding schedule: every extraneous conductive part entering the building, the main protective bonding conductor size (not less than half the main protective conductor, with the minimum), and supplementary bonding in locations with simultaneously accessible parts. Verify continuity of both with a test and record the readings.

For each RCD location state four parameters: rated residual current, type, number of poles, and time delay (S type for upstream). Match the type to the load: Type A as a general minimum for socket circuits, Type B or A-plus-DC-detection where the manufacturer specifies it for drives and chargers. For selectivity, use a time-delayed upstream device with a higher rated residual current and verify the manufacturer's selectivity table rather than assuming it.

Treat life-safety circuits as a distinct category from the first draft: dedicated final circuits from a supply point that is not affected by the failure of normal lighting, fire-resisting cable where the route passes through a fire compartment, physical separation from other services or a fire-resisting barrier, and monitoring so a single open circuit is annunciated. For the fire alarm, provide the dedicated unswitched supply with its own isolating device marked and secured, and record the cable type and route on the drawing.

Build a one-page map for each standard: title, edition in force, scope, and the ten clauses you most often need (conductor sizing, protection, earthing, locations, special installations). For every practice question, write the clause number next to your answer even when you are not asked for it. When two documents appear to conflict, note the hierarchy and the more specific requirement. Review the actual text of the clauses you cite, not a summary.

Before finalizing, check the design against a short buildability list: spare capacity of at least the project's stated percentage, spare ways in each board, socket outlets positioned relative to wet zones and furniture layouts with the required protection, circuit numbering consistent between drawing, schedule and physical board, and a legend that lets a person standing at the board identify every circuit. Walk the layout room by room as if installing it.

Keep a written instruction and approval log. For any substitution or deviation, require the contractor's written request, check it against the design calculation and the code, record the decision and its basis, and issue it in writing with a reference number. Never approve a change that alters current-carrying capacity, protection coordination or fire rating without redoing the relevant calculation. Retain as-built records and test certificates as part of the supervision file.

Establish the classification first, from the process data or the client's hazardous-area drawing, and record zone, gas or dust group, and temperature class. Then select equipment whose marking covers all three, choose the permitted wiring method and sealing arrangement for the zone, and check that the protection concept (flameproof, increased safety, intrinsically safe, encapsulation) is maintained by the glands and enclosures as installed. Put the classification and the equipment marking on the drawing so the inspector can verify it at the point of installation.