Iran Nezam Electrical Design Exam key terms

The ratio of the maximum demand of an installation, or part of it, to the total connected load of that installation or part. It is always less than or equal to one and is used to size feeders and equipment realistically.

The arithmetic sum of the rated input capacities of all apparatus connected to the installation, regardless of whether all operate simultaneously. It is the starting point for demand and diversity calculations.

The ratio of the sum of the individual maximum demands of the various parts of a system to the maximum demand of the whole system. Values above one show that consumer peaks do not coincide, allowing smaller upstream capacity.

The reciprocal of diversity factor: the ratio of the maximum demand of a group of loads to the sum of their individual maximum demands. It expresses how much of the total peak occurs simultaneously and drives upstream sizing.

The greatest average load over a specified period, usually measured over 15 or 30 minutes. It determines the rating of service equipment and the tariff category applied by the utility.

A tabulated list of every circuit in a panelboard showing its description, connected load, demand load, circuit breaker rating, cable size and phase allocation. It is the core working document for balanced panel design.

The deliberate distribution of single-phase loads across the three phases of a supply so that phase currents and neutral current remain as equal as possible. Good balancing reduces losses, voltage unbalance and neutral heating.

An enclosure containing busbars, overcurrent devices and terminal bars that distributes power to outgoing circuits from a single incoming feed. It may be a main-lug-only type or include a main breaker and metering.

A rigid copper or aluminium conductor, usually rectangular in section, used to carry and distribute large currents inside switchgear and panelboards. Its current rating depends on section, material, ventilation and allowable temperature rise.

Coordination between protective devices such that a fault is cleared by the device nearest to it, leaving upstream devices closed. It is achieved by grading current ratings, time settings or energy-limiting characteristics.

A systematic analysis comparing time-current curves of fuses, breakers and relays to verify that the intended device trips first for every fault level. It produces settings and confirms selectivity margins between upstream and downstream devices.

A logarithmic plot of a protective device's operating time against prospective current. Overlaying curves for series devices reveals whether and where selectivity is lost at high fault currents.

The intentional time margin between the operating curves of two series protective devices at a given current. It absorbs tolerances and breaker opening times so the downstream device clears the fault before the upstream one reacts.

The maximum prospective fault current a device or assembly can carry for a stated short time without damage, often quoted with an associated duration such as one second. It must exceed the calculated fault level at the point of installation.

The maximum prospective current a switching device can safely interrupt at a stated voltage and power factor. It is normally expressed as a symmetrical rms value and must be at least the fault current at its terminals.

The current that would flow at a given point if a short circuit of negligible impedance occurred there, calculated from source impedance, transformer impedance and conductor impedance. It sets the required ratings of protective devices and earthing conductors.

The total impedance of the path taken by fault current from the source through the phase conductor, the fault, the protective conductor and back to the source. It governs whether automatic disconnection occurs within the permitted time.

A protective measure in which a fault is cleared by a device opening the circuit within a specified time, so touch voltages are not sustained. It relies on adequate earth fault loop impedance and verified protective conductor continuity.

The deliberate connection of exposed conductive parts, extraneous conductive parts and the main earthing terminal so that potential differences between them are reduced to safe values during faults or lightning events.

A conductor or group of conductors in direct contact with the soil that provides an intentional electrical connection to earth. Its resistance depends on soil resistivity, electrode dimensions, depth and moisture, and is measured to verify compliance.

A protective device that trips when the vector sum of currents in the live conductors exceeds a set residual value, typically 30 mA for socket circuits and 300 mA for fire protection. It detects earth leakage currents that overcurrent devices may not.

The reduction in voltage along a conductor caused by its impedance carrying load current, usually expressed as a percentage of nominal voltage. Excessive drop causes dim lamps, stalled motors and unreliable electronics, so limits are imposed on final circuits and feeders.

The nominal metallic area of a cable conductor, in square millimetres, chosen to satisfy current-carrying capacity, voltage drop and fault withstand requirements. It is selected from standard sizes and verified against each criterion.

The maximum continuous current a cable can carry under stated installation conditions without exceeding its rated conductor temperature. Tabulated values are corrected by ambient temperature, grouping and installation method factors.

A multiplier below one applied to a tabulated current rating to account for adverse conditions such as high ambient temperature, soil thermal resistivity, buried depth, enclosure or protective covering. Several factors may apply simultaneously.

A correction factor reducing a cable's current rating when several loaded cables are installed close together, because mutual heating restricts heat dissipation. The value depends on the number of circuits and the spacing or enclosure arrangement.

A rigid open support system of side rails and rungs or perforated bottom on which cables are laid. Its fill, loading and support spacing are limited so cables are not crushed and heat can escape.

An enclosed metal or insulating channel with a removable lid, used to route and support cables, usually with a defined fill ratio. It offers mechanical protection and can incorporate compartmental barriers for separation of duty and emergency circuits.

The sealing of openings and penetrations where cables, trays or conduits pass through fire-rated walls and floors, restoring the fire resistance of the element. It must be executed with tested systems and documented for inspection.

Physical or compartmental separation between circuits of different duty, voltage or criticality, such as emergency lighting and normal power, so a fire or fault in one does not disable the other. It is achieved by separate trays, conduits or barriers.

A calculation that estimates average maintained illuminance on a horizontal working plane from the total lamp lumens, the utilisation factor and the maintenance factor divided by the room area. It is used for general uniform lighting design.

The proportion of lamp lumens that reaches the working plane, accounting for room geometry, reflectances of ceiling, walls and floor, and luminaire light distribution. It is read from manufacturer tables indexed by room index.

A divisor applied in lighting calculations to allow for lamp lumen depreciation and dirt accumulation on luminaires and room surfaces over time. It ensures maintained illuminance is met at the end of the cleaning cycle.

A dimensionless ratio of room length times width to mounting height times the sum of length and width, describing room proportions for lighting calculations. It selects the correct utilisation factor from tables.

The luminous flux incident on a surface per unit area, measured in lux. Target values are set by task and are verified by measurement on the working plane after installation.

The luminous intensity of a surface per unit of projected area, measured in candela per square metre. It is the physical quantity that determines visual brightness sensation and is the basis of glare evaluation.

A single-number glare metric for interior lighting that combines luminaire luminance, background luminance and observer position. Lower values indicate less discomfort glare, and limits are specified per space type.

The percentage of rated voltage that must be applied to one winding, with the other short-circuited, to circulate rated current. It determines the transformer's prospective short-circuit current and influences voltage regulation.

The large transient magnetising current drawn when a transformer is energised, which can reach many times rated current for a fraction of a second. Protection settings and generator sizing must tolerate it without nuisance tripping.

The change in secondary voltage of a transformer or generator between no load and full load, expressed as a percentage of rated voltage. Poor regulation causes dimming and equipment malfunction at times of peak load.

Selection of a generating set's kVA and kW rating to supply running loads plus starting transients of motors, while respecting acceptable voltage dip and the prime or standby rating basis. Load steps and harmonic content must be considered.

A device that senses loss of the normal supply and transfers the load to a standby source, then restores it when normal power returns. Its transition type and neutral configuration affect continuity for sensitive loads.

Circuits that must remain energised for safety or business continuity, such as fire pumps, emergency lighting, alarms, lifts and critical IT. They are separated from normal loads and supplied from standby or uninterruptible sources.

A static converter system that maintains power to a load without interruption during mains failure, typically via a battery and inverter. Double-conversion types also condition voltage and frequency.

A cable tested to maintain circuit integrity for a stated period under fire conditions, used for fire pumps, alarms, emergency lighting and smoke control. Its support system must also be fire-rated to preserve performance.

A fire detection architecture in which each device has a unique address, letting the panel report the exact device in alarm or fault. It reduces wiring and speeds identification compared with conventional zone systems.

A defined area of a building covered by one or more detectors reported as a single identifiable group on the fire alarm panel. Zone size and area limits are set by code to allow rapid location of a fire.

The central unit that receives signals from detectors and call points, processes them, and activates sounders, relays and transmission to responders. It monitors circuit integrity and displays fault and alarm conditions.

A design table listing each input event, such as a detector alarm or flow switch operation, against the required outputs, such as sounders, dampers, lifts and door releases. It is the specification that fire system software must implement.

The terminal or bar that connects the earthing conductor, protective conductors, bonding conductors and functional earthing to the earthing system. It is the reference point for verifying earth continuity and loop impedance.

A schematic drawing showing the electrical system with one line per circuit, including sources, transformers, switchgear, protective devices, busbars and ratings. It is the primary reference for operation, coordination and modification.

A drawing revised to record the installation as actually constructed, including deviations approved during site work. It forms part of the handover documentation used for future maintenance and alteration.

A contractor-prepared detail drawing showing fabrication, assembly and installation dimensions for approval by the designer before work proceeds. It translates design intent into buildable detail, such as panel layouts and support frames.

A written description of how a specific construction activity will be carried out, including sequence, plant, personnel, safety controls and inspection points. It is submitted for approval before the work starts.

The structured process of verifying that installed systems operate correctly and safely, covering insulation resistance, continuity, loop impedance, RCD trip tests and functional checks. Results are recorded in certificates and schedules.

The declared nominal voltages and earthing arrangement of a distribution system, such as 400/230 V three-phase four-wire with a defined neutral point treatment. Equipment ratings and protection assumptions are based on it.

An earthing arrangement in which the supply neutral is earthed at the source and the installation's exposed conductive parts are connected to that earthed point via protective conductors. Faults become earth faults cleared by overcurrent devices.

Connection of exposed conductive parts to earth so that fault current has a low-impedance return path and touch voltage is limited. Its effectiveness is proven by loop impedance measurement and continuity testing.