Egypt Engineering Consultant Electrical flashcards

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

Per-unit impedance base change

When changing bases, Z_new = Z_old × (V_old/V_new)² × (S_new/S_old). This keeps equipment impedances comparable on a common system base for fault and load-flow studies.

Symmetrical component transformation

Any unbalanced three-phase set resolves into positive, negative and zero sequence components using the operator a = 1∠120°. Fault analysis then uses sequence networks connected according to fault type.

Single line-to-ground fault current

For a solid SLG fault with negligible fault impedance, I_f = 3·V_phase / (Z1 + Z2 + Z0). The zero-sequence impedance path, including earthing, strongly controls the magnitude.

Transformer short-circuit MVA

The short-circuit capacity at a transformer secondary is approximately S_rated divided by the per-unit impedance. A 1 MVA transformer with 5% impedance gives about 20 MVA fault level.

Diversity factor application

Maximum demand equals connected load multiplied by the diversity factor, which is less than one. It accounts for the fact that not all loads operate simultaneously, reducing transformer and feeder sizing.

Cable derating correction factors

Tabulated current-carrying capacity must be multiplied by factors for ambient temperature, grouping, thermal resistivity and installation method. The corrected capacity must satisfy I_b ≤ I_n ≤ I_z.

Voltage drop three-phase formula

ΔV = √3·I·L·(R·cosφ + X·sinφ) for a three-phase feeder, with R and X per unit length. The result is compared against the permitted percentage drop at the load.

Ring main unit operation

Ring main units allow feeding a load from either direction on a ring. Interlocks and, where needed, directional protection prevent parallel back-feed and unsafe operation during maintenance.

IDMT relay operating time

For IEC Standard Inverse, t = TMS × 0.14 / ((I/Is)^0.02 − 1). Time multiplier setting shifts the curve vertically; pickup setting Is shifts it horizontally on the time-current graph.

Discrimination time margin

Upstream and downstream protection curves should be separated by roughly 0.2–0.4 s at the maximum through-fault current. The margin covers breaker clearing time, relay overshoot and CT errors.

Circuit breaker Icu and Ics

Icu is the rated ultimate short-circuit breaking capacity; Ics is the rated service breaking capacity, expressed as a percentage of Icu. Ics reflects the breaker's ability to continue service after clearing.

Earth fault loop impedance criterion

In a TN system, Zs must not exceed U0 divided by Ia, where Ia is the current that trips the protective device within the specified disconnection time. This ensures touch voltage is removed quickly.

Lumen method formula

Number of luminaires N = (E × A) / (Φ × UF × MF). E is maintained illuminance, A is area, Φ is luminaire lumens, UF is utilisation factor and MF is maintenance factor.

Inverse square law illuminance

For a point source, illuminance at normal incidence is E = I / d², where I is luminous intensity in candela and d is distance in metres. Illuminance falls with the square of distance.

Unified glare rating limit

UGR quantifies discomfort glare from a lighting installation. Offices typically require UGR ≤ 19, while precision industrial tasks may require UGR ≤ 16 to limit visual fatigue.

Emergency escape route illuminance

Escape routes require at least 1 lux along the centre line at floor level, maintained for 60–180 minutes. Luminaires must reach the required output within five seconds of supply failure.

Egyptian distribution voltage levels

Egypt operates at 50 Hz with LV 220/380 V (increasingly 230/400 V), MV at 11 kV and 22 kV, and HV transmission at 66 kV and 220 kV. Design studies must use these nominal levels.

Egyptian electricity law scope

The Electricity Law and its executive regulations set licensing, installation, inspection and safety requirements. Compliance is verified by the distribution company before connection and energisation.

Harmonised cable designation

Cable markings such as 0.6/1 kV XLPE/PVC describe voltage rating, insulation and sheath. Conductor classes and construction follow IEC 60228 and IEC 60502, adopted in Egyptian standards.

TN system disconnection time

For final circuits up to 32 A in TN systems, disconnection must occur within 0.4 s at 230 V. TT systems require 0.2 s, usually achieved with an RCD as the protective device.

Earth electrode rod resistance

A vertical rod in homogeneous soil has resistance approximately R = ρ/(2πL)·(ln(4L/d) − 1). Increasing rod length reduces resistance more effectively than increasing diameter.

Rolling sphere protection radius

Lightning protection levels use rolling sphere radii of 20 m (level I), 30 m (II), 45 m (III) and 60 m (IV). A smaller radius gives a more stringent and reliable protection volume.

RCD sensitivity selection

A 30 mA residual current device protects people against indirect and direct contact hazards. A 300 mA device is used for fire protection, where leakage current could ignite combustible material.

FIDIC contract book roles

The Red Book covers construction works, the Yellow Book plant and design-build. The Engineer administers the contract, issues determinations, and the Dispute Adjudication Board resolves disputes.

Earned value cost performance index

CPI equals earned value divided by actual cost. A CPI below one indicates the project has spent more than the value earned, signalling a cost overrun that requires recovery action.

Critical path total float

Total float equals latest start minus earliest start, or latest finish minus earliest finish. Activities on the critical path have zero total float, so any delay extends project completion.

Contract variation valuation order

Variations are valued first at contract rates, then at analogous rates, and finally by a reasonable build-up of cost plus profit. The Engineer must issue the instruction in writing.

Standard test conditions for PV

PV modules are rated at 1000 W/m² irradiance, 25 °C cell temperature and air mass 1.5. Real output deviates because of temperature, soiling, shading and inverter losses.

PV temperature derating

Module power drops roughly 0.3–0.45% per °C above 25 °C cell temperature. Cell temperature is estimated from ambient temperature, irradiance and the module's nominal operating cell temperature.

Wind turbine power equation

Power equals half the air density times swept area times cube of wind speed times power coefficient Cp. Cp cannot exceed the Betz limit of 0.593, and practical values are 0.35–0.45.

Polarisation index of insulation

The ratio of insulation resistance at ten minutes to that at one minute. A value above two indicates healthy, dry insulation; a low or falling value suggests moisture or contamination.

Fall-of-potential earth test

Earth electrode resistance is measured with a current stake and a potential stake placed at about 62% of the current stake distance. The result is read from the resistance curve plateau.