Engineer Electric Work flashcards

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

Ohm's law

Voltage equals current times resistance (V = I·R). In AC circuits, RMS values and impedance replace resistance, so V = I·Z and phase angle matters.

Three-phase power formula

For a balanced three-phase load, active power P = √3·V_LL·I_L·cosφ, where V_LL is line-to-line voltage, I_L is line current, and cosφ is power factor.

Resonant frequency

A series or parallel LC circuit resonates at f_r = 1/(2π√(LC)). At resonance, inductive and capacitive reactances cancel, leaving only resistance in the series case.

Power factor correction kvar

Capacitor reactive power needed equals P·(tanφ1 − tanφ2), where P is active power and φ1, φ2 are the pre- and post-correction power factor angles.

Per-unit impedance

Per-unit impedance is actual impedance divided by base impedance, where Z_base = V_base²/S_base. Per-unit values stay continuous across ideal transformers, simplifying calculations.

Heat rate

Heat rate is fuel heat input per unit of net electrical output, often in kJ/kWh. It equals 3600 divided by overall thermal efficiency, so lower heat rate means better plant performance.

Surge impedance loading

SIL equals line-to-line voltage squared divided by surge impedance. A line loaded near SIL has a nearly flat voltage profile and minimal reactive power flow.

Radial distribution system

A radial system has a single supply path from source to load. It is inexpensive and simple but offers no alternate path, so any upstream fault interrupts all downstream customers.

Short-circuit fault level

Fault level at a bus approximates V²/Z_thévenin or S_base/Z_pu. Switchgear interrupting rating must exceed the calculated maximum fault current at its location.

Induction motor slip

Slip s = (n_s − n_r)/n_s, where n_s = 120f/p is synchronous speed and n_r is rotor speed. Rotor frequency equals s·f, and slip increases with load.

Star-delta starting

Star-delta starting connects the motor in star during run-up, reducing phase voltage and therefore starting current and torque to one-third of direct-on-line values.

Transformer maximum efficiency

A transformer reaches maximum efficiency when copper loss equals core loss. Core loss is constant with load, while copper loss varies with the square of load current.

Synchronous speed

Synchronous speed n_s = 120f/p, where f is supply frequency and p is the number of poles. It is the speed of the rotating magnetic field in AC machines.

DC machine EMF

Generated EMF E = (p·Φ·N·Z)/(60·a), where p is poles, Φ is flux per pole, N is speed in rpm, Z is conductors, and a is parallel paths.

Differential protection

Differential protection compares currents entering and leaving a protected zone. It trips on the difference current, giving fast, selective protection for transformers, generators, and busbars.

Distance relay zones

Distance relays measure impedance Z = V/I and set zones as percentages of line impedance. Zone 1 covers most of the line, while Zone 2 and 3 provide time-delayed backup.

Symmetrical components

Unbalanced three-phase quantities are resolved into positive, negative, and zero sequence networks. Zero-sequence current flows only where a ground return path exists.

Coordination time interval

The coordination time interval is the intentional time margin between downstream and upstream protective device trips, typically 0.2 to 0.4 seconds, ensuring selective operation.

Critical clearing time

Critical clearing time is the longest fault duration for which a power system remains transiently stable. Longer faults cause loss of synchronism between generators.

Design current and cable capacity

Installation design requires I_b ≤ I_n ≤ I_z: design current no greater than protective device rating, and device rating no greater than cable current-carrying capacity.

Adiabatic equation

The adiabatic equation S = √(I²t)/k checks that a protective conductor survives fault current I for time t, where k depends on conductor material and insulation.

Disconnection condition

Automatic disconnection of supply requires Z_s ≤ U_0/I_a, where Z_s is earth fault loop impedance, U_0 nominal voltage, and I_a the current tripping the device in time.

Voltage drop limits

Common design limits are 3 percent voltage drop for lighting and 5 percent for other loads, measured from the origin of the installation to the load.

Cable derating factors

Tabulated cable current capacity is reduced by factors for ambient temperature, grouping of circuits, thermal insulation, and installation method such as buried or in conduit.

IEC 60364

IEC 60364 is the international standard for low-voltage electrical installations. It covers protection, earthing, conductor sizing, and verification requirements for building electrical systems.

Hazardous area zones

IEC 60079 classifies gas atmospheres as Zone 0, 1, or 2 by likelihood of explosive mixture, and dust as Zone 20, 21, or 22, determining required equipment protection.

Earthing system types

IEC 60364 defines TN-S, TN-C, TN-C-S, TT, and IT systems by how neutral and protective conductors connect to earth, affecting protective device and RCD requirements.

NFPA 70E

NFPA 70E is the US standard for electrical safety in the workplace. It covers arc flash risk assessment, approach boundaries, PPE selection, and energized work permits.

Lockout/tagout procedure

LOTO requires isolating the energy source, applying locks and tags, verifying absence of voltage with a tested instrument, and applying temporary protective grounds before work.

ISO 50001

ISO 50001 is the international standard for energy management systems. It requires an energy baseline, energy performance indicators, and continual improvement of energy performance.

SF6 greenhouse impact

Sulfur hexafluoride is a potent greenhouse gas with a global warming potential around 23,500 times that of carbon dioxide, so recovery and leak reporting are regulated.

Cost performance index

CPI equals earned value divided by actual cost. A CPI below 1.0 means the project is spending more than the value earned, indicating a cost overrun.

Critical path

The critical path is the longest duration path through the project network. It has zero total float, so any delay on it delays the whole project.