LEW flashcards
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32 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 = IR). In AC circuits, use RMS values and replace resistance with impedance magnitude for reactive loads.
- Three-phase power formula
- For a balanced three-phase load, real power equals √3 times line voltage times line current times power factor. Reactive power uses the sine of the angle.
- Per-unit system
- A method of normalizing voltages, currents, and impedances to base values so transformer ratios disappear and multi-voltage networks simplify. Base impedance is V²/S.
- Series resonance
- Occurs when inductive and capacitive reactances are equal; impedance is minimum and current is maximum. Resonant frequency is 1/(2π√(LC)).
- Conduit fill limit
- For three or more conductors, the total cross-sectional area may not exceed 40% of the conduit interior area. One conductor is limited to 53%, two to 31%.
- Box fill calculation
- Conductor volume allowances plus device, clamp, and ground allowances must not exceed the box's cubic-inch capacity. Allowances vary by conductor size.
- Equipment grounding conductor color
- The equipment grounding conductor is identified by green, green with yellow stripes, or bare copper. The grounded neutral is white or gray.
- NM cable restrictions
- Nonmetallic-sheathed cable is prohibited in wet locations, in most commercial occupancies, and where subject to physical damage unless protected.
- Standard OCPD sizes
- Common standard ratings include 15, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 125, 150, 200, 225, 250, 300, 350, and 400 amperes.
- Next-size-up rule
- A breaker may exceed conductor ampacity only if the ampacity is nonstandard, the next standard size does not exceed 800 A, and the load does not exceed ampacity.
- Concrete-encased electrode
- A grounding electrode consisting of at least 20 feet of ½-inch rebar or 4 AWG bare copper encased in at least 2 inches of concrete at the bottom of a footing.
- GFCI trip threshold
- A ground-fault circuit interrupter trips when it detects a current imbalance of about 5 milliamperes between the ungrounded and grounded conductors, protecting people from shock.
- Motor FLC tables
- Motor full-load current for sizing conductors and overloads comes from NEC tables based on horsepower and voltage, not the motor nameplate current.
- Motor overload sizing
- Overload protection is typically set at 125% of nameplate current for motors with a service factor of 1.15 or less, and 115% for higher service factors.
- Seal-in contact
- A holding contact wired in parallel with the start pushbutton keeps the motor starter coil energized after the button is released, maintaining the circuit.
- Motor rotation reversal
- For a three-phase motor, interchange any two supply leads. For a single-phase motor, reverse the leads of the start winding to change rotation.
- Transformer turns ratio
- The voltage ratio equals the turns ratio, while current ratio is inverse. Apparent power is approximately conserved, ignoring losses and magnetizing current.
- Delta-wye transformer
- A transformer connection providing a neutral and a 30-degree phase shift. It is common for distribution because it supplies both three-phase and single-phase loads.
- Voltage drop formula
- Single-phase voltage drop equals 2 × K × I × L divided by circular mils. Use √3 instead of 2 for three-phase circuits.
- Triplen harmonics
- Odd multiples of the third harmonic add in phase in the neutral of wye systems, causing neutral overheating and requiring oversized or separate neutrals.
- Arc-flash boundary
- The distance at which incident energy equals 1.2 cal/cm², the threshold for second-degree burn. Inside it, arc-rated PPE is required.
- Limited approach boundary
- A shock-protection boundary that unqualified persons may not cross. It depends on voltage and whether the exposed conductor is fixed or movable.
- Lockout/tagout
- A procedure that isolates energy sources, applies locks and tags by each worker, and verifies de-energization before work begins to prevent unexpected energization.
- Classified locations
- Areas with flammable gases, vapors, dust, or fibers. Class I covers gases, Class II dust, and Class III fibers; divisions or zones define hazard likelihood.
- Insulation resistance test
- A megohmmeter applies a DC voltage to measure insulation resistance. Values are temperature-corrected and trended; a decreasing trend indicates deterioration.
- CAT ratings
- Meter safety categories: CAT III for distribution circuits and CAT IV for service entrances. The rating must match or exceed the environment being measured.
- Phase rotation meter
- An instrument that verifies the ABC phase sequence of a three-phase supply before connecting a motor, preventing reverse rotation and equipment damage.
- Thermographic inspection
- Infrared imaging reveals loose connections and overloaded conductors by their elevated temperature. Compare similar phases and investigate significant temperature differences.
- RMS value
- The effective value of an AC waveform equal to the DC value producing the same heating. For a sine wave, RMS is peak divided by √2.
- Wye versus delta
- In a wye connection, line voltage is √3 times phase voltage. In a delta connection, line current is √3 times phase current.
- Expansion fitting
- A fitting installed in a raceway to absorb thermal expansion and contraction, required where raceways cross expansion joints or where temperature changes are significant.
- Selective coordination
- Coordination of overcurrent devices so that the device nearest a fault trips first, limiting outage scope. Time-current curves and adjustable settings are used to verify.