gh-ctvet-cert-ii-electrical 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
- The current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance: V = I × R, where V is volts, I is amperes and R is ohms.
- Power formula
- Electrical power in a resistive circuit equals voltage multiplied by current: P = V × I. It can also be written as P = I²R or P = V²/R depending on which quantities are known.
- Parallel resistance
- In a parallel circuit the reciprocal of total resistance equals the sum of the reciprocals of the individual resistances: 1/R = 1/R1 + 1/R2 + ... The total is always less than the smallest branch.
- Power factor
- The ratio of true power in watts to apparent power in volt-amperes, equal to cos φ. A low power factor means more current is drawn for the same useful power.
- Three-phase power
- Total power in a balanced three-phase system equals √3 × line voltage × line current × power factor. This applies to both star and delta connected balanced loads.
- Synchronous speed
- The speed of the rotating magnetic field in an induction motor: N = 120f/P, where f is supply frequency in hertz and P is the number of poles.
- Motor slip
- The difference between synchronous speed and actual rotor speed, expressed as a fraction of synchronous speed: s = (Ns − Nr)/Ns. Typical full-load slip is 2 to 5 percent.
- Star-delta starting
- A method of starting an induction motor by connecting windings in star for starting, then delta for running. It reduces starting current and torque to one third of direct-on-line values.
- Transformer turns ratio
- The ratio of primary to secondary voltage equals the ratio of primary to secondary turns: V1/V2 = N1/N2. Current ratio is the inverse of the voltage ratio.
- Transformer kVA rating
- Transformers are rated in kilovolt-amperes because heating depends on voltage and current, not on load power factor. The rating defines the maximum apparent power the transformer can supply continuously.
- TN-S earthing system
- A system where the protective earth conductor is separate from the neutral throughout the installation, supplied from the source as distinct conductors. Fault current returns via the metallic protective conductor.
- TT earthing system
- A system where the installation has its own local earth electrode and the supply neutral is earthed separately at the source. Earth fault loop impedance is high, so an RCD is required.
- RCD type A
- A residual current device that trips for sinusoidal alternating residual currents and for pulsating direct residual currents. It is more sensitive than type AC and suits modern electronic loads.
- Earth electrode resistance
- For TT systems the electrode resistance Ra must satisfy Ra × IΔn ≤ 50 V, where IΔn is the RCD rated residual current. A 30 mA RCD therefore requires Ra ≤ 1667 ohms.
- Insulation resistance test
- A test applying a DC voltage, commonly 500 V for 230/400 V installations, between live conductors and earth to verify insulation integrity. The minimum acceptable result is generally 1 megohm.
- Proving unit
- A portable device that supplies a known voltage to verify that a voltage tester is functioning correctly before and after proving a circuit dead. It is essential for safe isolation.
- Clamp meter use
- A current-measuring instrument that clamps around a single conductor and measures the magnetic field. Clamping around a two-wire cable gives zero because currents cancel.
- Loop impedance tester
- An instrument that measures the earth fault loop impedance Zs to confirm that the protective device will disconnect within the required time. Results are compared with the maximum permitted values.
- PVC insulation rating
- Polyvinyl chloride insulated cables have a maximum continuous conductor temperature of 70 degrees Celsius. Exceeding this accelerates ageing and reduces insulation life.
- XLPE insulation
- Cross-linked polyethylene insulation withstands a maximum continuous conductor temperature of 90 degrees Celsius, giving higher current ratings and better dielectric strength than PVC.
- Copper resistivity
- Copper has a resistivity of about 1.72 × 10⁻⁸ ohm-metres at 20 degrees Celsius, giving a conductivity of about 58 megasiemens per metre. It is the reference conductor material.
- SWA cable
- Steel wire armoured cable has a bedding layer and galvanised steel wire armour beneath the outer sheath. The armour provides mechanical protection and can serve as the protective conductor.
- Current-carrying capacity rule
- The cable capacity Iz must be at least the protective device rating In, which must be at least the design current Ib: Iz ≥ In ≥ Ib. All correction factors must be applied to Iz.
- Grouping factor
- A derating factor applied when cables are bunched or grouped together, because mutual heating reduces each cable's ability to dissipate heat. Values are tabulated against the number of circuits.
- Protective conductor sizing
- For phase conductors up to 16 square millimetres the protective conductor is the same size. From 16 to 35 square millimetres it is 16 square millimetres, and above that half the phase size.
- Fire-rated cable
- A cable designed to maintain circuit integrity during fire, used for life-safety systems such as fire alarms, emergency lighting and smoke extraction. It must be installed to the manufacturer's instructions.
- Single-line diagram
- A drawing that shows the distribution system from the origin to final circuits using single lines for multi-phase connections, annotated with device ratings, cable sizes and circuit references.
- Schematic diagram
- A drawing that shows the electrical function and logical connections of a circuit without regard to physical layout or terminal positions. It uses standard graphical symbols.
- Cable schedule
- A tabulated document listing each circuit's cable type, size, length, origin and destination. It supports installation, testing and handover documentation.
- Lockout tagout
- A safety procedure that isolates energy sources, applies personal locks and tags, and verifies de-energisation before work. Only the person who applied a lock may remove it.
- Hierarchy of control
- The order of risk control measures from most to least effective: elimination, substitution, engineering controls, administrative controls, and finally personal protective equipment.
- Break-even point
- The sales volume at which total revenue equals total costs, calculated as fixed costs divided by the difference between selling price per unit and variable cost per unit.