EWRB Theory Exam Prep Questions With Complete Solutions!!!

What is the minimum value of the permitted test result for the insulation resistance test of a three-phase, 400V, mains cable? - ANS 1MΩ When carrying out testing on a low voltage electrical installation, which document details the tests and checks required to be carried out? - ANS AS/NZS 3000 What is the minimum or maximum test result value you would consider acceptable for an Earth Continuity test? - ANS 1Ω or less State TWO types of electrical protection provided by a RCBO. - ANS ANY TWO: - Earth Leakage - Overload - Short Circuit If the voltage and current are known, what TWO other factors are required to calculate the power consumed by a three-phase motor? - ANS √3 & Power Factor What type of single-phase motor can be reversed by reversing the connections to the run winding? - ANS ANY ONE: - Split-Phase Motor - Capacitor Start Motor - Cap-Start, Cap-Run Motor What type of single-phase motor can be reversed by reversing the connections to the armature brushes? - ANS ANY ONE: - Universal Motor - Series Motor State ONE reason why the metal frame of a Class 1 electrical appliance is earthed. - ANS ANY ONE: - To ensure the operation of a protective device under fault conditions. - To keep the frame of the appliance at earth potential under fault conditions. State TWO factors that determine the severity of electric shock upon the human body. - ANS ANY TWO: - Voltage Applied - Shock Current Level - Contact Duration - Skin Dryness - Current Path You have carried out a test on a low voltage final subcircuit in an electrical installation, the result value was 20MΩ. What type of test has been carried out? - ANS Insulation Resistance Test State the main reason why interlocks are used in a star/delta starter. - ANS To prevent the star and delta contractors closing simultaneously - and causing a short circuit. Calculate the power factor of a small three-phase printing factory with mainly motor loading. Metering instruments have recorded the following: Power = 30000W Voltage = 406V Current = 57A per phase - ANS 0.748 pf = P/V×I×√3 Define the term CUT OFF TIME as it refers to HRC fuses. - ANS ANY ONE: - Pre-arcing Time: Time taken to melt the fuse elements. - Arcing Time: The time taken between the initiation of the arc, and the arc extinguishing. - Cut Off Current: The fault current that will cause the fuse to blow. - Total Clearing Time: The time it takes to interrupt the flow of current and extinguish the flame. State the purpose of the slip-rings on a wound rotor induction motor (or slip-ring motor). - ANS To connect the external resistance to the wound motor. Calculate the fault current if a short circuit occurs on a 230V final subcircuit with an earth loop impedance test result of 0.32Ω. - ANS 718.75A MEN switchboards have characteristics that are NOT FOUND on a distribution board. Name TWO of those characteristics. - ANS ANY TWO: A MEN switchboard: - Has a MEN link between the neutral and earth busbars. - Has a connection to earth via an earthing lead to an earth electrode. - Is electrically closest to the point of supply. - Can supply to another MEN switchboard. Why do three-phase induction motors run smoother and quieter than single-phase induction motors? - ANS They have a constant rotating magnetic field. State TWO reasons for using reduced voltage starting for a three-phase induction motor. - ANS ANY TWO: - Reduced starting current. - Reducing voltage fluctuation/surges. - Limiting mechanical shock. What is the permitted protective earthing conductor test result for a test between the earth pin of an appliance supply plug and the earth led accessible metal? - ANS 1Ω Max State the reason why the secondary voltage of a transformer is less at full-load than at no-load. - ANS When current is flowing there is an internal volt drop due to the impedance of the windings. What is the minimum value of the permitted test result for the insulation resistance test of a three-phase, 400V, mains cable? - ANS 1MΩ What is the maximum value of the permitted test result for an earth leakage test of a 230V, Class 1, plug-in appliance? - ANS 5mA State TWO types of fault that a MCB provides protection for. - ANS - Short-Circuit - Overload What type of fault does a RCD provide protection for? - ANS Earth Faults State the full name for a PRCD. - ANS Portable Residual Current Device. State the full name for a RCBO. - ANS Residual Circuit Breaker with Overcurrent protection. State the full name for a SRCD. - ANS Socket Residual Circuit Breaker. State ONE practical method used to reduce iron losses in the construction of a transformer. - ANS ANY ONE: - Laminate the iron core. - Use special types of alloy for the core. State TWO methods of reducing the stroboscopic effect of a fluorescent fittings installed across three-phases. - ANS ANY TWO: - Connect fittings on different phases. - In double-tube fittings, connect a capacitor in series with one ballast or install lead/lag fittings. - Intersperse incandescent lighting with the fluorescent lighting.