Protection for Safety

What are the two categories of protection against electric shock in BS 7671?

• APrimary and secondary protection
• BBasic protection and fault protection✓
• CInsulation and bonding
• DGrounding and neutraling

Answer: BS 7671 defines basic protection (against normal operation contact) and fault protection (against fault conditions).

What is a typical maximum voltage for extra low voltage (ELV) circuits?

• A12V
• B24V
• C50V✓
• D100V

Answer: ELV is defined as AC up to 50V or DC up to 120V, providing inherent protection against shock.

What does basic insulation protect against?

• AMechanical damage
• BContact with live parts under normal operating conditions✓
• CWater ingress
• DOvervoltage

Answer: Basic insulation is the standard insulation protecting against accidental contact with live parts during normal operation.

What is double insulation?

• ATwo cable jackets
• BBasic insulation plus supplementary insulation✓
• CGrounding and bonding
• DTwo separate circuits

Answer: Double insulation consists of basic insulation plus an additional layer of insulation, eliminating the need for earthing.

In a TN earthing system, how is fault protection achieved?

• AThrough an RCD only
• BVia automatic disconnection using protective conductors and overcurrent devices✓
• CUsing insulation only
• DManual disconnection only

Answer: TN systems use the low-impedance protective conductor path and overcurrent devices for automatic disconnection.

What is the maximum permissible earth fault loop impedance for a 32A circuit breaker in a TN system?

• A1.44 Ohms✓
• B0.9 Ohms
• C2.5 Ohms
• D0.6 Ohms

Answer: For a 32A circuit breaker with 400V supply, the maximum Zs is 1.44 Ohms to achieve 5x instantaneous trip.

What is the primary function of an RCD (Residual Current Device)??

• AOvercurrent protection
• BDetection of earth leakage current and automatic disconnection✓
• CVoltage regulation
• DCurrent boosting

Answer: RCDs detect imbalance between phase and neutral currents (earth faults) and disconnect the circuit.

What is a typical RCD operating current sensitivity for general protection?

• A300mA
• B30mA✓
• C100mA
• D500mA

Answer: General protective RCDs operate at 30mA (Type A or Type AC), providing shock protection for most circuits.

What is AFDD (Arc Fault Detection Device) designed to protect against?

• AOverload only
• BSeries arc faults that may cause fires✓
• CEarth faults only
• DVoltage spikes

Answer: AFDDs detect series arcing (between conductors or phase-earth) that can cause fires without always triggering RCDs.

In BS 7671, what is 'simultaneous contact' protection?

• AContact between two people
• BProtection against simultaneous contact with two conductive parts at different potentials✓
• COnly applies outdoors
• DIs achieved by insulation alone

Answer: Simultaneous contact protection ensures that touching multiple conductive parts does not create a shock hazard.

What is the purpose of protective bonding conductors?

• ATo carry load current
• BTo maintain exposed conductive parts at the same potential during fault conditions✓
• CTo supply neutral current
• DTo measure voltage

Answer: Bonding conductors maintain equipotential surfaces to prevent dangerous potential differences during faults.

What is the minimum conductor size for main protective bonding in TN-S systems?

• A1mm²
• B2.5mm²
• C6mm²✓
• D10mm²

Answer: Main bonding conductors should be a minimum of 6mm² copper or 10mm² aluminum, or half the main earth conductor.