Certified Apartment Supplier (CAS)

Correct: The first step in auditing or assessing an electrical installation in a special location is to define the zones. According to BS 7671, the requirements for equipment selection and erection are entirely dependent on whether the equipment falls within Zone 0, 1, 2, or outside the zones. Without establishing these boundaries, it is impossible to accurately evaluate compliance or risk.

Correct: According to BS 7671 Regulation 528.3, electrical services must be installed so that they do not cause harm to non-electrical services and are protected from the effects of those services. The best practice is to ensure independent support so that maintenance on one service does not compromise the other, and to maintain adequate spacing or physical protection to prevent issues like electrolytic corrosion, heat transfer, or mechanical damage.

Incorrect: Using other service supports is a violation of installation standards as it creates a dependency that can lead to mechanical failure or safety risks during maintenance. Placing conduits near water pipes for cooling is a safety hazard due to potential condensation or leaks. Thermal expansion coefficients are not the primary selection criteria for proximity to other services; rather, separation and independent mounting are the regulatory priorities.

Takeaway: Electrical wiring systems must be independently supported and adequately separated from non-electrical services to prevent mutual detrimental influence and ensure safe maintenance.

Correct: According to Regulation 411.3.3 of BS 7671, additional protection by means of an RCD with a rated residual operating current not exceeding 30 mA must be provided for socket-outlets with a rated current not exceeding 32 A. In non-domestic installations, an exception can be made if a documented risk assessment determines that RCD protection is not necessary, which aligns with the professional audit requirement for documented safety justifications.

Incorrect: A 100 mA RCD is used for protection against fire or for earth fault protection where impedance is high, but it does not meet the 30 mA requirement for ‘additional protection’ against electric shock. Restricting RCD protection only to sockets near windows is an outdated or incorrect interpretation of the regulations, as general-use sockets in an office require protection regardless of location. The circuit type (radial or ring) or the specific rating (20 A vs 32 A) does not exempt the socket-outlets from the 30 mA RCD requirement under current standards.

Takeaway: BS 7671 requires 30 mA RCD protection for all socket-outlets up to 32 A in standard installations to provide additional protection against electric shock.

Correct: According to BS 7671 (specifically Section 740 for temporary structures and fairgrounds), a PME (TN-C-S) earthing terminal shall not be used as the means of earthing unless the installation is under the supervision of a skilled or instructed person who can ensure the integrity of the earthing and bonding. This is due to the risk of the metalwork becoming live relative to the ground in the event of an open-circuit supply neutral, which is a significant hazard in outdoor or temporary environments.

Incorrect: The suggestion that PME can be used freely with 6mm² bonding is incorrect because standard bonding does not protect against the specific risks of a broken neutral in a TN-C-S system. Claiming that PME is mandatory is false; in many temporary or high-risk outdoor scenarios, a TT system is preferred to isolate the installation from supply neutral faults. Using a 100mA time-delayed RCD does not mitigate the risks associated with PME neutral faults and is not the regulatory condition for using a PME terminal in this context.

Takeaway: PME earthing terminals are restricted in temporary installations to prevent electric shock hazards arising from potential differences between the installation’s metalwork and the true earth during supply neutral faults.

Correct: According to BS 7671 and the Electricity Safety, Quality and Continuity Regulations (ESQCR), a PME (TN-C-S) earthing terminal should generally not be used for temporary construction site installations. This is because a fault in the supply neutral could cause the metalwork on-site to become live relative to the ground. Converting the site to a TT system using an earth electrode and RCD protection ensures that the installation is independent of the supply neutral and provides safety through automatic disconnection of supply.

Incorrect: Utilizing the PME terminal is restricted due to the danger of exported potentials during a neutral fault, and simply increasing bonding size does not remove this risk. Connecting extraneous-conductive-parts to the neutral conductor effectively creates a TN-C system, which is prohibited within consumer installations. Relying on the cable armor as a TN-S system is only permissible if the supply is confirmed as TN-S by the distributor; it cannot be used to ‘convert’ a PME supply safely without proper isolation.

Takeaway: For temporary construction installations, a TT earthing system is the preferred method to prevent dangerous voltages from appearing on exposed metalwork in the event of a supply neutral failure.

Correct: BS 7671 Regulation 722.531.3.101 specifies that RCDs for EV charging must be Type A (with 6mA DC detection) or Type B, because Type AC RCDs can be ‘blinded’ (rendered inoperative) by DC leakage current from the vehicle. Furthermore, Regulation 722.411.4.1 restricts the use of PME (TN-C-S) earthing for outdoor EV charging because a fault in the supply neutral (open-PEN) could cause the vehicle’s metalwork to reach a dangerous potential relative to the ground.

Incorrect: Option B is incorrect because 30mA RCD protection is mandatory for additional protection, and while TT is a common solution for EV earthing, it is not the only permitted method. Option C is incorrect because cable length does not mitigate the risk of RCD blinding or PEN failure. Option D is incorrect because testing must be performed at the charging point using an EVSE adapter to simulate the vehicle’s presence and ensure the entire circuit, including the socket/connector, is safe.

Takeaway: EV charging installations require specialized RCDs (Type A with DC detection or Type B) and specific earthing arrangements to protect against DC leakage and open-PEN conductor faults.

Correct: According to BS 7671 Regulation 542.2.1, any earth electrode must be constructed of materials capable of withstanding corrosion and must have sufficient mechanical strength. In a TT system, the integrity of this connection is vital for safety, as it forms a critical part of the fault return path. The selection must account for the specific chemical and physical properties of the soil at the credit union site to prevent premature failure.

Incorrect: Specifying a fixed length of 4 meters is incorrect because the required length is dependent on achieving a sufficiently low resistance to Earth (Ra) based on local soil resistivity. Encasing a standard driven rod in concrete is not a regulatory requirement and is typically reserved for foundation electrodes. While the earthing conductor must be appropriately sized, a blanket requirement of 25mm² is not the universal minimum for all TT installations, as sizing is determined by the presence of mechanical protection and the characteristics of the protective device.

Takeaway: Earth electrodes must be selected based on their durability against environmental corrosion and their mechanical suitability for the specific installation site to ensure long-term electrical safety.

Correct: In Group 2 medical locations, BS 7671 Section 710 requires the use of a medical IT system for circuits supplying medical electrical equipment intended for life support or surgical applications. The medical IT system is an ungrounded system that uses an insulation monitoring device (IMD); it ensures that the first fault to earth does not trigger a protective device, thereby maintaining the continuity of the power supply which is critical for patient safety.

Incorrect: Option B is incorrect because while supplementary bonding is required in medical locations, BS 7671 does not mandate a ring topology for these conductors. Option C is incorrect because automatic reclosing RCDs do not meet the safety requirement for a medical IT system and do not prevent the initial disconnection of power. Option D is incorrect because the primary safety objective for Group 2 locations is the implementation of the IT system to prevent disconnection on the first fault, rather than simply reducing the disconnection time of a standard TN system.

Takeaway: Group 2 medical locations require a medical IT system to ensure that a single earth fault does not interrupt the power supply to critical life-support equipment.

Correct: According to BS 7671 Regulation 722.411.4.1, for a TN-C-S system, a PME earthing facility shall not be used for an EV charging point located outdoors unless specific protective measures are taken. One such measure is the use of a device that disconnects the phase, neutral, and protective conductors if the voltage between the neutral and the true earth exceeds 70V, preventing the vehicle chassis from becoming live in the event of an open-PEN conductor fault in the supply network.

Incorrect: Relying on existing bonding is incorrect because it does not prevent a potential difference between the vehicle and the ground it stands on during a PEN failure. Type AC RCDs are prohibited for EV charging as they are not designed to detect or withstand the DC residual currents often present in EV charging, which can ‘blind’ the RCD. Simply connecting to the MET of a TN-C-S system for an outdoor charger is generally prohibited due to the severe shock risk posed by the loss of the supply neutral.

Takeaway: For EV chargers on TN-C-S systems, specific protection against the loss of the PEN conductor is mandatory to prevent the vehicle chassis from becoming live relative to the ground.

Correct: Option A is correct because BS 7671 Part 5 (Selection and Erection of Equipment) requires that electrical equipment be selected and installed so that it is suitable for the intended use and the external influences of the environment. In an audit of lighting controls, a failure to map the detection zones of Passive Infrared (PIR) or microwave sensors against the physical obstructions in the room (like shelving) constitutes a failure to ensure the installation is fit for purpose and safe for occupants.

Incorrect: Option B is incorrect because the overcurrent protective device (OCPD) is designed to protect the cable from thermal damage, not to manage the detection range of sensors. Option C is incorrect because while voltage drop can affect performance, it typically results in flickering or failure to power on, rather than the specific detection ‘blind spots’ described. Option D is incorrect because the presence or absence of a neutral at the switch would affect the basic operation of the sensor rather than its ability to detect movement behind physical obstacles.

Takeaway: Effective lighting control design requires that sensor placement and quantity are coordinated with the room layout to eliminate hazardous detection gaps.