BS 7671 Special Locations — Level 3 Environmental Technologies Practice

Sample questions

1. 01Under BS 7671 Section 712, what is the maximum DC string voltage permitted for domestic PV systems?

   • A600 V DC
   • B1500 V DC
   • C230 V DC
   • D1000 V DC

   Why: For domestic PV the DC system is kept within the low-voltage band, conventionally up to 1000 V DC. Higher string voltages (up to 1500 V DC) are used on larger commercial/utility arrays with additional protective measures; BS 7671 Section 712 also caps the voltage to earth (Ugc) at 120 V DC.

2. 02Under BS 7671 Reg 712.411.3.2.1.2, where the inverter does not provide simple separation, what additional protective measure is required?

   • AAn additional DC isolator on the AC side of the inverter, adjacent to the consumer unit
   • BA protective conductor of double the normal size run with the DC string cabling to the array
   • CA transformer providing simple separation between the AC and DC sides, OR an RCD type B on the AC side
   • DA Type AC 30 mA RCD on the AC side, which Section 712 accepts as sufficient for any inverter

   Why: Section 712 requires that PV inverters either provide simple separation themselves or have a transformer or Type B RCD on the AC side, because non-isolated (transformerless) inverters can produce smooth DC fault current that defeats Type AC and A RCDs.

3. 03Under BS 7671 Section 722 (EV charging), what does Reg 722.411.4 require for a PEN-conductor fault detection on a TN-C-S supply?

   • AA second PEN conductor run in parallel with the first so the fault current is halved, together with continuity testing of both conductors at every periodic inspection
   • BA surge protection device adjacent to the charge point, which clamps the voltage appearing on exposed-conductive-parts if the supply PEN conductor becomes open-circuit
   • CA Type AC 30 mA RCD at the origin of the final circuit, which Section 722 accepts because an open PEN produces a residual current large enough to operate it
   • DEither an earth electrode of suitable resistance, or a device that disconnects the EVCP from the supply within 5 s of detecting a PEN open-circuit fault, or O-PEN protection

   Why: Section 722 addresses the risk of an open PEN making the vehicle's exposed-conductive-parts live. Reg 722.411.4 requires an earth electrode, a 5-second disconnection device, or integrated O-PEN protection (as offered by most modern EVCPs).