BS 7671 Special Locations — Level 3 Environmental Technologies Practice

Sample questions

1. 01In a typical domestic PV installation, what DC voltage do strings normally stay within?

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

   Why: Most domestic inverters and DC components are rated for 1000 V, so domestic strings are designed to stay below it, allowing for the rise in open-circuit voltage in cold weather. BS 7671's low voltage band itself extends to 1500 V DC, which larger commercial and utility arrays use. 120 V DC is the Band I limit for SELV and PELV (Regulation 414.1.1), a different thing.

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 722.411.4.1 require before a PME earthing facility can be used for an outdoor charging point?

   • 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
   • DAn installation earth electrode limiting the MET-to-Earth voltage to 70 V RMS on an open PEN, or a device that disconnects the vehicle within 5 s, or an equivalent alternative device

   Why: Section 722 addresses the risk of an open PEN making the vehicle's exposed-conductive-parts live. Reg 722.411.4.1 allows a PME earthing facility for an outdoor charging point only if one of its methods is used: (b) an installation earth electrode sized so the voltage between the MET and Earth cannot exceed 70 V RMS on an open-circuit PEN; (c) a device that disconnects the vehicle from the live conductors and protective earth within 5 s when the cpc-to-Earth voltage exceeds 70 V RMS; (d) a device that disconnects within 5 s when the line-neutral voltage at the charging point goes above 253 V or below 207 V; or (e) an alternative device giving no lesser degree of safety. Indent (a) was deleted by A2:2022. Equivalent functionality may be built into the charging equipment.