Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, about the 17th Edition and whether it is permissible work to minimum luminaire voltage tolerances rather than percentage drops, is answered by the NICEIC:
For example, with a luminaire having a HF ballast able to work down to a minimum voltage of 200V, and working to the lowest allowed supply voltage of 216.2V ( Uo - 6%), can it be allowed to have an absolute maximum voltage drop of 16.1V (Uo - 7%).
I would never design using maximum limits but, if I was to design ensuring that I am within the safe working limits of the luminaire, I could use a larger voltage drop than the 3% stated in BS7671. As far as I can see from looking through BS7671, I cannot find any note saying that I can work to the voltage tolerances of equipment supplied, instead of Table 12A.
Answer: Referring to Regulation 525.1 (p106) of BS7671 : 2008 requires the voltage at the terminals of equipment to be greater than the lower limit specified in the product standard for the equipment concerned. So, if these conditions were ensured, then this would comply with BS7671:2008. The percentage approach, given in Regulation 525.3, is usually applied where specific information concerning the load is unavailable at the time of design and hence is more onerous.
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