Published: 5 May 2011
Category: Q&A
Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, concerning a circuit fed from a 15A RCD and its testing, is answered by the NICEIC:
Question: When recently testing a circuit fed from a 15A RCD, I obtained an ELZ value of 1.54. This is fairly close to the BS 7671:2008 value of 1.60 for a BS 3871 Type 3 MCB.
Seeing that this is an RCD, and BS 7671:2008 states a value of 1667 ohms for the ELZ, would this be acceptable?
Answer: BS 3871 Type 3 MCBs are no longer detailed in BS7671:2008. 1667 ohms is the maximum Zs for EN 61008-1 and EN 61009-1 for final circuits not exceeding 32A (Table 41.5) TT systems - see Regulation 411.5. Further, Regulation 411.4.9 may apply.
Additionally, refer to the manufacturer's data for the maximum Zs values for its devices.
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