Published: 5 January 2015
Category: Q&A
Neil asks this question, which concerns what BS 7671:2008 requires of earthing at an outdoor swim spa, which is classed as a ‘Special Installation’. It has been answered by the NICEIC (Theme – Regulations and Legislation):
This Q & A is one of thousands posted in our Technical Expertise area, and answered on a daily basis by our Voltimum Experts.
Question: I’m currently installing two 32A single-phase electrical supplies for an outdoor swim spa unit, which will be protected by a 30mA RCD. The installation is a TT system, with the Ze (earth loop impedance external to the installation) recorded at 31 ohms.
With this being a ‘Special Installation’ under BS 7671:2008, do I need to lower the Ze below 20 ohms?
Alternatively, can I install an extra earth rod at the swim spa to try to get the Zs (earth fault loop impedance) of the 32A supply circuits below 20 ohms, which will also help reduce the Ze?
Answer: There is no formal requirement to have the Ze below 20 ohms. However, the lower the Ze, the more effective the arrangement is.
The TT arrangement relies on an RCD for earth fault protection on the system, and the BS 7671:2011 Table 41.5, Note 2 states that an electrode resistance of below 200 ohms or less would be deemed as a stable reading.
So, a reading of 31 ohms is a resistance that could be suitably relied upon in this case.
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