Q & A of the Day - Are these acceptable Zs readings for a TT system?

Published: 31 January 2011 Category: Q&A

Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This Question of the Day, concerning a new farm building electrical installation and the earth-fault loop impedance (Zs) reading values, is answered by the ECA:

Q & A of the Day - Are these acceptable Zs readings for a TT system?

Question: I've recently wired a new farm building. The supply is taken from the house via a u/g 16mm three-core SWA cable (earth core not connected to PME earth point). I have installed a 1200 x 16mm earth rod with 16mm to the distribution board - approximately 4m length (a reading of 47 ohms between earth rod and live supply). The system in the building is TT, and at the supply point I have installed a 63A 100mA RCD.

All eight circuits are protected with 30mA RCBOs having a disconnection time of around 28ms on all circuits but my Zs readings are between 5.16 ohms and 5.89 ohms - one reading is 6.13 ohms.

Are these acceptable readings for a TT system, as there is little guidance in the Regulations on TT systems.

Answer: The RCD/RCBOs will provide the earth fault protection that is required. The maximum recommended earth fault loop reading on an earth rod is 200 ohms. The 100mA RCD should be time delayed to give you the necessary discrimination with the RCBOs.

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