Published: 26 February 2014
Category: Q&A
John asks this question, which is about the external TT electrical supply to a shed, and its circuit protection arrangements. It has been answered by the NICEIC (Theme – Distribution Equipment including Cable Management):
This Q & A is one of thousands posted in our Technical Expertise area, and answered on a daily basis by our Voltimum Experts.
Question: Can you offer a solution to the following problem concerning a 17th Edition split distribution board using a 30mA RCBO to feed a shed via 6mm SWA cable?
- The earthing arrangement is TT at 10 ohms.
- The RCBO at the origin is required because of the TT earthing, not because the cable has less than 50mm cover in the wall - which is why it is in SWA from the origin consumer unit to the remote consumer unit.
- The SWA cable is earthed at the origin consumer unit and insulated at the remote consumer unit.
- The remote consumer unit has RCBOs and a main switch, and is also TT earthed separate to the origin earth - and is also 10 ohms.
The problem occurs in that when a trip happens at the remote end, it also trips the origin consumer unit RCBO.
Please note that using a Type C RCBO at the origin doesn’t help.
Would 100mA RCBO at the origin help solve this problem? And would it be outside the regulations, or is the only answer to use protective multiple earthing (PME) at the origin and MCBs?
Answer: A time delayed RCD at the front end would be acceptable and compliant rated at 100mA, as this will offer earth fault protection on the first TT arrangement.
Then, further down at the shed, the 30mA RCD arrangement could stay in place.
This would still offer the level of ADS and discrimination required by BS 7671:2008 for compliance.
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