Q & A of the Day – Can the RCD on this barge be reduced to 100mA to avoid nuisance tripping?

Question: I'm in the process of wiring up a barge restaurant, which will be berthed on the non-tidal part of the River Medway. It will be run by its own private 400V 100A three-phase supply direct from EDF's network, to a switched fused unit 150m away from its berth. Part of the SWA cable run is to be clipped directly to the river wall (which is in a public area). Permission has been obtained from the Environment Agency for this. The remaining length will be buried in the ground.

Having read Section 709 Marinas and Similar Locations of BS7671: 2008 (17th Edition), I believe that I'm right in stating that the incoming supply should be protected by a 30mA RCD. All power outlets onboard are being protected by their own 30mA RCBOs.

My customer's concern is that if a fault should occur during service, he will loose all power to his restaurant - causing major disruption and loss of business.

Can the RCD at the supply be reduced to a 100mA RCD to reduce the possibility of unnecessary tripping?

Answer: The incoming supply can be protected by a less sensitive RCD with an operating current greater than 30mA. However, some calculation will be required to ensure that the touch voltage does not exceed the 50V limit, which will be the product of the operating current and the earth fault path resistance.

As you indicate, the power outlets will then need to be protected by RCDs or RCBOs no greater than 30mA.

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