Construction Site Distribution Board
A temporary construction site uses a 400 V three-phase supply via a 63 A TP&N distribution board feeding four 32 A single-phase outlets for tools, a 16 A outlet for a site office, and a 16 A outlet for a 110 V transformer. The site is muddy, and trailing leads are common. The 110 V transformer has a 3 kVA rating and feeds a small 110 V distribution board with two 16 A outlets. The site manager wants to add a 230 V 16 A outlet for a cement mixer near a water butt. The earthing is TT with a measured electrode resistance of 180 Ω. The main protective bonding is 6 mm² to the incoming metallic water pipe.
1. Assess the adequacy of the earthing and protection arrangements for this site, and identify the changes required before the new outlet is added.
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TT system and disconnection. With a 180 Ω electrode, the earth fault loop impedance is far too high for a 63 A or 32 A device to disconnect within the required time using overcurrent protection alone. For a 32 A device, disconnection within 0.4 s requires Zs of roughly 1.1 Ω or less (depending on device type); 180 Ω cannot achieve this. The site therefore relies on 30 mA RCD protection throughout, and the RCDs must be verified by test. If the board has no 30 mA RCD protection on the 230 V outlets, it must be added, or the outlets must be supplied through 30 mA RCDs.
Construction site requirements. For construction sites, BS 7671 requires 30 mA RCD protection for all socket outlets up to 32 A, and reduced low voltage (110 V) for hand-held tools where practicable. The 110 V transformer arrangement is appropriate for tools, but the 230 V outlets must be RCD-protected.
Bonding. The 6 mm² main protective bonding to the water pipe is likely inadequate; for a TT installation the bonding conductor should be sized per the supply neutral/earthing conductor, and 10 mm² is a common minimum for main bonding. The water butt itself is not an extraneous-conductive-part unless it is metallic and connected to the general mass of earth; a plastic butt is not.
New outlet. The cement mixer outlet must be 30 mA RCD-protected, have a suitable IP rating for the wet location, and be positioned so the lead does not trail through water. The mixer's flexible cable and plug must be suitable for the environment.
Verification. The electrode resistance, RCD trip times, and continuity of protective conductors must be tested and recorded; the existing installation may need a periodic inspection given the site conditions.
2. The site manager proposes to improve the earth by driving a second rod and connecting it to the first with a 4 mm² conductor. Is this acceptable, and what else should be checked?
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Driving a second rod is a legitimate way to reduce electrode resistance, but the connecting conductor must be sized as a protective conductor, not arbitrarily at 4 mm². For a TT installation, the earthing conductor between the rod and the main earthing terminal must be sized to carry fault current and meet the adiabatic requirement; 4 mm² may be adequate for a small installation but must be justified by calculation or by the standard table (typically 16 mm² for a supply neutral up to 35 mm², or 10 mm² for smaller). The connection must be mechanically sound and protected against corrosion, and the rod must be accessible for testing.
Even with two rods, 180 Ω may not fall enough to allow overcurrent disconnection; RCD protection remains the primary means of fault protection. The RCD trip times and the effectiveness of the earth electrode must be re-tested after the change, and the results recorded on the schedule of test results. The main bonding to the water pipe must also be checked and upgraded if necessary, and the new cement mixer outlet must still be RCD-protected and suitably enclosed.