Buried SWA to a Detached Garage: Depth, RCD and Armour as CPC
A 6 mm² three-core SWA cable supplies a detached garage 35 m from the house. It is buried directly in garden soil at a depth of 450 mm along most of the route, with no warning tape or conduit, and rises into the garage through a plastic sleeve. The cable is protected at the house by a 32 A type B MCB. The garage has two 13 A socket outlets and a 3 kW heater. The armour is used as the circuit protective conductor. The measured Ze is 0.35 Ω and the measured R1+R2 using the armour is 0.45 Ω. There is no RCD in the garage or at the origin. The customer says the installation passed a 'visual check' and wants a certificate. The tabulated capacity of the cable is adequate for the load.
1. Identify the non-compliances with the buried cable and the garage socket outlets.
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The direct-buried cable is at 450 mm, below the usual 0.6 m minimum depth for direct burial unless it is provided with equivalent mechanical protection; no warning tape or conduit is present. The garage socket outlets rated at 13 A require 30 mA additional protection, and no RCD is installed. The cable entry through a plastic sleeve must be checked for mechanical protection and sealing against water and rodents. The armour must be correctly terminated at both ends with suitable glands and a reliable earth connection; a visual check cannot confirm continuity. The absence of an RCD also means the socket outlets do not meet the additional-protection requirement, even though the armour may provide a satisfactory earth-fault path.
2. Verify the earth-fault disconnection using Zs, and check whether the armour is adequate as a CPC by the adiabatic equation. Use k = 51 for steel armour and a disconnection time of 0.1 s.
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Zs = Ze + R1+R2 = 0.35 + 0.45 = 0.80 Ω. Prospective earth-fault current = 230/0.80 = 287.5 A. A 32 A type B MCB requires 5 × 32 = 160 A for magnetic operation, so 287.5 A is sufficient and disconnection should occur within 0.1 s. Adiabatic check: S = √(I²t)/k = √(287.5² × 0.1)/51 = √8265.6/51 = 90.9/51 = 1.78 mm². The steel armour has a much larger cross-section than 1.78 mm², so it is thermally adequate for this fault. However, the calculation depends on the armour being continuous and properly terminated; the measured R1+R2 of 0.45 Ω suggests it is, but the connection must be inspected and tested.
3. List the remedial work and tests required before a certificate can be issued.
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Remedial work: lower the cable to at least 0.6 m or provide equivalent mechanical protection such as conduit or a cable tile, and add warning tape or a marker; provide 30 mA RCD protection for the garage socket outlets, either at the origin or in the garage, selecting a device rated for the prospective fault current; inspect and remake the SWA glands and armour earth connections; seal the cable entry; and confirm the heater load is within the circuit rating. Tests: continuity of protective conductors, including the armour; insulation resistance; polarity; earth electrode or loop impedance as applicable; RCD operation at rated residual current and at half rated current; and a functional check of the socket outlets. The certificate must record the departures and the remedial work, because the existing 'visual check' is not a substitute for verification.