Detached workshop: MEN link, bonding, and what the builder's mate said
A 4-core 16 mm² submain is installed from the main switchboard of a house to a new detached workshop 35 m away, protected at the house by a 40 A MCB. The workshop has a concrete slab floor, a sink fed by a copper pipe that runs continuously under the driveway from the house, steel roof purlins, a 16 A socket circuit and a dedicated 20 A welder socket. The house has an overhead MEN supply with a 60 A point of supply. The builder asks whether the workshop needs its own earth electrode and MEN link 'like the house has', because a mate told him every building needs one, and whether the copper pipe from the house has to be bonded to anything.
1. Explain how the workshop should be earthed, including whether a second MEN link and electrode should be installed, and how the answer changes if the workshop had its own point of supply.
Read the worked answer
Same point of supply - do not install a second MEN link.
The workshop is part of the same installation, fed from the same point of supply. The neutral-earth link belongs at the main switchboard only. A second link at the workshop would:
- parallel the neutral with the earth conductor, so part of the neutral current returns through the earth/PE system;
- put a standing voltage on exposed metalwork and on the submain earth conductor;
- provide an alternative return path that can defeat RCD operation and make fault current readings meaningless.
Correct arrangement: run a PE conductor in the submain (the fourth core), terminate it on an earth bar in the workshop board, and connect all circuit protective conductors and any bonded metalwork to that bar. The workshop board needs no electrode and no link.
If the workshop had its own point of supply (its own supply from the network, its own metering), it would be a separate installation: it would then need its own main earthing system, its own MEN link at its main switchboard, and its own electrode as required for that supply arrangement.
Judgement point: the deciding question is not 'how many buildings' but 'how many points of supply'.
2. Is the copper water pipe from the house an extraneous conductive part requiring bonding in the workshop? Set out how you would decide, and what you would do either way.
Read the worked answer
Decide by test, not by appearance.
- Is it earthed already? The pipe originates in the house, whose plumbing is likely bonded to the house earthing system. Test continuity from the pipe at the workshop back to the workshop earth bar.
- Is it an extraneous conductive part? Test the resistance from the pipe to the workshop earthing system. If the pipe is reliably connected to the installation earthing system and to the general mass of earth, it is already at earth potential and is not introducing a potential - no separate bond is needed, but record the test result.
- If it is not reliably connected (plastic sections, insulated joints under the drive, corrosion, or a high resistance reading), it can introduce a potential into the workshop and must be treated as an extraneous conductive part and bonded to the workshop earth bar with a correctly sized bonding conductor.
- Supplementary bonding consideration: the workshop has a concrete slab floor and earthed steel purlins. Touch voltage between the earthed steelwork and an unbonded pipe is the risk being managed - the concrete floor makes a person a better earth contact, so the risk is higher, not lower.
What would I do? Test, record the reading, bond if the test shows it is not reliably earthed, and record the reasoning on the certification. Do not bond 'just in case' without a test - bonding a pipe that is deliberately isolated can create a path that did not exist.
3. List the verification you would carry out before certifying the workshop, and state any design checks the 4-core 16 mm² submain on a 40 A MCB still needs.
Read the worked answer
Design checks on the submain
- Volt drop: 2.8 mV/A/m (approx., 16 mm² single-phase) × 40 A × 35 m / 1000 ≈ 3.9 V = 1.7 % - acceptable, and leaves room for the final circuits.
- Coordination: 16 mm² derated capacity comfortably exceeds 40 A, so the MCB protects the cable.
- Earth fault loop impedance at the workshop board must suit the 40 A device's disconnection time.
- Neutral and earth core sizing - 16 mm² actives with a reduced PE is acceptable only if the PE meets the minimum ratio and adiabatic check for the fault level.
Verification tests before certification
- Continuity of the protective earthing conductor and of the main earth.
- Insulation resistance of the submain and each new final circuit.
- Polarity.
- Earth fault loop impedance at the workshop board and at the furthest point of each circuit.
- RCD operation (trip time and current) where RCDs are installed.
- Functional check of socket outlets, and confirmation that the MEN link exists at the house only.
Certification: the registered electrician issues the CoC for the work with test results recorded. If any part of the work is high-risk prescribed electrical work (for example, the new submain connection and livening), then inspection and certification by a licensed electrical inspector before the supply is connected is required.