Neutral and Earth Combined in a Sub-board
During a periodic inspection of a small retail unit, you open a sub-distribution board fed from the main board by a four-core SWA cable. The sub-board has a separate earth bar, but you notice a short link between the neutral bar and the earth bar. The main board is TN-C-S with the PEN split at the origin. The sub-board earth bar is also connected to the armour of the SWA. Earth loop impedance at a socket is 0.62 Ω and the 32 A type B MCB feeding the sub-main has a measured disconnection time within limits. The installer says the link is needed 'because the earth was reading high'.
1. Explain why the neutral-earth link in the sub-board is dangerous, even though the loop impedance looks acceptable.
Read the worked answer
The link creates a parallel path for neutral current through the earth conductor and the SWA armour. Under normal load, part of the neutral return current flows in the CPC and the armour, raising the potential of exposed conductive parts above earth. Anyone touching a metal enclosure and true earth (a radiator, damp floor, or earthed structure) can receive a shock even with no fault present. The armour and gland are not rated to carry continuous neutral current and can overheat.
It also defeats RCD protection: any residual current diverted through the earth path may be seen as imbalance by an upstream RCD and cause nuisance tripping, or a downstream RCD may be blinded. The acceptable loop impedance reading is misleading because the link lowers the measured loop by providing a parallel return, so the test result no longer represents the real fault path. The correct fix is to improve the earthing and protective conductor arrangement properly, not to bridge neutral and earth downstream of the origin.
2. Outline the corrective actions and the verification tests you would carry out before re-energising.
Read the worked answer
Corrective actions:
- Isolate and lock off, then remove the neutral-earth link.
- Confirm the sub-main has a proper CPC sized for the sub-main (armour alone may be adequate only if its cross-sectional area and conductance meet the requirement; otherwise a separate CPC is needed).
- Verify the earthing arrangement at the origin is intact and the main protective conductor is sound.
- If earth loop impedance is genuinely high, address it by increasing CPC size, improving the main earth, or adding RCD protection where permitted, not by linking.
Verification tests before re-energising:
- Continuity of protective conductors and main bonding.
- Insulation resistance between live conductors and earth with the link removed.
- Polarity.
- Earth fault loop impedance at the sub-board and at the furthest point, with the link removed, to get a true reading.
- RCD operation if fitted (½×, 1×, 5×).
- Functional check under load, then record results on the schedule of test results and issue a minor works or EIC as appropriate.