Periodic Inspection Findings at an Ageing Factory Intake

Welding bay submain - highest priority

• A 63 A type D device trips magnetically at 20 × In = 1260 A. Maximum permitted Zs = (230 × 0.95) / 1260 = 0.17 Ω.
• The measured 1.6 Ω corresponds to a fault current of roughly 137 A, about 2.2 × In. The breaker will not trip instantaneously; disconnection depends on the thermal element and may take tens of seconds or never occur at all.
• This is a shock and fire risk on every earth fault in the bay. It is a danger-present finding requiring the circuit to be taken out of service or restricted until corrected.

32 A socket - second priority

• A 32 A type C device trips at 10 × In = 320 A; maximum Zs = 218.5 / 320 = 0.68 Ω. The measured 0.9 Ω fails, so disconnection is not assured.
• The absence of 30 mA additional protection on a socket rated up to 32 A is a second, independent deficiency on the same circuit. Potentially dangerous, requiring improvement.

Main protective bonding - third priority

• A 6 mm² bonding conductor is not adequate for a 25 mm² earthing conductor on a TN-C-S supply, where the bonding conductor must be sized from the earthing conductor with a substantially larger minimum. This is potentially dangerous but is a slower-acting risk than the two above.

Documentation - nine years without records is a compliance failure in its own right and limits what can be certified.

In the report

• Record each observation with its location, the measured value, the maximum permitted value it is compared against, and the calculation behind that maximum - so the finding is evidence, not opinion.
• Classify in the terms the report uses: danger present (requiring immediate action), potentially dangerous (requiring improvement), and improvement recommended. The welding bay is danger present; the socket circuit and the main bonding are potentially dangerous; the absent records are an improvement item.
• State the extent and limitations of the inspection, that the installation has not been inspected for nine years, and that sampling was constrained by the absence of any previous results.
• Recommend a reduced interval to the next inspection given the age, condition and duty of the installation, and record the departures identified.

To the client

• A satisfactory certificate cannot be issued while danger-present and potentially dangerous items remain. What can be issued is an unsatisfactory report that lists the defects and the remedial work - that is exactly what an insurer needs, and it protects the client's cover.
• Falsifying or omitting the welding bay finding to meet a deadline is not an option; the measured Zs is a matter of record and the liability would sit with whoever signed.
• Practical route: isolate or restrict the welding bay now, correct the Zs by increasing the circuit protective conductor, shortening the run, changing the device type or adding a 30 mA device, upgrade the bonding, add RCD protection to the socket, re-test, then issue the certificate with the schedule of results.

Precautions

• Safe isolation, lock off, prove dead at the point of work, and treat the bay's own earth electrode or structural bonding as live until proved otherwise - welding return currents frequently travel through building steel and bonding conductors.
• Never disconnect main protective bonding while the installation is energized; a diverted return current or a simultaneous fault elsewhere will make isolated metalwork live.
• Check that the welder's work return conductor is dedicated and clamped to the workpiece, not to the building structure or the earthing system. A return path through the bonding is itself a defect and will invalidate any Zs measurement taken afterwards.
• Coordinate with production: the bay must be out of service, not merely switched off at the local isolator.

Verifying the repair

• Re-measure Zs at the furthest point of the bay with a loop impedance tester and compare against the maximum for the device actually fitted; if an RCD has been added, verify the new Zs against the RCBO's requirements and note that the RCD does not remove the need for a viable CPC.
• Confirm CPC continuity with a low-resistance ohmmeter, including the armour or separate CPC and all accessible exposed conductive parts.
• Trip-time and ramp test any residual current device installed, at 1× and 5× IΔn, at both half-cycles.
• Record the new values with the ambient and conductor temperature, the instrument used, the date, and the previous values side by side so the improvement is auditable, then update the schedule of test results and the labels at the board.