Gas integrity and safe working¶

Most of what proves the gas network safe is not a live log but a periodic measurement and a piece of paper: a cathodic-protection potential read at a test point, an odorant check, a leak survey walked down a street, a permit to open a live main. The record is thinner and slower than the electricity net’s, and it is meant to be, so its signatures are as much about missing, stale or overdue records as about anomalous ones.

Cathodic protection¶

Part of the network is steel, held clear of corrosion by a small protective voltage that keeps the pipe’s potential more negative than the standard criterion (around minus 850 millivolts against a copper/copper-sulphate reference). The evidence is that potential, read at test points and drainage points, together with the output of the rectifiers that supply it. The measurement was made by hand at something like ten thousand points once a year; remote sensors now report several times a day and have cut the physical visits to roughly one in three years.

Normal is potentials sitting within criterion and rectifiers holding steady. The anomaly that counts is a loss of protection potential, the reading drifting above the criterion, which leaves the steel exposed and, in time, invites a leak. Its innocent cause is usually a failed rectifier or a broken bond; its cross-check is the potential against the rectifier’s output and against a field reading. A remote potential that stays flat and healthy while a field measurement disagrees is a stuck or misreported sensor rather than a protected pipe, the same lesson the electricity side draws from a measurement no independent record confirms.

Odorisation and gas quality¶

Distribution gas is odorised with THT, an added odorant, so that a leak can be smelled; the nose is the public’s primary detector. The odorant is added upstream, by the national transport operator at the handover, so the network generally receives gas that already carries it, and assurance is periodic rather than continuous: random-sample measurements of odorability and THT content by gas chromatograph at the transfer points.

Green gas is the exception that puts odorisation inside the operator’s own record. Injected locally into the distribution net, it is odorised before it enters, under a regime of checks, at start-up, at twenty-four hours, at four weeks, and monthly, with the safety regulator pressing for continuous monitoring of gas quality and THT at the injection points. Normal is THT within band and the checks made on schedule. Under-odorised gas is a safety signature, leaks that will not be smelled, and it reads as a THT measurement below band, a check missed, or an injection point’s quality drifting, set against the point’s own dosing record and against customer reports of gas smelled or not smelled.

Leak surveys¶

The low-pressure net is surveyed for leaks on a cycle, roughly two-yearly across the estate and at least yearly on the brittle-gas mains still awaiting replacement, walked with a hand detector or a sleepmat or driven with a vehicle sniffer. A leak that is found is located and put into a severity class set by its reading and its distance to buildings, and the class fixes how quickly it has to be repaired. The artefact is a classified survey record, not a live sensor stream.

Normal is surveys run on schedule, leaks found, classed and closed within the window their class allows. The signatures are an overdue survey, a leak found but left unclassed or unrepaired past its window, or a repair booked with no survey behind it. Each is read against the next record along, the survey against the leak class, the class against the repair work order, the whole against the brittle-gas replacement map that says where the worst pipe still lies.

Working a live main¶

Work on a live gas main is done gas-free and pressure-free wherever it can be. The line is verified drukloos and unable to re-pressurise, a plug is set, and the steps follow the national gas safety instruction (VIAG) and a bedieningsplan. The record is procedural: the permit, the safety-instruction sign-offs, the drukloos verification, the as-left. Normal is the permit, the plan and the verification all present and agreeing.

The unsafe reading of the same work is the one the Zoetermeer investigation set out: a gas-detector alarm overridden rather than obeyed, a supplementary procedure for non-standard work absent, and a line worked while it could still carry gas. It is a safety failure first, but it is an evidence one too, the same act, safe or unsafe, told apart by whether the gas-free verification and the instruction sign-offs are there and consistent or missing.

The register and the map¶

All of this rests on the asset register and the network drawings being right. When the register is stale the crew works to a picture that no longer matches the ground, which is what the Zoetermeer investigation found in the outdated drawings, so a divergence between the register and the field is a signature in its own right. A change to the register, meanwhile, carries the same authorised-or-not question as any other record: a network drawing altered with no work behind it is as much a question as a relay setting changed the same way.

What the gas record cannot always show¶

The gas net’s evidence is periodic and paper-heavy by design, so its hardest cases are not falsified logs but absent ones. A cathodic-protection point unread, a leak survey overdue, an odorant check skipped, a gas-free verification not filed: each is a hole where a record belongs, and a hole is easy to make and hard to tell from ordinary lag. What separates a benign gap from a covering one is corroboration the operator does not control, a field potential, a customer’s report of smell, a survey the neighbouring section shares, the physical state of a valve or a plug, and, underneath, the register agreeing with the ground. The gas record is thin enough that the physical world often has to be the witness the paperwork cannot.

On the gas side the record moves at the pace of scheduled work, not live telemetry. Measurements are sparse and scheduled, records lag the field as a matter of course, and an overdue check or a stale drawing is common and mostly innocent, so a single gap proves little; what indicts is a gap that lines up with something else, an unsmelled leak, a corroded main, a job a permit does not cover.

Last updated: 14 July 2026