What EML is for, and what it is not
Electromagnetic locating follows current. A live cable radiates a field. A conductive pipe can carry an applied signal. A sonde can be pushed along a duct. The locator then gives a peak or null that a trained surveyor interprets as a line on the ground. That line becomes a PAS 128 segment only when it is surveyed into the site grid and given a quality level.
EML is not a complete picture of the UK subsurface. Plastic water, many fibre routes and abandoned non-metallic ducts will not play. PAS 128:2022 therefore pairs EML with GPR for Type B. The CICES types-and-quality-levels article is explicit about that minimum toolkit.
On the day of excavation, HSG47 still expects competent use of locating tools before the first cut. A PAS 128 EML survey does not retire that duty. It gives the designer a documented trace so the excavator is not discovering the service for the first time in a trench.
Quality levels from a locator, not from hope
A continuous, supported trace with a plausible depth and a matching GPR response can justify a high QL-B on that segment. A broken signal that only exists because a record said a cable “should” be there is a lower quality level or an assumed route. Writing that down is the professional act. Smoothing every line to look confident is how drawings become dangerous.
Congested London streets are the usual stress test. Multiple live services, street furniture and return paths create ghost signals. Manchester industrial estates add process pipework and older metallic networks that still respond, sometimes too well. In both settings we would rather publish a cautious quality level than a pretty one.
How EML is briefed
Tell us where you can isolate, where you cannot, and which chambers we may open. A sonde survey needs access. A clamp on a live service needs a competent person and a reason. If the site is a night possession on a Birmingham inner ring or a live campus spine, say so in the enquiry; the method is the same, the logistics are not.
Read quality levels explained if you are deciding whether EML evidence is enough or whether a trial hole is cheaper than an argument later. Then commission the Type B pair, not a locator-only pass dressed as PAS 128.
Modes, sondes and the ethics of a continuous line
Passive modes listen to what is already radiating. Active modes apply a signal with a clamp or a direct connection. Sondes travel where a person can gain access. Each mode produces a different kind of evidence and should be named in the method statement. A drawing that does not say how a cable was followed is asking the next person to trust a gesture.
Direct connection is powerful and not always available. Live networks, missing earth points and owners who will not attend all limit the method. We will not apply a signal we are not authorised to apply. That sentence belongs in a proposal, not in a heroic claim that “we can trace anything metallic in the UK”.
Sonde work needs a chamber, a duct that is not collapsed, and time. It is often the only way to follow a non-metallic duct that GPR sees as a smear. It is also how you discover that the duct you were promised is blocked ten metres from the cover. That discovery is a useful QL-B limitation, not a failed day.
London construction density and Manchester industrial estates were mentioned as environments because they stress the method in opposite ways: too many live signals in the first, too many forgotten metallic runs in the second. In both cases a continuous pretty line can be a lie. We would rather break the line, drop the quality level, and tell you to buy a trial hole on that crossing.
EML remains half of Type B. Commission it with GPR. If a quotation separates them to look cheaper, you are not buying PAS 128:2022 Type B, whatever the header says.
On the morning of excavation, the same tools come out again. A PAS 128 EML trace is yesterday’s evidence. HSG47 still expects a competent person to locate before the bucket. Treat the drawing as the plan, not as a substitute for that last check.