EML (Electromagnetic Locator) Techniques

Electromagnetic locators (EMLs) detect buried metallic services by sensing electromagnetic fields around cables and pipes.
EMLs work in two modes: passive mode detects signals from live cables, and active mode uses a signal transmitter.
A CAT (cable avoidance tool) is the most common EML used on UK construction sites for service detection.
The Genny (signal generator) is connected to a service at an access point to apply a traceable signal.
EMLs cannot detect non-metallic pipes such as plastic water mains, clay drains, or fibre optic ducts.
HSG47 (Avoiding Danger from Underground Services) sets out the UK guidance for safe detection and excavation.
All operatives using EMLs must hold a valid competency card and have received refresher training.
False readings can occur from reinforcement, metallic fences, nearby vehicles, and parallel services.
EML detection must always be supplemented by reference to utility plans and trial hole investigation.
Equipment must be checked for correct function using a known signal source before each use.

Prevent service strikes	Failing to detect buried services leads to cable strikes causing electrocution, explosions, and gas leaks.
Legal duty	HSG47 and CDM 2015 require competent use of detection equipment before any ground disturbance.
Protect workers and the public	Gas main strikes and HV cable strikes endanger workers and nearby residents with immediate life-threatening hazards.

Do

Don't

Check the EML is functioning correctly using a known signal source before every use
Use both passive and active (with Genny) modes to maximise detection of buried services
Scan the full work area systematically in a grid pattern, not just the planned dig line
Cross-reference EML readings with up-to-date utility plans for the area
Mark all detected services clearly on the ground with paint or markers before excavation
Dig trial holes by hand to confirm the exact position and depth of detected services
Hold a valid EML competency card and attend refresher training as required
Record the scan results and service positions in the site records before excavation begins
Report any unexpected readings or anomalies to the supervisor before proceeding
Keep the EML away from vehicles, generators, and steel structures that cause interference

DON'T rely solely on EML detection — always cross-check with utility record plans
DON'T assume no signal means no service — plastic pipes and fibre are not detectable by EML
DON'T use an EML without testing it on a known source to confirm it is working
DON'T scan in one direction only — services at angles to the scan line may be missed
DON'T excavate with machines until EML scanning and trial holes confirm service positions
DON'T use the equipment near running engines or steel structures that distort signals
DON'T allow unqualified persons to operate EML equipment for service detection
DON'T ignore weak or intermittent signals — they may indicate deep or partially shielded services
DON'T skip the Genny mode — passive mode alone does not detect all metallic services
DON'T treat EML results as definitive — they indicate probable positions requiring confirmation

See also: CAT and Genny Safe Use | Safe Digging Practices (HSG47)

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