Finding Plastic and Non-Metallic Pipes and Cables
A CAT and Genny finds metallic services well. Plastic and rubber pipes — increasingly the norm for water and gas — are the hard case: they carry no signal for a detector to hear. This covers how crews trace them anyway, where that fails, and the safe answer when a service simply can't be located. It pairs with how a CAT and Genny works and the wider cable avoidance guide.
Contents
Why Plastic Is the Hard Case
A CAT detects electromagnetic signals, not material — it isn't a metal detector. A metallic service either radiates a signal (a live cable), re-radiates one (radio mode), or can have one applied to it (the Genny). Plastic and rubber pipes do none of those. Pass a CAT over a plastic water main and it stays silent, not because the ground is clear but because there is nothing for the tool to hear.
That matters more every year, because so much modern water and gas service pipe is plastic. It's the single biggest reason a clear scan is not proof of empty ground — and the reason cable avoidance always ends in safe digging, not just detection.
The Trace-Rod Method
The standard field workaround is to give the locator something to detect: introduce a metallic conductor into the pipe. If you can reach an open end — a stop tap, a chamber, an exposed length — you feed a flexible metallic trace rod (or a length of cable) along the inside of the pipe, connect the Genny directly to that conductor, and trace it with the CAT in Genny mode. You get the route and an approximate depth from the same reading.
It works only as far as the rod will travel. Bends, joints and length all stop it, so you may trace the first few metres and no further — and the marked route ends where the rod stopped, not where the pipe does. Useful, often enough, but plainly partial: treat the end of the trace as "this is where I lost it," not "this is where the pipe ends."
Tracer Wire — When It's There
Newer plastic services are often laid with tracer wire — a metallic conductor run alongside the pipe, or detectable marker tape above it — put there precisely so the line can be located later with a CAT and Genny. Where it exists, you connect to or induce a signal onto the wire and trace as normal. The catch is the older and undocumented stuff: plenty of plastic pipe went in without any tracer wire at all, and there's nothing to detect on those. You can't assume a plastic service has it.
When You Can't Locate It
Sometimes there's no open end to feed a rod into, no tracer wire, and no signal to be had. When a service can't be located, the answer is not to drive and hope — it's to treat the area as though the service is there. That means breaking ground by hand-digging or vacuum excavation rather than driving a stake or machine-digging blind, and keeping well clear where you can.
For staking, the practical takeaway is simple: near suspected plastic services, a clear CAT reading buys you nothing, so widen your margins, reposition stakes away from the likely route, and expose anything you're unsure about before you drive. If the risk can't be managed on site, get the utility or a specialist locator involved. An un-locatable service is a reason for more caution, not less — see scanning a marquee site before you stake for how that fits the wider survey.
Talk It Through
We're bringing cable-avoidance kit into the Hogan range. If you're weighing it up, or want to talk through a site where plastic services are the worry, get in touch.
Email: hoganuk [at] hoganstakes.co.uk
Contact form: hoganstakes.co.uk/contact
Safety note — this is an overview, not training. Locating non-metallic services is difficult and uncertain. This guide explains the common approaches and their limits; it is not a substitute for competent, trained use of a cable locator (HSE HSG47) or for safe-digging practice. Where a service cannot be established as clear, do not break ground until it has been.
Background drawn from HSE HSG47 and publicly available CAT & Genny material (Paragon, Sygma Solutions, Proper DIY — including a demonstrated trace-rod method on a plastic pipe). Used as reference; adapted for UK practice.
Common Questions
Can a CAT and Genny find a plastic water pipe?
Not directly. A CAT responds to electromagnetic signals, and a plastic or rubber pipe carries none. You can only find one with a CAT and Genny if you can get a signal onto it — for example by feeding a metallic trace rod or cable along the inside of the pipe and applying the Genny's signal to that. With no way to introduce a signal, the pipe stays invisible to the locator.
How do you trace a plastic pipe?
The common field method is to introduce a metallic conductor the locator can detect. If the pipe is accessible at one end, you push a flexible metallic trace rod or a length of cable down it, connect the Genny directly to that conductor, and trace it with the CAT in Genny mode — which also gives an approximate depth. It works as far as the rod will travel; bends and length limit how far you get.
What is tracer wire?
Tracer wire (or detectable marker tape) is a metallic conductor laid alongside a plastic pipe or duct when it is installed, specifically so it can be located later with a CAT and Genny. Where it exists, finding the service is straightforward. The problem is older or undocumented plastic services that were laid without it — there is nothing to detect.
Why doesn't a CAT pick up plastic pipes?
Because a CAT is not a metal detector — it detects electromagnetic signals, not material. Metallic services either radiate a signal (live cables), re-radiate one (radio mode), or can have one applied (the Genny). Plastic and rubber do none of these, so unless a signal is introduced via a trace rod or pre-installed tracer wire, there is nothing for the CAT to hear.
What if you can't locate a plastic pipe at all?
Then you treat the area as if the service is there and break ground safely — hand-digging or vacuum excavation rather than driving a stake or machine-digging blind. An un-locatable service is a reason for more caution, not less. If the risk can't be managed, get the utility or a specialist locator involved before you commit.