The Signal Clamp: What It Does, and When You Need One

A Genny and CAT will find most buried services, but not all of them. Some cables carry no usable signal by induction. Others sit among so many services that the trace becomes a guess. In both cases there is a tool that puts a clean, reliable signal on the exact service you are trying to find: the signal clamp. Plenty of teams leave it at the bottom of the kit bag, or do not carry one at all.

What a signal clamp is

A signal clamp — sometimes called an induction clamp — is an accessory for the Genny. It is a split core that opens and clips around a cable or pipe at any point you can reach: a cable riser, an inspection chamber, a stop valve, a footway access box, or where a service enters a building.

When the Genny drives a signal through the clamp, the clamp induces that signal onto the service running through it, the way a transformer passes energy from one winding to another. It does this without cutting into the cable and without interrupting the supply, so it is safe on live services where a direct connection would not be. The CAT then works as normal, following the signal now travelling along that one service.

How it compares with direct connection and induction

• 01Direct connection — the cleanest, strongest signal there is, but it needs safe access to the conductor, which you rarely have on a live buried service, and it is not possible on HV.
• 02Induction — the Genny sits on the ground and radiates into nearby metal. Fast, needs no access, useful for a first sweep. But it radiates into every service in range, so in congested ground it lights everything up and the target is lost among them.
• 03The clamp — the signal goes onto one specific service. That service carries a strong signal and the ones around it only a weak return, so you can follow the target with confidence. It works on live cables, and it gives access to 131 kHz.

Put simply: induction is broad and fast but indiscriminate. A clamp is precise. It puts the signal where you want it and nowhere else.

Where a clamp earns its place

Congested ground and trial holes. Footpaths packed with cables are the hardest case. A shallower or better-conducting cable takes the induced signal and screens the deeper ones behind it. To clamp a buried cable you first have to expose it — so carrying a clamp encourages proper trial holes rather than guesses from a surface scan. Once the services are exposed, a clamp goes onto each one in turn: every cable positively identified, each given its own clean signal, each traced out individually. One trial hole, worked with a clamp, turns an uncertain survey into a set of clean, separate, confident traces.

Communications cables. BT and cable TV are high-resistance lines, so a 33 kHz induction signal cannot follow them at all. At a footway access box the cable is reachable, so it can be clamped and traced with the 131 kHz signal these lines need. Without a clamp there is often no way to put a usable signal on them.

Cables you can reach above ground. A cable coming down a pole, or running down a wall before it enters the ground, can be clamped there in seconds and traced cleanly along its buried route. No excavation, no delay.

HV. There is no way to make a direct connection to an HV cable, and induction struggles because HV sits deeper and is often masked by shallower cables above it. The reliable way to get a signal onto HV is to expose it in a trial hole and clamp it. From that point it traces cleanly for the rest of the job.

The cost case

A signal clamp is not expensive kit — list price is around £331, and with the discounts usually available it works out closer to £215, one per Genny and CAT set. Set that against the cost of a single strike: the repair, the standby, the programme delay, the reinstatement, the client penalty. A clamp pays for itself many times over the first time it prevents one.