A Simple Guide to the Different Underground Cable and Pipe Locating Methods

Underground utility location is a crucial process in a wide range of industries. It’s typically done by applying a signal to the pipe or cable using a transmitter, and then reading the return signal with a receiver. The signal is applied using three modes – inductive clamp, inductive and conductive mode. Let’s explain these modes:

• Inductive Clamp – Clamps are connected to the transmitter, thus encircling the utility at a point of access
• Inductive – The transmitter is equipped with an inductive antenna on the ground just above the utility
• Conductive – Direct connection to the utility with the help of clips and cable

No matter which one of these modes you use, the key to successfully using an underground cable and pipe locator is applying the right frequency. The signals applied to the buried lines wane down into the ground, meaning they get weaker with depth. The key factors that impact signal strength include signal frequency, cable diameter, type of soil, and transmitter power level.

Out of all these possible variations, only two are under your control – power level setting and signal frequency. Frequencies are categorised into four different classes – low, medium, high and very high.

Frequencies

• Low Frequencies – These frequencies are less than 1kHz. And since they don’t propagate to other lines, they’re ideal for use in congested areas where several utilities can be located. They’re also ideal for tracing utilities over long distances. However, you can’t use these frequencies with inductive and inductive clamp locating modes.
• Medium Frequencies – These frequencies are in the range of 1-10kHz. They’re typically used for location, as they can propagate to other utilities. Medium frequencies are used with conductive and inductive clamp modes. While they don’t travel as far as low frequencies, they can still trace up to about 1-2km.
• High Frequencies – High frequencies are considered those in the range of 10-100kHz. They wane down quickly compared to low and medium frequencies, and they propagate to closeby conductors. However, they’re ideal for locating small diameter cable telecommunication and TV cables and lines. These frequencies are best applied using inductive clamp and inductive modes.
• Very High Frequencies – Even more than high frequencies, these frequencies wane very quickly, and propagate to other conductors even more easily. They’re used for quick sweeping of utilities in very congested areas. They provide great signals across gaps in a line, making them ideal for scanning for cable breaks and insulated pipe joints.

Power Level Settings

As far as power level settings go, having more watts to work with is beneficial to covering longer distances on small conductors such as tracer wires, and electrical and telecommunication cables, because you’ll be able to push through high-resistance conductors or go over non-conductive joints such as iron pipes. However, using more watts will propagate to nearby buried utilities, so lower the wattage to reduce propagation.

Conductive Locating

This method requires a direct connection to the cable or pipe being located, thus isolating the signal to just one utility. In this method, proper grounding is paramount. In other words – the better the grounding, the better the signal. The transmitter is connected at one end electrically, the signal is sent through the desired line, goes to ground, and comes back through the ground stake. Improper connection or grounding will result in a weak signal. The ground stake should be placed far away from the far-end ground and trace path. That being said, extend the stake away from the trace path at about 90° for optimum results.

Inductive Locating

This method is the most simple out of the three locating methods, used by most underground cable and pipe locator experts, as long as one point of the trace path is known. All you have to do is put the transmitter on the ground right above the buried utility and line it up with the transmitter arrows parallel to the direction of the utility line. Ideally, the transmitter should be about 3 metres from the line for a good signal. If several conductors overlap, this method won’t work, because typically, inductive locating requires high-frequency usage. Furthermore, if the receiver is within 25-30 metres of the transmitter, it will pick the signal through the air, which is why you should start at least 30 metres away from the transmitter when tracing.

Inductive Clamp Locating

This method applies the signal with an inductive clamp. This is done by closing the jaws of the clamp around the cable or pipe, allowing it to be induced directly into the line. In the process, the chances of other conductors picking up the signal are reduced. However, the clamp must be closed fully, and the conductor needs to be properly grounded on the far and near end.