Locating Methods Are Not All Created Equal

Many of you will be aware of how to find a cable underground; there are several methods, such as:

• Referring to plans and maps, and hoping for the following:

  1. that the plans were accurately drawn

  2. that any references used are still there

  3. and that if GPS-located, your continent hasn't drifted too far (yes, it's a real problem).

• Digging carefully, starting at one known end, and following the cable to the point you need to find. But this is almost always tedious, expensive, and potentially dangerous. (Reminder - Always contact other utilities for details of nearby services before picking up a shovel.)

• Using "electromagnetic induction" by applying a known signal to the cable at some accessible point and allowing that to create a current in the cable, which will develop a magnetic field around the conductor. This magnetic field can be followed using a "locator" tool.

Tempo has been producing cable locators that operate using "electromagnetic induction" for many years, and you may already know of these products. Our 501 is a great general-purpose locator; its smaller brother, 508S, is an almost-pocket-sized version, ideal for tracing indoor cables in walls or shallow buried cables. BLL-200 is a more advanced locator that can even measure the depth of the cable, when conditions are good, and detect "RF noise" on the cable that you may not even know is there (e.g. power hum or induced currents from nearby conductors). Please see Tempo's locators here for more information.

The Problem

Those skilled in the art of cable location and those who've paid attention to their training will know that cable location relies on the magnetic part of electromagnetic induction to work. But to generate an electromagnet, you need a flowing current. If a cable in the ground becomes broken open circuit, then it will not carry a current from one end to the other. So you will not be able to locate it.

Well, all is not lost. If you apply an alternating signal to any conductor in the soil, that conductor is surrounded by conductive soil. Therefore, there will always be some level of capacitance between the conductor and the soil, allowing for some "capacitive current" to flow along the wire. However, this is frequency-dependent (high frequencies go through capacitors easier than low frequencies) and as you approach the broken end of the cable, the remaining capacitance can diminish towards zero, meaning the actual broken end of the cable may not be precisely located.

In many situations, this is NOT a huge problem, because you can also see the evidence on the surface left by the recent work of other contractors who broke your cable. 

But what about the many situations where a cable is broken simply due to soil movement, corrosion, or similar? No surface evidence.

Solutions

Tempo has a tool for that. We have tools for most things related to wires, cables, optical fibres, etc. (check our catalogue).

When an underground cable is broken, the soil likely has some moisture; therefore, the end of the cable will be grounded. Okay, maybe not for a 1V, 5V, or even 30V signal, but with a little "encouragement" in terms of added electro potential, we can make those electrons and ions in the soil flow.

To this end, Tempo's 521A Wire and Valve Locator can output signals at up to 250V onto a cable, causing plenty of currents to flow. This makes irrigation control cables, which are often only shallow-buried and hardly ever in ducts, easy to find with the inductive locator.

But then there are those "problem" cables - they're buried deep and, for some reason, don't seem to "break down" to ground at lower voltages - or they're still connected end to end but have a "ground fault"; and yet, you've still got to find that broken end or ground fault and re-splice it.

PE2003 Pulser Ground Fault Locator

If you can apply a signal to the cable that will leave the cable at the "open end" into the soil, how can you find that above ground? Well, if it is a good and solid "ground", as mentioned above, you can continue to use your existing electromagnetic locator because current will flow to ground at the fault and the characteristic of the signal will change at the fault. But if the ground fault is subtle, or if there is a complete "open", then finding the exact end is tricky.

Enter the "pool of potential". Instead of applying an alternating signal and relying on the magnetic field set up by the flowing current, if we apply a DC signal to the cable where this DC signal "leaks" from the broken insulation or open end into the nearby soil, it will develop a "potential gradient" in the soil, back to the origin.

Have questions? Watch this video, it will make things a heap clearer...

Think of applying a voltage at the start of the cable, relative to a local ground point. That potential will travel to the "ground fault" and enter the soil, and then make its way back to the source. Between the "fault" and the "source", there is a "gradient of potential"; you can follow that towards the fault using the A-Frame. The indicator will reverse as you pass the fault. Directly above the "fault", the "pool of potential" is circular (assuming soil conductivity is uniform) so you can easily "pinpoint" the break on the surface directly above the fault.

Conclusion

As is the case in many situations, you can often save a heap of time and money on a job by applying the correct tools and methods. PE2003 is just one example of a low-cost, specialist tool that can be a "career saver" at the right time.

If you want more information on PE2003, please visit its webpage here.

For a more general introduction to electromagnetic cable location, watch this online seminar I made.