The Hill Of Potential
Method
Of Underground Electric Cable Fault Locating
The Hill of Potential method is probably the oldest method used for underground electric cable fault locating. Not long after utilities began installing electric cables underground those cables developed faults, and not long after that workers discovered that they could locate these faults by probing the ground and taking voltage measurements. Using the Hill of Potential is nothing more than a fancy way of saying that you are detecting voltage in the ground over the cable path.
Sadly, many fault locators rely solely on using this method to locate underground faults, and even more use this method as their primary fault locating method. While it is true that the Hill of Potential method is effective, it is not always accurate and it is not always foolproof. I would much rather use a more consistent system to pinpoint an electric cable fault than risk excavating for no reason.
Why It Works
A fault, or break in a buried electric cable allows the current to escape at the point of the break as the electricity follows the path of least resistance. In most cases with underground cable, the path of least resistance is directly into the earth that surrounds the cable. By utilizing a sensitive and accurate multimeter connected to a pair of probes, this "escaping" voltage can be detected on the surface. The probes that are used with the Spitfire are especially useful for this method.
How It Is Performed
The Hill of Potential method is best used after the circuit has been traced and marked. Knowing the exact path of the cable produces the best results and eliminates the detection of other, unrelated underground utilities. The circuit must be energized. If a standard source is not available the cable can be energized by using a generator with a set of adapter clamps.
Attach the leads from the Spitfire probes to the multimeter, turn the meter on, and set it to AC voltage at the most sensitive range.
Place the RED probe into the ground directly over the cable path.
Place the BLACK probe into the ground away from the cable path and parallel to the cable path.
Move the probes along the circuit, keeping the RED probe on the cable path and the BLACK probe off to either side. Keep an eye on the measurements being detected by the multimeter.
As you work your way closer the fault, the measurements on the multimeter should slowly increase. When a point is reached where the measurements begin to decrease you may have passed the fault. Leave the BLACK probe stationary and check various areas along the cable path with the RED probe. The fault could be located at the point where the highest measurement is detected.
Be sure to check the entire length of the section, a much higher measurement could occur at a different point along the cable path. If this is the only fault locating method that you are using you will have to consider all of the measurements noted and decide which areas to excavate.
Why It Does Not Always Work
As I mentioned earlier, the Hill of Potential method is not always accurate or reliable. These days there are so many facilities buried underground that any one of these could influence your measurements. Invisible dog fences, cable television cables, phone lines, or other buried electric circuits all carry voltage of some sort, and therefore could cause a measurement to be detected. There are also many situations where the fault cannot be detected at all by using the Hill of Potential method. Some soil types, concrete, asphalt, or deep cable will all prevent the voltage from the fault from being detected on the surface.
When The Hill Of Potential Method
Should Be Used
I have seen many fault locators who have been rather successful with using the Hill of Potential method before using other underground cable fault locating equipment. However, this is dependent on the area, the type of cable, and a host of other variables. I recommend using this method for proving the location of an underground fault, or eliminating areas where other equipment may have indicated a fault to be.
Proving a Fault is simply using a second method to back up the findings of the first equipment used. For instance, if you have pinpointed a fault location by using the Spitfire simply plug the probes in to your multimeter and check the same location for voltage. It is always reassuring to have multiple results that coincide with each other.
Eliminating Areas by using the Hill of Potential method is useful in circumstances where the Spitfire may have indicated more than one area where a fault may exist. Check each area where the Spitfire indicated a fault for voltage. Chances are good that one location will stand out above the others, and that is where you will find the fault.
next - Utilizing Test Holes