Locating Short Drops and Fault Locating
Anyone who has been cable locating will be quick to point out a few useful facts that help locate a cable or pipe.
By Steve Benzie
Anyone who has been cable locating will be quick to point out a few useful facts that help locate a cable or pipe. These may include:
• Always try to use a low frequency,
• Make sure the line has good continuity,
• Make sure both ends are well earthed, and
• Make sure you make a good earth connection.
These are all good suggestions, but we know life is not always so simple. Take for instance the third suggestion, “make sure both ends are well earthed.” Many cables will not be grounded at the far end—or the beginning for that matter. Applying a low frequency signal locate tone in these circumstances can be fruitless.
If there is no earthing or the earthing is poor, making a good ground connection at the application point and checking for continuity is not going to help because there is nowhere for the signal to travel if there is no earth at the far end. A different technique should be used to get the signal to travel to the end of the cable. This is where a locator such as the VM-560 is useful. Typical locate frequencies tend to be in the range of 512 Hz to 32 kHz. These frequencies offer good signal to noise locates where good grounding is present. The VM-560 from Vivax-Metrotech offers 480 kHz for poor grounding situations.
So why 480 kHz? The answer is at 480 kHz the signal will “bleed off” the cable due to the distributed capacitance of the cable to ground. There’s no need to go into explanations of what this is, but suffice to say that the higher the frequency the greater these effects. This helps with ungrounded cables because the capacitance creates a sort of pseudo ground that allows the signal to pass from the conductor into the ground and completes the circuit back to the transmitter. Using the 480 kHz mode helps detect these short, unearthed cable drops.
In addition, 480 kHz can also help detect older cast iron pipelines. Many of these pipelines will have insulated joints. This electrical insulation may be a result of corroding joints and nuts and bolts, or because the sealing material used to join the pipes can create an insulated joint. Either way, this is bad news for someone trying to detect the position of the pipeline. Capacitive effects help here, also. The jointed ends, although possibly insulated, will have some capacitance across the joint. Using low frequency pipe detecting techniques does not get a good signal path, but switching to 480 kHz allows the signal to use the capacitance of the joint to jump over the insulated section.
A word of warning—as we have seen, the use of high frequencies such 480 kHz can be beneficial in many circumstances, but like all good things there is a downside. Understanding these limitations can still allow the user to appreciate the benefits of high frequency locate tones. The two main downsides are:
1. If there are any utilities nearby, there is a danger the bleed off effect can also result in a bleed on effect. So, retuning signal currents traveling through the ground may hitch a ride on other utilities, resulting in multiple signal paths. These tend to be smaller signals, so careful locating techniques can help identify the correct one.
2. Because the signal will bleed off over the entire length of the cable, the distance the signal will travel along the cable or pipe will be less than that of a low frequency signal.
It should be noted that using low frequency locate tones is always best where good grounds are present; for this reason, the VM-560FF also has 512 Hz, 8 kHz locate modes and can detect 50/60 Hz power signals, also.
Fault Finding Using the VM-560FF With the VM-510
Locating the route of a cable is sometimes just the start of the process. The operator often needs to find the route so repair work can be undertaken on the cable. There can be many different types of faults, but where there is damage to the cable and the conductor is in contact with the ground, the optional addition to the VM-560 locator kit, the VM-510FFL Standalone A-frame fault locator can be used to identify the exact position of the fault.
The process of detecting a fault to ground can be a complicated and laborious task. The VM-510FFL simplifies the process and provides an accurate pinpoint of the fault—within a few cm/inches.
The process is simple. First, remove all ground bonds from the cable. The idea is to make the fault the only contact the cable has with a ground. Having isolated the cable, the transmitter can be connected to the faulty cable and the other connection cable connected to an independent earth. A small ground rod is supplied for this purpose. The transmitter is switched on and set to the 8kFF mode. This is a complex signal, but the main frequency is 8 Hz—which is used to detect the fault.
The low frequency component of the fault find signal (8 Hz) travels along the cable and leaks into the earth at the fault location. This sets up ground voltages around the defect in a similar way a garden watering hose would squirt water in all directions from a hole in the pipe.
The fault currents that cause the pooling of voltage at a fault return through the ground and complete the circuit back at the earth stake.
The VM-510FFL is designed to locate and measure the fault currents in the ground. To do this, the A-frame is switched on and the spike of the A-frame inserted into the ground over the cable. A dB reading indicates the level of fault current/voltage at that point. The spikes are placed in the ground at regular intervals along the cable. As the fault is approached, the signal level will increase as the ground currents increase as the fault is neared. The signal reaches a maximum just before and after the fault.
Phase sensitive measurements are made on the fault currents, which are interpreted as simple forward/back arrows so the user is steered to the fault. A left/right indicator is also shown, ensuring the operator keeps to the center line of the cable.
The VM-560FF kit, with the addition of the VM-510FFL stand-alone A-frame, is the ideal short drop and fault finder combination. UP
About the author: Steve Benzie is Technical Director for Vivax-Metrotech Corp. He trained as an engineer in the Ministry of Defense and achieved higher national certification in electrical and electronic engineering. He has more than 30 years’ experience in the locating buried utilities industry.