Buried cables sometimes fail in many different ways for various reasons. Lightning strike, overload or surge, installation problems, shovel and rodent damage are some common causes of cable failures. Over time, any discontinuity in the cable sheath will cause moisture to corrode the conductor. Cable grounding and/or another conductor in the cable is open, short-circuited, or there is a fault between them. The type of fault should be determined, because different faults require different methods. Usually, the cable fault (grounding fault) can be found most accurately and easily by using the A-type rack. It is best for TDR to find open circuit and short circuit. The "flicker" fault that only occurs at high voltage usually requires a high voltage surge generator or a "thump". Most transmitters in fault finding systems will indicate whether there is a grounding path in some way, such as ammeter or ohmmeter, while some have both. If there is a path to the earth, then when the conductor is not enclosed in the pipeline, A-frame is still one of the most commonly used methods.
We can detect persistent soil faults based on the basic location method. We show that the cable location is realized by generating alternating current (AC) on the cable and tracking the generated alternating electromagnetic field (EM) with a tuned receiver. The difference of the rack system is that in addition to the positioning signal, we also add a pulsed DC current (for grounding fault location) to the cable under test (cutting). Using direct current can detect the direction of current, which will lead to user failure. Where it is in contact with the earth, the current will flow out of the notch at the fault point and flow back to the grounding pile of the transmitter. The current will be concentrated near the fault and grounding pile, but from these points, it will flow very wide and deep to find the path with the least resistance. The current through the resistor produces a voltage. The pulse DC current flowing through the earth impedance will produce a slight DC voltage, which is how we find the fault. Adding an A-shaped frame to the positioner can basically turn the positioner into a very sensitive voltmeter, and follow the pulse DC of the A-shaped frame to give the direction and magnitude of the fault passing through the earth.
Because we follow the path through the earth, the cable in the conduit will cause us trouble. Even if we find that the signal flow is faulty, it does not necessarily lead to our fault. If the cable problem happens to be at the pipeline damage point, there may be enough current flowing through the ground from this position. Although there is probably moisture in the pipeline, we also have some current flowing through this path, which limits the signal that causes the fault. If the cable problem is completely in a good conduit (for example, what happens when the sheath is peeled off when it is pulled into the conduit), all our current can flow into the conduit until it reaches the grounding path (for example, when there is a fault), and we can't find the fault.
Setting up and using A-frame system correctly requires us to prevent the current at the fault location from flowing to any place other than the fault. This can ensure the maximum number of signals to follow and can guide us to the right position. Therefore, all other grounding paths must be disconnected, including neutral and ground wires. This may require completely disabling the power cable before testing the fault.
When faults are found, it is more important to use good grounding than to use conventional cables for positioning. There is a big clip at the end of the extra spool, which is a valuable tool and can use long-distance independent grounding piles. Stop signs, insulated anchors and existing but isolated piles usually improve the performance of positioning equipment. The actual limit of fault detection depends on the ground conditions, which is about 0.5-2 MW DC resistance. As the A-frame is measuring voltage, it needs to make electrical contact with the ground below it. Concrete, asphalt, dry or sandy soil are all high-resistance grounding paths, which will limit the voltage we detect through the ground. Sometimes, wetting the ground or even the sidewalk in the target conductor path will help fault detection. Even better, after determining that this is a ground fault and the resistance is low enough to detect its position, we can start tracking the cable with A-frame. Most A-frame systems have two signs: signal strength and fault direction. Signal strength is best expressed by logarithmic value, because a wide range of signals can be detected. Near the grounding pile where the current is concentrated, the receiver should indicate that the fault amplitude is large. It is best to write this value as a "reference number". The fault location should also indicate the detected current value.
Between the fault and the transmitter, the amplitude usually drops sharply, because the current has spread to a certain extent, so it is "diluted" The behavior of the arrow may also change. Near the fault and grounding pile, it will show a good hard "locking" fault direction, but in the middle of cable span, weak signal will lead to fluctuation or no direction indication. The correct technology is to continue to cross the weak area. When we approach the fault, the signal will be enhanced and the normal operation will resume.
A fault will be detected when two situations occur. When we approach the fault, the signal strength increases due to the increase of current concentration; The arrow suddenly reversed when we crossed the fault. Rotating the A-frame 90 degrees and passing through the cable path will improve the accuracy of fault location. The final test of fault location is to execute the "pothole" or circle of fault. Leave the front leg of the A-frame on the ground and put it directly at the fault position, then circle this point and put the rear leg on several points on the ground. If all signal indications point to the fixed leg, the optimal position is indicated. If the number in the receiver at this location does not reflect the reference number you see on the transmitter ground post.
If possible, remove the fault, repair the fault or at least make its contact with the ground disappear, and then retest the cable with the cable fault tester. Remove all contact points, including the outside of the dry cable, to prevent current from passing through moisture. If the cable is tested well now, we can be sure that there are no other faults on the cable.
The concentric neutral point without sheath may cause problems, because our fault location current sometimes finds that the neutral point is easier to return to the grounding pile area than the ground. This influence can be minimized by keeping the grounding pile as far away from the cable as possible and using a good (low resistance) grounding.
A-frame is the most accurate tool, but it is not necessarily the fastest tool, because the operator must move the cable from the transmitter to the grounding point. Usually, the grounding fault will make water penetrate into the water, and the conductor will also corrode the open circuit or short circuit. It is usually the most effective use of technicians' time to estimate the approximate location of the fault with a time domain reflectometer and then find the exact point through a frame.
Defendant: Huatian Electric Power