Pre-location
Time Domain Reflectometry (TDR)
The TDR method is the most established and widely used measuring method for establishing the total length of a cable and the distance of low resistive faults, cable interruptions, and the location of joints along a cable. When a low voltage pulse is sent into a cable that has a parallel path of two conductors, reflections will be seen at points along the cable that exhibit different impedances.

Secondary Impulse Method / Multiple Impulse Method (SIM/MIM)
SIM/MIM is also known as surge arc reflection and is based upon a surge generator or thumper being coupled together with a TDR. A high voltage impulse is sent down the cable causing the fault to break down and temporarily transforms a high resistive fault into a low resistive fault, which can be detected by a TDR signal to measure the fault distance. Fault distance evaluation is conducted fully automatically.

Impulse Current Method (ICM)
ICM is the conventional location method for high resistive cable faults in very long cables. A surge generator/thumper is coupled to a TDR via an inductive coupler. The breakdown in the fault generates a current impulse, which travels along the cable sheath between the surge generator/thumper and the cable fault causing reflections that are detected by the TDR.

Cable Sheath Fault Location (Bridge Measurement)
When a fault occurs between two defined cores or parallel wires the theory of time domain reflectometry (TDR) can be applied, as is the case for all the pre-location methods described above. However, certain cable structures can allow faults to happen from a core to the outside ground or soil, such as in unshielded cables or when there is damage to the outer sheath of a cable. Cable sheath faults will not immediately cause cable failures, but over time the performance of the cable can be compromised because water can penetrate into the cable causing corrosion and the growth of possible water trees. For these types of faults measuring bridge techniques must be employed.

Decay Method
In certain cables the breakdown voltage of the fault may be higher than the rated output of the surge generato. In this case a VLF or DC source with higher voltage output needs to be used as the high voltage source. The decay method is based on voltage decoupling by a capacitive voltage divider. The faulted cable is charged by applying high voltage VLF or DC up to the breakdown voltage. Compared to the ICM method above, the decay method is based on a transient voltage wave that is continuously recorded by the capacitive coupler.

Differential Impulse Current Method / Differential Decay Method
The differential ICM or decay methods can be used for cable faults that are very difficult to locate, such as in very long cables, in a T-branch network, or in overhead transmission lines. For this method, two cables are required for the pre-location process – the faulted cable and a healthy auxiliary cable.
In a first step, the HV impulse is released into the healthy cable and faulted cable simultaneously, giving a first differential picture. Second, a linking bride is connected at the far end of the two cables.

Pin-point Fault Location
Cable Tracing
A successful cable fault location relies on knowing the position of the cable and other lines laid in the soil. If the exact route of the underground cable is not known magnetic frequency procedures can be used to establish the position and depth of the cable using the minimum or maximum method. An audio frequency generator can be connected to a healthy phase of the faulted cable via galvanic connection, inductive connection with a clip-on CT clamp, or inductive connection with a frame antenna. Galvanic connection is considered the best method because the best signal values can be obtained.

Acoustic Fault Pin-Pointing
The acoustic fault location method is used for pin-pointing of high resistive or intermittent faults in buried cables in which the cable is “thumped”, i.e., a series of high voltage surge pulses are sent down the cable causing the fault is break down. During a flashover an audible acoustic signal is generated that can be detected on the ground surface by using a ground microphone, receiver, and headphone. The closer the distance to the fault, the higher the amplitude of the flashover sound.

Step Voltage Fault Pin-Pointing
Cable sheath faults or short circuit faults to ground do not allow for a flashover when thumping a cable, and therefore, acoustic fault pin-pointing cannot be applied. In this case, a sequence of voltage impulses (step voltages) is sent into the faulted cable, which produces a voltage drop to ground. The voltage drop results in a voltage gradient, which can be measured with the use of two earth probes above ground. When walking towards the fault, an increasing voltage should be detected and immediately over the fault a detectable polarity change will be measured and the resulting voltage will be zero when the earth probes are placed symmetrically above the fault.

Twist Field Location
Despite reverse current directions, a magnetic field is produced and using a detecting rod the maximum and minimum of the signal can be detected due to the twist or steady change of geometrical position of the cores in the cable. Since the audio signal returns at the fault position, the position at which no signal can be detected can be determined as the cable fault. The twist field method can also be used to detect cable joints, as the twisted field is interrupted according to the length of the joint.

Cable Identification
If a violent cable fault has occurred and is visible, then it is fairly easy to distinguish which cable needs to be repaired. However, in other circumstances, especially when multiple cables are bundled together, the correct cable has to be identified first to reduce the chances of cutting a section of healthy cable that does not require any repair.
Cable identification is conducted by connecting a transmitter onto the suspected faulted cable, either galvanically or inductively. The transmitter contains a capacitor that is charged and then discharged into the cable. A flexible coupler (Rogowski coil) is then used to measure the current pulse in the target cable.