Cable fault location
Cable fault location is a multi-step process that must be performed as quickly and as safely as possible because customers will be without power.
Step 1: Fault identification; once a fault has been identified, the first step is to determine the phase in which the fault occurred and if it is of low or high resistance. Doing so determines the correct technique and equipment that should be used to diagnose the fault. A low voltage pulse (e.g., 40 V) is used via a TDR (time domain reflectometer). If the fault is a higher resistance one (> 100 Ohm), a low voltage pulse will likely not see it. For these types of faults, an insulation tester or a VLF test instrument will be necessary.
Step 2: Pre-location; a reliable and precise pre-location is necessary to locate a cable fault quickly and efficiently; without this, the general proximity of the fault may be unknown and somewhat elusive. A good pre-location can determine the fault position within a few percent of the cable length and will reduce the required time for pinpointing to a few minutes. If the fault is a low resistance one, pre-location was likely completed together with fault identification via the TDR in Step 1. For high resistance faults, ARM (arc reflection) or ICE (impulse current) methods with a SWG (surge wave generator), or the Decay method with an HV DC tester (bridge), are necessary for pre-location.
Step 3: Cable tracing; this test is done to determine the route of the cable at the site (see the EasyLoc/Ferrolux)
Step 4: Pinpointing; As an underground cable rarely runs in a straight line and rather tends to meander in depth and direction, an exact transposition of pre-location results to a site is not practical. Even with a potential pre-location accuracy of 0.1 % or better, the deviation in the field can be around 5 %. Exact localisation of the fault with pinpoint accuracy is essential for preventing unnecessary excavation work. In most cases, shock discharge generators are used for pinpointing; these create a loud flashover crack by means of a capacitive discharge at the location of the fault. The cable is pinpointed precisely using an acoustic pinpointing device, such as the digiPHONE+, which evaluates the time difference between the acoustic signal and the electromagnetic impulse of the shock discharge. When searching for the cable run in the pre-located fault area, the exact fault location is reached as soon as the shortest time difference is displayed.
Short circuits or sheath faults do not cause audible breakdowns and require different technology for their location. For short circuits, audio frequency systems can be used, whereas sheath faults are mostly pinpointed using a pulsed, DC voltage and a step voltage receiver such as the ESG NT.
Today, sheath testing is one of the most important tools for the preventative maintenance of supply networks. A damaged sheath is a considerable hazard because sooner or later it will result in a cable fault, which will usually cause interruptions to supply. In addition, sheath damage accelerates water penetration and cable aging, reducing the cable's useful life. Quick detection and rectification of sheath defects contributes considerably to reducing maintenance costs.
Step 5: Cable Identification; this test yields the phase identification once the cable is unearthed and can determine the faulty cable amongst several cables. On live systems, clear identification of a cable before it is cut or fitted is a task with absolute relevance to safety. The Megger CI transmitter sends a safe 50V pulse at 100A which is then analysed through a corresponding receiver. Any mistakes here can result in fatal consequences for the cable fitter and may cause outages for the connected customers.