napster.zatac

 Profile Answers by napster.zatac 

• Jan 23rd, 2010

 

Sectionalizing: it is risky and reduces cable reliability, because it depends on physically cutting and splicing the cable. Dividing the cable into successively smaller sections will enable you to narrow down the search for a fault.

For example, on a 500-ft length, you would cut the cable into two 250-ft sections and measure both ways with an ohmmeter or high-voltage insulation resistance (IR) tester. The defective section shows a lower IR than the good section. You would repeat this "divide and conquer" procedure until reaching a short enough section of cable to allow repair of the fault. This laborious procedure normally involves repeated cable excavation.

Thumping: When you supply a high voltage to a faulted cable, the resulting high-current arc makes a noise loud enough for you to hear above ground. While this method eliminates the sectionalizing method's cutting and splicing, it has its own drawback. Thumping requires a current on the order of tens of thousands of amps at voltages as high as 25kV to make an underground noise loud enough for you to hear above ground.

The heating from this high current often causes some degradation of the cable insulation. If you're proficient in the thumping method, you can limit damage by reducing the power sent through the cable to the minimum required to conduct the test. While moderate testing may produce no noticeable effects, sustained or frequent testing can cause the cable insulation to degrade to an unacceptable condition.