Long power cables and large machines require different application techniques from most electrical wiring. The first topic discussed is a new technique for evaluation of insulation for continued use. The second area is a different method for selection of conductor size considering temperature and voltage drop. The hi-pot evaluation is primarily a maintenance function, while the voltage drop procedure is primarily a design practice.
For the first topic, high potential (hi-pot) tests are often conducted on power cables and large machines. However, there has been no standardized method of determining if the insulation is acceptable. Various methods have tried to compare leakage current between conductors, use a fixed limit on leakage current, or compare with previous tests. Each has its advantages and limitations.
From empirical data and diverse experience, we have developed a numeric technique that will indicate impending failure. The procedure is mathematically rigorous but can be practically applied. It is very applicable to computer controlled hi-pot systems as well as manual systems.
The second topic is the voltage drop for long conductors. We have applied numerous considerations to develop a simple relationship that can be readily applied. It incorporates wire diameter, length, current, number of phases, and temperature correction, with permissible voltage drop.
A variety of test devices and procedures are used in an effort to determine the quality of insulation. A number of test methods and devices use dc voltages. Nevertheless, there are extensive data to indicate limitations of dc. [1,2].
Despite all the limitations, if field tests are performed, dc testing is still the method of choice. Although other methods show promise, at this time their limitations exceed their perceived advantages .
The high potential dc tester is a machine which, given the present state of the art, provides the most information about insulation quality . Field machines typically can apply up to 60,000 volts to energize the wire. Some machines, such as at our research facility, are rated up to 200,000 volts or more.
Elevated voltage can be used to cause virtually any insulation system to fail at its weakest point. However, it is very difficult to interpret the readings so the quality can be determined without taking the insulation to destruction. The most valuable information is comparison between historical data from previous evaluations and data from present evaluations. Experience, skill, and knowledge of local conditions taken in conjunction with test results are major aids in analyzing the suitability of equipment for reuse .
It is apparent that the test voltage 'V' plays a key role in the relationship. For wire insulation, the resistance varies with the length. As the length increases, the megohm value of the insulation decreases. This is a non-linear change. The insulation behaves as a string of parallel resistances. For a fixed test voltage, the leakage current must increase exponentially as the length increases.
Length has to be incorporated in the Ohm's law relationship. If the resistance is multiplied by length, the appropriate units of resistivity are megohm-thousand feet (MOhm-kft). The reciprocal is called conductivity and has units of micromhos per thousand feet (umho / kft).
Alternatively, the conductivity can be expressed in units of microamps per volt - thousand feet (uA/V-kft). This term is often called the "leakage current" although technically it includes other terms. It can only be considered to be leakage current when the value includes the test voltage.