In this test, the voltage is raised in equal steps and time is allowed between each step for leakage current to become stable.the current is relatively high as a voltage is applied owing to capacitance
charging current and dielectric absorption currents. As time passes, these transient currents become minimum with the steady-state current remaining, which is the actual leakage current and a very small amount of absorption current. At each step of voltage, the leakage current reading is taken before proceeding to the next step. Usually, it is recommended that at least eight equal steps of voltage be used and at least 1–4 min be allowed between each step. The leakage current versus voltage are then plotted as a curve. As long as this plotted curve is linear for each step, the insulation system is in good condition. At some value of step voltage, if the leakage current begins to increase noticeably, an increase in the slope of the curve will be noticed, as shown in Figure 1.1. If the test is continued beyond this test voltage, the leakage current will increase even more rapidly and immediate breakdown may occur in the cable insulation. Unless breakdown is desired, the test should be stopped as soon as the increase of slope is noticed in the voltage versus leakage current curve.
Maximum leakage current allowable for new cables acceptance can be determined from the ICEA formula for minimum allowable insulation
Figure 1.1 Step-voltage hi-pot test current.
resistance discussed earlier. The formula for leakage current then can be written as follows:
IL is the conduction or leakage current
E is the test voltage impressed
K is the specific insulation resistance megohms per 1000 ft at 60°F
D is the diameter over insulation
d is the diameter over conductor
The typical specific insulation resistance (K) for various commonly used insulations for cables are given under discussion of insulation resistance measurement test.
In order to explain the use of this formula, an example is given below for determining the maximum leakage current allowable for a 15 kV, 500 kcmil cable for an acceptance test.
A 15 kV cable 500 MCM 220 Mil XLPE insulation conductor OD = 0.813 Class B strand. The circuit is 2500 ft long. Calculate the maximum leakage current at maximum test voltage of 65 kV.