Cable testing is conducted to chart the gradual deterioration over the years, to do acceptance testing after installation, for verification of splices and joints, and for special repair testing.
It is always appropriate to conduct the cable insulation resistance measurement test first, and if the data obtained looks good, then proceed with the DC overpotential test.
After the DC overpotential test is completed, then perform the insulation resistance test again to assure that the cable has not been damaged during the DC overpotential test.
Cable Insulation Resistance Test
The insulation resistance is measured using a Megohmmeter. This is a nondestructive method of determining the condition of the cable insulation to check contamination due to moisture, dirt, or carbonization.
The insulation resistance measurement method does not give the measure of the total dielectric strength of cable insulation or weak spots in the cable. Generally, the following voltages can be used for the indicated cable voltage rating
The following is the general procedure when using a megohmmeter for resistance measurement tests.
- Disconnect the cable to be tested from other equipment and circuits to ensure that it is not energized.
- Discharge all stored capacitance in the cable by grounding it before testing, as well as after completing tests. To discharge a cable, Connect a ground to the cable and leave it connected for at least four times the length of the test time. Do not touch the cable until it is fully discharged.
- Connect the line terminal of the instrument to the conductor to be tested.
- Ground all other conductors together to sheath and to ground. Connect these to the earth terminal of the test set.
- Similarly measure other insulation resistance values between one conductor and all other conductors connected, one conductor to the ground, and so on. The connections are shown in Figure given below.
- The guard terminal of the megohmmeter can be used to eliminate the effects of surface leakage across exposed insulation at the test end of the cable or both ends of the cable for leakage to the ground. The guard terminal is provided to bypass the current due to corona and surface leakage around the megohmmeter so that corona and surface leakage currents are not included in the test readings.
The insulation resistance measurements should be conducted at regular intervals and records kept for comparison purposes. Keep in mind that, for a valid comparison, the readings must be corrected to a base temperature, such as 20°C. A continued downward trend is an indication of insulation deterioration even though the resistance values measured are above the minimum acceptable limit.
Cable and conductor installations present a wide variety of conditions from the point of view of the resistance of the insulation. These conditions result from the many kinds of insulating materials used, the voltage rating or insulation thickness, and the length of the circuit involved in the measurement.
Furthermore, such circuits usually extend over great distances and may be subjected to wide variations in temperature, which will affect the insulation resistance values obtained. The terminals of cables and conductors will also affect the test values unless they are clean and dry, or guarded.
The Insulated Cable Engineers Association (ICEA) gives minimum values of insulation resistance in its specifications for various types of cables and conductors. These minimum values are for new, single-conductor wire and cable after being subjected to an AC high voltage test and based on a DC test potential of 500 V applied for 1 min at a temperature of 60°F. These standard minimum insulation resistance (IR) values (for single conductor cable) are based on the following formula:
IR = K log10(D/d)
Where, IR is in meg-ohms per 1000 ft of cable, K is a constant for insulating material, D is the outside diameter of conductor insulation and d is the inside diameter of conductor.
DC Overpotential Testing of Cable
In the past, this test has been extensively used for the acceptance and maintenance of cables. Recent studies of cable failures indicate that the DC overpotential test may be causing more damage to some cable insulation, such as cross-link polyethylene, than the benefit obtained from such testing.
It can indicate the relative condition of the insulation at voltages above or near operating levels. This test can be used for the identification of weakness in the cable insulation and can also be used to break down an incipient fault. Generally, it is not recommended that this test be used for the breakdown of incipient faults even though some test engineers use it for this purpose.
Therefore, the incipient fault breakdown probability should be anticipated before and during the hi-pot test. The impending cable failure will usually be indicated by sudden changes in the leakage current, and before insulation is damaged, the test can be stopped.
The test voltage values for DC hi-pot tests are based upon the final factory test voltage, which is determined by the type and thickness of insulation, the size of conductors, the construction of the cable, and applicable industry standards.
The DC test values corresponding to AC factory proof test voltages specified by the industry standards are usually expressed in terms of the ratio of DC to AC voltage for each insulation system.
This ratio is designated as K, which when multiplied by the acceptance test factor of 80% and maintenance factor of 60% yields the conversion factors to obtain the DC test voltages for hi-pot tests. These recommended test voltage conversion factors are shown in following Table .
Also, the IEEE standard 400.1–2007 lists the voltage values for conducting hi-pot acceptance and maintenance tests in the field for laminated shielded power cables, which are shown in Table 2.5.
* Maintained for a duration of 15 min.
Note: Voltages higher those listed, up to 80% of system BIL for installation and maintenance testing may be considered in consultation with the suppliers of cable and the accessories. When equipment, such as transformers, motors, etc., is connected to the cable circuit undergoing a test, voltages lower than recommended values may be used to comply with the limitations imposed by the connected equipment.
Many factors should be considered in selecting the right voltage for existing cables that are in service. As a general rule, for existing cables, the highest values for maintenance should not exceed 60% of the final factory test voltage, and the minimum test value should be not less than the DC equivalent of the AC operating voltage. The hi-pot test can be conducted as a step-voltage test as discussed next.
Voltage versus Leakage Current Test (Step-Voltage Test)
In this test, the voltage is raised in equal steps, and time is allowed between each step for the 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 is 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 given below. The test should be stopped as soon as the increase of slope is noticed. 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.
Maximum leakage current allowable for new cables acceptance can be determined from the ICEA formula for minimum allowable insulation resistance discussed earlier. The formula for leakage current then can be written as follows:
Where, IL is the conduction or leakage current, E is the test voltage impressed, K is the specific insulation resistance meg-ohms per 1000 ft at 60°F, D is the diameter over insulation and d is the diameter over conductor.
Leakage Current versus Time Test
When the final test voltage of leakage current versus voltage test is reached, it can be left on for at least 5 min, and the leakage current versus time can be plotted for fixed intervals of time as the leakage current during this step reduces from an initial high value to a steady-state value.
A curve for good cables will generally indicate a continuous decrease in leakage current with respect to time or steady-state value without any increase of current during the test. This curve is shown in following Figure.
Go, No-Go Overpotential Test
The hi-pot test can be conducted as a go, no-go overpotential test. In this test, the voltage is gradually applied to the specified value. The rate of rising of the test voltage is maintained to provide a steady leakage current until the final test voltage is reached.
Usually, 1–1.5 min is considered sufficient for reaching the final test voltage. The final test voltage can then be held for 5 min, and if there is no abrupt increase in current sufficient to trip the test set, the test has been successfully passed.
This test does not provide a thorough analysis of cable condition but provides sufficient information as to whether the cable meets a specific high-voltage breakdown strength requirement.
This type of test is usually performed after installation and repair, where only cable that can withstand strength verification without a breakdown is to be certified.
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