The Importance of Shielded Power Cable Testing

Your electrical cable system provides critical links throughout your operation, and cable faults can cost your operation in repair costs and downtime. Proper shielded cable assembly and power cable testing are crucial for a safe, reliable power system. New shielded cables must demonstrate that they are reliable and will hold up during use. Cables in service must be tested to gauge their remaining useful life while also ensuring ongoing reliability. A NETA certified technician will deploy the proper power cable testing methods and have the skill to identify corrective action quickly and efficiently.

Power Cable Testing Methods

Proving that new shielded cables are ready for operation and gauging the remaining life of an older cable in service is an important step. The current standards in place by ICEA, IEC, and IEEE state that testing can be performed using direct current, power frequency alternating current, or very low frequency alternating current. Power cable testing standards are currently moving toward using very low frequency (VLF) testing as it places less stress on your cables. 

When using a DC high potential test set, added stress and capacitance on the cable will shorten the cable’s lifespan as the test is running at a constant positive voltage. A VLF test set, however, runs at 0.1 hertz, a voltage of far lower frequency. This places much less stress and capacitance on the cable while still proving the cable can withstand a constant voltage.

Two tests are generally recommended for power cable testing for new installations (acceptance testing) and service aged cables (maintenance testing). The first is insulation resistance testing using a Megger IR test set, and the second is withstand testing using a Megger VLF test set. You can find power cable testing standards for all tests on IEEE 400.2.

Insulation Resistance Test

The purpose of the insulation resistance test is to see the health of the insulation and identify trends in power cables. It will also check whether the insulation has been contaminated by moisture, dirt, or carbonization.

The test is performed with a Megger IR test set and will immediately let you know whether the cable is suitable by holding the full voltage applied by the test set. Should the cable not hold the full voltage, it is faulted and will require further investigation. Testing at regular intervals is also recommended so trends can be evaluated for deterioration before resistance values drop below the minimum acceptable limit.

VLF Withstand Test

The purpose of the VLF withstand high-potential test is to ensure the shielded cable can hold AC voltage at or beyond it’s rated capacity. The test is performed using a Megger VLF test set and will also demonstrate whether the insulation, semiconductor, and conductor have any breakdowns between them.

During the power cable testing procedure, if there is even a tiny nick in the insulation or if the cutback is not at the proper length, the test will create a fault in that area. The withstand voltage applied by this test is typically much greater than the operating voltage, and the cable must withstand that voltage for a specified period without a breakdown to pass the test.

Partial Discharge & Tan Delta Testing

Tan Delta testing is used to provide an assessment of the overall condition of a shielded cable and its insulation and is frequently performed along with insulation resistance and withstand testing during both acceptance and maintenance testing. It can be used for both new and in-service cables, but is most typically employed for aged cables during maintenance testing. Because highly degraded cables can be detected at voltages at or below the normal operating voltage, test failures can be avoided.

Partial discharge testing is used to detect the location of a partial discharge in the insulation, termination, or splices of a shielded cable and is also performed in conjunction with insulation resistance and withstand testing. This test is critical for new cables during acceptance testing as it will find manufacturing or installation errors so they can be corrected before energization. It can also be useful during maintenance testing so defects can be detected in a controlled environment rather than during an operation failure.

Proper Cable Assembly

Proper shielded cable assembly is critical for ensuring your system will start up and stay running. Assembly is completed prior to powering up equipment for the first time or while installing or splicing a new cable on existing equipment. Cable manufacturers will provide guidelines for proper cutbacks on the jacket, concentric neutral, insulation, semiconductor, and conductor, and it’s imperative that these guidelines are followed.

Cable assembly requires skill and experience and should be performed carefully. If any cutback is shorter than the manufacturer’s recommendations for the voltage you are applying, or if you cut too deeply into any of the layers, the cable can fail, or added voltage stress over time can lead to sudden downtime. All cable preparations should have a quality check performed before taping over or adding insulated wrap, as 42 percent of cable failures can be attributed to improper termination and splicing.

Cable Fault Location and Tracing

Whether a fault is detected during testing or your equipment begins to fail during operation due to cable issues, locating and tracing the fault on shielded cables is the next step. Finding cable faults is a tedious, but informative task. The process starts by identifying the faulting cable using a Megger IR test set or high potential testing. Then, time-domain reflectometry (TDR) or the arc reflection method (ARM) is used to pre-locate the problem.

ARM is typically used in conjunction with thumper testing, which puts high-voltage, high-energy pulses into the cable while operating an attached radar set. This type of testing is necessary as most cables are located underground. The radar allows the technician to see the full picture of the cable and will show where the fault is located on the cable. In most cases, the fault can be pinpointed with an acoustical device, but in more complicated cases, advanced location techniques will be necessary.

Expert Power Cable Testing

The InterNational Electrical Testing Association (NETA) has developed training and certification protocols for technicians who evaluate equipment and systems to ensure they will carry out testing safely and effectively. The NETA certified technicians at Quad Plus have the training and experience to perform the correct tests using proper power cable testing equipment, and interpret the data collected and provide actionable recommendations. We can locate, isolate, and trace a cable fault on shielded cables using multiple testing types during both acceptance and maintenance testing to keep your facility operational and avoid costly downtime.

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