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Sheath testing is an important test that is performed before commissioning. The test inspects the insulation ability of the outer sheath against the ground, ensures that the cable is not damaged during transportation or installation and that all joints are sealed. If there is a sheath fault, it will be located and repaired in conjunction with testing and troubleshooting.
The utilities have increased the requirements to ensure quality of cable systems. One way to ensure the installation is to perform a PD mesuring. PD measuring is a method for detecting partial discharges. PD are electrical discharges that occur in tears caused by incorrect installation, dirt, etc.
It is important to prepare and install a cable, joints and termination correctly. To ensure that the cable has no mechanical damage, the cable must be sheath tested before the PD measuring is started.
VLF test (Very Low Frequency)
The VLF test is an isolation test for PEX-insulated cable using very low frequency AC voltage. The method is not destructive for good isolation but is destructive for bad isolation. This means that weakened isolation should break down during the test in a controlled manner and not in service with power failure as a consequence. The test replaces isolation testing with DC voltage. It does not give a measurement result, it will only show whether the cable is OK or not.
Tangens Delta Measurement (Tan δ)
The Tangens Delta measurement is used to control the presence of water trees (treeing) in PEX-insulated cable. Treeing is weakening of the insulation that eventually leads to a cable fault. A Tangent Delta measurement assesses the status of the insulation and identifies affected cables, which can form the basis for future action.
Earth connection test
The test involves inspection of the cable shield and longitudinal earth conductor along with other external ground connections. The test is also called a Ymer measurement or return current measurement. For new installations, it is primarily a method that ensures the condition of the longitudinal earth conductor. With older cables, moisture beneath the outer sheath can cause corrosion or even interruption of the cable screen, which affects the ability of the cable to handle earth fault currents.
Nexans can measure, assess and provide action plans for cables that fail the tests above.
After all testing, protocols are sent to the network owner or the subcontractor.
Cable fault location
With a portable and powerful cable fault location system (surge generator and TDR), Nexans is well prepared for locating most types of cable fault in the ground. Our sheath fault location equipment can also be very effective in locating other types of cable fault.
Mounting cable accessories
Nexans offers joint assembly and termination of medium and high voltage cables, both after installation or for delivery on specially adapted drums.
Condition check for older cables in the 12-24 kV networks.
Cables to focus on:
- Cables from the 70s (outer conductive layer of graphite and conductive bands)
- Cable sections that don't pass the earth connection test
- Cables with PVC sheath (AXKJ)
- Plowed cables where sheath testing has not been performed
- Corroded screen
Detecting the occurrence of punctual corrosion, (metallic connection is missing)
- The Delta*** measurement
Detects the prescence of treeing
Measurements usually result in three action proposals:
- Cable OK = No action
- Partially aged cable = further investigation or renewed testing in 3 to 8 years
- Cable has bad values = plan for exchange
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