Cable Termination and Joint Testing After Field Assembly
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A factory-made cable length arrives with its insulation quality established. The joint that connects it to the next length is made in a trench, and the insulation system around that joint is created on site. Cable joint termination testing exists because the assembly is part of the insulation, and its quality is determined by conditions a factory controls and a trench does not.
The test scope after jointing is therefore not simply a check on the cable. It is a verification of the completed circuit, in which the weakest element may be an assembly made hours earlier.
What changes when a joint is assembled on site
The cable ends prepared for the joint are stripped back, cleaned and built up. The interface between the cable insulation and the joint body is created in the field, and that interface determines the dielectric performance of the assembly.
The variables a factory controls are present in the field in a weaker form. Cleanliness is harder to maintain in a trench, humidity varies with the weather, temperature affects the materials, and the personnel have varying experience with the particular joint type.
The result is that the joint condition is not fully determined by the cable type and the joint design. Two identical joints installed on the same day can have different dielectric behaviour, and the difference is in the assembly rather than in the components.
Testing scope after termination and jointing
| Test | What it establishes after jointing | Practical point |
|---|---|---|
| Insulation resistance | No gross conduction path through the assembly | Fast screening; insensitive to localised defects |
| Withstand test | The assembly survives the test voltage for the specified duration | Method chosen for the insulation type; acceptance values after a repair |
| Partial discharge measurement | Whether an active site exists at the test stress | Most informative when taken during the withstand test |
| Sheath and bonding checks | Continuity and condition of the metallic protection and its bonding | Often omitted, and a common source of early deterioration |
| Dielectric loss where applicable | Loss behaviour of the completed insulation | Depends on the cable type and the arrangement available |
The withstand test is normally applied to the whole circuit rather than to the joint alone, because the test voltage is applied from the ends. That is also the more useful test, because it re-proves the cable ends disturbed during assembly.
Cleanliness and workmanship as test variables
Cleanliness is a dielectric parameter. Contamination at the interface between the cable insulation and the joint body creates a site where the field concentrates, and the defect it produces may not appear at the test voltage at all.
The same applies to moisture. A joint assembled in high humidity traps water in the interface, and the trapped moisture changes the dielectric behaviour over time as it migrates. A joint assembled in the rain is a joint whose test result should be interpreted with that condition in mind.
Workmanship is harder to quantify but equally relevant. The preparation of the cable end, the application of the stress control elements and the tightening of mechanical components all have correct procedures, and the assembly record is the evidence that they were followed.
Recording those conditions is not administrative. Where a joint fails in its first years, the assembly record is the first thing examined, and a record that does not state the conditions cannot support the investigation in either direction.
Partial discharge after jointing
A joint defect that is dormant at the test voltage can become active under service stress. The discharge measurement taken during the withstand test provides evidence about whether such a site exists, and it addresses the risk the joint introduces more directly than the withstand result does.
The measurement has to be configured for the test frequency being used, because discharge behaviour at a very low frequency differs from that at power frequency. The detection threshold achieved in the field environment has to be established, and it belongs in the record because it determines what the measurement can and cannot find.
Where the circuit is critical or has a history of joint failures, the discharge measurement is worth taking even where it is not required. It costs little extra time in a test already being performed, and it is the only evidence about whether the assembly contains an active site rather than merely surviving the test. The measurement method is defined in IEC 60270.
Interface with the acceptance programme
The joint test sits inside the circuit acceptance programme rather than alongside it. The circuit is tested once it is complete, and the joint is part of what is being tested.
That means the joint schedule has to be complete before the test runs, because an unfinished joint leaves the circuit untestable. It also means the assembly records have to be available at the time of the test, so that any anomaly can be considered against the conditions under which the joint was made.
The acceptance criteria applied after a repair are normally the acceptance values for the cable class rather than the moderated values sometimes used for aged circuits. Where the operator chooses otherwise, the decision and its justification belong in the test plan and in the record. The field test requirements are set out in IEC 60502-2 for medium voltage extruded cables, in IEC 60840 for the higher voltage classes and in IEC 62067 for the highest extruded classes, within the general field test framework of IEC 60060-3.
Repeat failures at joints: causes and evidence
A repeat failure at a joint should be investigated as a pattern rather than as a coincidental event. Two failures of the same joint type, installed by the same team, in the same period, point at a systemic cause.
The evidence to compare is the assembly record for both, the fault location relative to the joint, the condition found when the joint was opened, and any discharge or gas evidence from the period before the failure. The comparison usually distinguishes between an installation issue and a design or material issue.
Where the cause is installation, the response is procedural: revised work instructions, additional training or a change in the conditions under which the work is permitted. Where it is design or material, the response is technical and may affect other joints of the same type elsewhere on the network.
Condition monitoring after energisation
A joint that has been tested and energised is not monitored again until the next scheduled test unless something draws attention to it. For critical circuits, partial discharge monitoring or periodic measurement provides the evidence between tests.
The value of monitoring depends on having a baseline. The discharge measurement taken during the commissioning or repair test is that baseline, which is the strongest argument for taking it even where it is not required.
Where monitoring is installed, its alarm threshold should be set so that it triggers an investigation rather than an immediate shutdown, and the response to an alarm should be defined in advance. The trade-offs between monitoring and periodic measurement are addressed in the accompanying article on online partial discharge monitoring and periodic measurement.
Documents to close out
The close-out should contain the joint schedule with positions and types, the assembly records with their conditions, the test results with the method and acceptance criteria applied, and the diagnostic results where measured.
It should also record the state in which the circuit was left: the bonding and earthing restored, any temporary arrangements removed and the route record updated with the new joint position.
Construction and installation documentation is published by the cable manufacturers such as Nexans, and field experience with joint failures and their diagnosis is coordinated through CIGRE study committees. The cable test and location range is grouped on the cable fault testing hub, with the withstand test systems on the high-voltage insulation withstand test hub.
A joint is an insulation assembly made in field conditions, and the assembly record is part of its test evidence.
Send your joint schedule and assembly procedure to our engineering team and we will mark the conditions and records the close-out needs. Cable test and withstand test systems are grouped on the cable fault testing and high-voltage insulation withstand test hubs.
FAQ
What changes when a joint is assembled on site?
The insulation system around the joint is built in field conditions rather than in a factory, and its dielectric behaviour depends on the cleanliness of the work, the moisture present during assembly and the skill applied. The cable ends prepared for the joint are also part of the new insulation assembly, so the joint and the cable ends have to be tested together.
What test scope applies after jointing?
A withstand test applied to the completed circuit using the method appropriate to the insulation type, plus a diagnostic measurement where the circuit is critical or has a history of joint failures. The withstand test establishes that the insulation survives the applied stress; the diagnostic measurement indicates whether there is an active site at that stress level.
Why does partial discharge measurement matter at a joint?
Because a joint defect that is dormant at the test voltage may become active under service stress and thermal cycling. The discharge measurement taken during the withstand test provides evidence about whether the joint contains a site that is active at that stress, which the pass-or-fail outcome alone does not.
What in the assembly conditions is worth recording?
The ambient temperature and humidity, the cleanliness of the prepared surfaces, the time the cable ends were exposed, the joint type and the personnel. Those conditions determine the quality of the assembly, and they are also the explanation for a joint that behaves unexpectedly in its first years.
How should a repeat failure at a joint be investigated?
By establishing whether the failures share a cause before treating them as separate. Two failures at joints of the same type installed by the same team point at a systemic issue rather than at two coincidences. The investigation should compare the assembly records, the fault locations and the discharge evidence from both events.
