MV Terminations | Incompetency & Substandard Jointing Plumbs Another New Depth
Published 11 Jul 2019
Contributed By: Watkins and Jacomb Construction Power & Consultancy
Uploaded By: Chris Dodds – Thorne & Derrick: Distributors of LV, MV & HV Cable Jointing, Cable Terminations, Substation & Electrical Eqpt
WJCPC are specialist LV HV Cable Termination & Jointing contractors with over 20 years experience in the High Voltage Electrical Industry covering the utilities, renewable energy, rail, data centre and general industry sectors – UKPN competent for LV & HV Termination and Jointing Of Cables, Confined Spaces, Substation Entry and Cable Fault Finding.
In the following post Ben Jacomb, Director of WJCPC, highlights several shortcomings and the dangerous consequences of clients failing to carefully vet and selectively employ Competent Cable Jointers.
WJCPC were asked to attend site to remove a total of 16 incorrectly specified and installed cable glands and install correct type brass glands. Due to the cable termination being a larger diameter to permit the existing and replacement cable glands to be changed WJCPC had to carefully remove the anti-track heat shrink (red insulation) from the heat shrink terminations.
After first inspection WJCPC were unsatisfied with the cable jointing technique undertaken to remove the semi conductive layer from the MV cable: this was done by using a semi-con stripping tool, the semi conductive layer is semi bonded so the stripping of the layer should be stripped by using a depth knife and the ringing of the stress control point by a soft file.
Any marks left in the XLPE insulation by the Jointer will influence the magnetic fields inside the MV cable and should be removed by emery cloth to leave a smooth finish eliminating any voids. If the heat shrink tube is shrunk on top of a void there will be moisture and where there is moisture there will be arcing, burning and then failure.
If the semi conductive layer is not removed correctly at the crucial point discharges occur and can damage the medium voltage cable and safe operation of the power network.
Below are WJCPC’s site observations.
Incorrect cable gland installed
The cable installed is a 11kV single core power cable with XLPE insulation and 120sqmm stranded copper conductors (16sqmm earth wires) – the original cable glands installed were 50mm plastic glands which are over-sized and incorrect for the cable diameter consequently failing to provide adequate mechanical retention.
Also, due to the cable being a backup supply from generators the vibrations will evidently loosen the cable gland until it is effectively and dangerously removed from the gland plate of the cable box which will then allow the cable to move around within the termination box – this could potentially loosen the cable termination connection.
This could do serious damage within the generator.
New 40mm brass cable glands were installed to the correct specification and that corrective action by WJCPC now has eliminated any vibration and loosening of the connection issues.
Ensure that when ordering jointing materials that manufacturers recommendations and specifications are met to eliminate incorrect installations.
Over-Crimped Cable Lugs
All 16 of the site 11kV cable terminations had been over compressed or “crimped” due to the Jointer using an incorrect crimping die set.
Over compression of the cable lugs caused a distinct gap between the cable lugs and the cable insulation.
|Incorrect Cable Lug Compression||Correct Cable Lug Compression|
- Choose a manufacturer providing crimp tooling die set and lugs matched to work together to give assured crimping performance.
- Each lug should be clearly marked with a reference on the barrel or palm which enables the user to verify that the lug selected is the correct size and type for the conductor. The manufacturer’s name or logo also allows the correct die set and crimp tool to be selected from their recommended list.
- Prepare the conductor by stripping the insulation back so that, when assembled, it cannot be trapped in the barrel of the lug. The strip length should be equal to that of the barrel. Take care to avoid damage to the conductor strands during the stripping operation. Exposed conductor strands should be cleaned to remove any particles of insulation.
- Fully insert the conductor into the barrel. If an inspection hole is provided, check the conductor is fully inserted.
- Crimp the lug in accordance with the manufacturer’s instructions. Pay careful attention to the positioning of the die on the barrel and, if multiple compressions are required, the sequence in which they are made.
- For further information visit Cembre | Cable Lugs & Crimping Tools.
Examine The Completed Cable Crimp Lug To Check
- Marking on cable lug shows it is correct for the conductor type and size.
- The marking on the cable lugs show the correct die set was used.
- No insulation is trapped in the cable lug barrel.
- Excess grease is removed from insulation and lug.
- If an inspection hole is provided, the conductor has been fully inserted
- It is NOT to be over-compressed causing excessive flash or burrs, which are detrimental to the performance of the joint.
It is NOT under-compressed, as this is detrimental to the performance of the joint.
If any doubt exists, samples should be produced for test purposes.
Poor Cable stripping
of semi conductive layer
The semi conductive layer of the MV cable has been stripped by a stripping tool and on one occasion the stripping tool has impeded past the manufacturer’s requirements for their insulation diameter which will cause a void within the cable termination – the void could cause a hotspot and potential flash over which will render the connection irreparable.
The semi conductive layer is semi bonded (easy peel) so the semi conductive layer should be removed using a specialist Jointers tool or depth knife – here the semi con layer has been removed from the cable using a bonded semi-con layer stripping tool and it has left a poor finish to the XLPE insulation.
Even if the semi con layer is bonded type then the cable jointer should use 3 stage emery cloth ( 80, 120, 240 grit) to sand out any ridges and voids and then clean down the cable with a non- conductive cleaning cable wipe.
The XLPE insulation finish on easy peel type cables should be a nice smooth finish without ridges, undulations or jagged surfaces.
The semi con round off point should be a perfect finish as this is the most crucial point of the cable termination – if this is not perfectly smooth and the void filler mastic is not installed properly this can affect the stress relief from electrical fields.
It is essential this is undertaken by the Jointer with due care and attention.
Below is a photo of potential damaged caused by above non-conformances.
Missing earth strands from cable
On 50% of the cables WJCPC found missing earth strands. This will degrade the earthing protection for that cable.
Poor outer sheath strip
The outer sheath on occasions was poorly removed and not stripped according to the manufacturers jointing instruction – this may affect the damp seal which is installed on the outer sheath and the cable terminations measurements may come out of sync and affect the electrical connections durability.
After full inspection WJCPC found that poor Jointing and sub-standard workmanship without quality control had introduced serious and potentially catastrophic operational issues into the medium voltage cable and power system network.
WJCPC recommend that a survey onsite is conducted to check more cable terminations as they suspect this will be a continuous issue onsite – using a thermal camera and partial discharge tester can help determine any cable terminations that need to be looked at and maybe pick some at random to strip down and visually inspect.
Due widespread concerns about industry workmanship, lack of Jointer training and declining standards, WJCPC can provide an audit service to visually audit jointers and ensure they are completing their termination to the manufacturers jointing instruction – this may not eradicate Jointer incompetency but it will contribute to a reduction in the volume of poorly installed cable joints.
There is also another process you can introduce and this is a step-by-step photo sequence of the termination installation at its crucial stages – this does not take long and captures the poorly installed cable joints or terminations before they are energised.
➡ Please Note: The cable glands WJCPC have changed and the re-termination of the cables will not be guaranteed by WJCPC due to the terminations not being installed to the manufacturers instructions. WJCPC have terminated them to their best ability but only as a temporary measure to allow the generators to be commissioned.
WJCPC strongly advise these cables are removed and new cable and new terminations installed immediately.
HV Cable Terminations
Thorne & Derrick distribute the most extensive range of HV Cable Terminations & Joints to suit 11kV/33kV medium and high voltage power cables, including indoor cold shrink and heat shrink terminations, outdoor pole-mounted terminations or separable connectors for gas insulated equipment applications.
THORNE & DERRICK are national distributors of Cable Installation, Jointing, Substation & Electrical Safety Equipment MV HV – we service UK and global businesses involved in cable installations, cable jointing, substation, overhead line and electrical construction at LV, 11kV, 33kV and EHV.
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