Dangers Of Substandard Heat Shrink Cable Terminations On High Voltage Systems
Published 02 Jan 2018
Uploaded By: Chris Dodds – T&D Distributors of LV, MV & HV Cable Terminations & Cable Joints
The following LinkedIn Comments provide some expert guidance, insight and advice on how to avoid the dangers of substandard installation of heat shrink cable terminations on high voltage systems. Generally, when categorising voltages we include 11kV and 33kV power systems into the high voltage (HV) definition.
However contractors, manufacturers and the industry would also argue that 11/33kV should be classified as medium voltage (MV) and HV commences at 66kV.
The Comments below were in response to the following site installation photograph of an 11kV cable termination. Let us know your thoughts and we will include to the thread in order to improve readers knowledge and reduce industry risk.
Scott Wilkie HV Power Distribution Engineer at Scottish Power – cable entry points through the cable gland plate would be better if centralised. Crossing cores with increased stress over insulation, clearly used to phase out the cable sections in the box. Would prefer to see 11kV single cores with a trifurcating joint outside of the cable box if that was possible.
♦ See: Cable Glands MV HV
Brendan Preece, P.E. HV Electrical Engineer – in addition to previous comment, use a high voltage two-hole compression lug that matches the two-hole terminal pad for good low-resistance metal-to-metal surface area contact.
Tony Haggis Director at Tony Haggis Consulting Ltd – the HV cable box is designed for single core cable terminations only. It’s impossible to use 3 core cable and maintain clearances between the cores – hence the black discharge marks. Not sure why anyone would use 3 core XLPE cable these days – single core triplex is cheaper and much easier to joint and terminate. Most UK DNOs and European utilities use it. There is no way to recover the heat shrink cable terminations other than cut all the cables out and replace with single cores with trifurcating joints outside. Also I’m amused by the use of the outdoor rainsheds – not only unnecessary but they also make the clearances worse.
Colin Woodman Team Leader & HV Cable Jointer DELPRO – I could be wrong but looking at it I am struggling to see where, other than down near the bottom of the heat shrink cable terminations, that the semi con screen had been cut and the mastic installed…cables must only cross where the semi con is still in tact. This is why HV cable terminations like this should be designed from the top down not from the cable strip up.
♦ See: Cable Jointing Tools
Tony Haggis Director at Tony Haggis Consulting Ltd – in view of Colin Woodman’s comment I’ve looked a bit closer at the picture and it could be salvaged by remaking the heat shrink termination using what we used to call a “crossed core” kit that we used to use with 11kV belted paper cable. This kit included heat shrink semicon heat shrink tubes that we used to screen the cores up to the stress control tube which was placed up near the cable lug.
Perhaps stripping off the existing cable termination and extending the core screen with semicon tubes could then place the stress control tubes up between the perspex phase barriers. I personally wouldn’t guarantee it as we don’t know what discharge damage has already been caused. If one was determined to use 3 core 11kV cable in this box then a top down screen off measurement would work but I would want the termination manufacturer’s view first. Looks like the semicon has been terminated taking a measurement from the crutch leaving unscreened core in the areas to be crossed.
I’ve always advocated taking the screen-off measurement from the HV cable lug thus leaving maximum screened core in the box. However, most termination manufacturers give a bottom up dimension but will give a top down dimension when asked.
♦ See: MV HV Cables 11kV 33kV
Michael White CEO at Campbell White – Colin you are spot on where a cross or a roll on end termination you must know creepage distance on these lineal stress terminations: on all Raychem terminations 90mm from top of black semiconductor heat shrink to open conductor then you can roll and cross on the semicon screen and have no trouble. There is no science taught these days on stresses and zero potential for cable jointers. I can understand third world jointers making mistakes but not educated ones.
Dan James MIET Technical Services Engineer – Senior Authorised Person at NG Bailey – complete mess. As suggested above this can’t be saved. Cut it all back, do a cable joint to single cores, get rid of them heat shrink sheds and perspex.
Richard Poulter SPS Managing Director – looks like a 33kV termination because of the phase barriers but more so the sheds per phase and the length of the stress control tubes which you can see the outline of below the anti-track tubes (they look about 260mm long). You can make out the semi-conductive screen ends because of the small bulge where the yellow stress tape is applied but most of the stress control tubes are fitted below this point rather than above it. So the majority of the stress tube is not doing anything.
I suspect the heat shrink cable terminations are fitted this low because the cable jointer was probably trying to achieve a distance of 250mm from the top of the stress tubes to the bottom of the lug barrels. The cable end termination box just does not look big enough for this voltage class termination, probably better to have used single core cable and they might have had a chance of achieving the cable spacings. Lots of discharge going on. That phase on the right looks like totally different anti-track but the intense heat has discoloured the other phases by the looks of things. Has it blown yet?
Maybe do a Trif joint below the cable box and take single cores in. If little space, use Raychem IXSU heat shrink kits or a cold shrink termination. Life expectancy is hard to pinpoint but there is a lot of discharge going on so they should schedule this within a week otherwise the damage will be a lot more. The temperature at the discharge can be as high as 1000 degC and once the carbon deposit increases on the cores, the quicker it will fail.
Andy O’Malley Allteck IBEW 258 EHV Cable Jointer/Splicer – they could have broke this 3 core cable into single cores outside of the 11kV box utilising jacket reintroduction kits and glanded the single cores individually and this would of been a perfect installation.
John Thompson High Voltage Cable Splicer – Okay, honestly it looks like hell. But let’s pick this apart. There are 12 cables in this picture (4 per phase). The 11kV terminations appear to be in some kind of a protected cabinet so they could operate for a length of time if this was some form of a temporary hookup. In my opinion this is more of an engineering catastrophe than a cable splicers mishap.
Joe Kinnane HV Electrician/ Transmission Cable Jointer With Western Power – they clearly needed to be “broke out” way lower than they have been with longer spaces between the stress point on the different phases! Heat shrink tubes weren’t pushed down fully on the breakout either.
Evan Paul Galleozzie Approved Electrician – bad design of the entry of the 11kV cables which has resulted in a mess of the cable terminations. And a few of the the cables look in a poor condition. And the CPC’s could have been installed properly using crimping tools, too much copper showing. Bad, bad, bad.
Christopher Williams Head of Services and Support at IPEC Ltd – looks like a severe case of surface tracking on the heat shrink cable terminations (a form of partial discharge). This is likely caused by the inadequate installation of the feeder cables increasing the electrical stress across the surface of the high voltage cable insulation. Any dirt/dust/contamination would then have helped create small electrical discharges across the insulation causing the carbonisation you see in the image. Over time this will compromise the insulation further, finally resulting in a catastrophic failure. Interestingly, this could have been detected via PD monitoring, in this case, ultrasonic emission would have been emitted and sensors could have detected the inception and location of this defect long before it got to this stage!!!
Heat Shrink Cable Terminations
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HV cable terminations are available for indoor (switchgear, transformers, motors) and outdoor installations where single or 3 core type power cables with polymeric or paper insulation must be effectively terminated into MV-HV electrical equipment.