Shorted Cables on OHL

Hi to all!

I just want to know what are the reasons of shorted cables on overhead lines. Currently our phase conductors are insulated and the messenger is bare. They are all assembled together.
50% of our electrical shorts came from the overhead lines.

Thanks to all!

Hello jprs_ie,

Well you sure aren't giving us very much information to work with but we have some clues from your IP number. You live in a very hot, dry country with a lot of sunlight.

1) Is this cable of low voltage design and used for service entrances drops to buildings and houses?

2) Is the stranded (regular round?, compressed?, compact?) conductor aluminum and insulated with a single layer of virgin, (no regrind), black polyethylene specifically formulated for outside use?

3) How old is this cable since it was installed and is it the oldest cables that are failing.

Obviously the cables are failing because the phase insulation has degraded in service perhaps because:

a) Of the poor quality of the original virgin polyethylene supplied by the cable manufacturer.

b) The virgin polyethylene could have been mixed by the cable manufacturer with with very low quality and contaminated regrind polyethylene at extrusion.

c) The virgin polyethylene used by the cable manufacturer was not originally formulated for outdoor use and therefore may be missing adequate UV (ultraviolet) protection chemicals.

Regards
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com
519 641- 3212

Thank you Mr. Stewart-Hay for your reply I really appreciate it.

This is my first time to post a discussion.

Regarding about your reply, yes we are doing low voltage design and it is used for service entrance drops to buildings and houses. Below are the processes that we do in producing OHL.

For 3x50+50

Drawing - 3mm,Al H14 (phase) and 3.37,Al H14(neutral)

Stranding - 7wires,round compacted (phase) and 7 wires, non-compacted (neutral) (For the neutral we use 1 ACSR/AW with a diameter of 3.37mm at the middle)

Insulation - All phase conductor are insulated with 2.03mm XLPE/2.5%carbon black (all virgin). we also put a torch in front of the cross-head. For the neutral, we do not insulate it.

Curing - After insulation process we do not cure because as per our technical department, it is OHL and it will be self cured.

Assembly - we assemble the phases and neutral all together using our drum twister machine or bow twister machine.

Testing - During the high voltage test, we use 3.5kV ac for testing the cable.

50% of our shorted cables come from OHL 3x50+50 and 3x120+120.

Hello again and please call me Peter from now on.

- I was the one that transferred your initial query to the Electrical Division as this is indeed an electrical issue.

- From the way your original message was written, I thought I was dealing with a Hydro engineer and not a cable manufacturer. I see I was mistaken.

- Sorry about the error message you get when you post something. The message is incorrect so please just ignore it. A bug report requesting corrective action has been issued.

- Your conductor all looks fine so we have to focus on the insulation and the stranding process itself.

- I have just a few more questions if you don't mind.

A) It seems that you are using a sioplas cross linking process. This is usually comprised of a base of polyethylene compounded with liquid silane and liquid peroxide. (Process 1)

The resulting pellets are then extruded after being accurately mixed by a gravimetric mixer at the extruder with both the catalyst and a color masterbatch. (Process 2)

Are you doing both Process 1 and Process 2 at your plant? Please clarify and confirm precision gravimetric mixing at Process 2.

Have you carefully examined all your bow twister eyelets to see if any are badly grooved or cracked? Ditto for all guides and the closing die. How about the capstan?

C) Do the operators carefully check the inside flanges of the reels and the drums of the reels before using both at insulating and stranding? You wouldn't be the first to have the finished product damaged by protruding nails on wood reels or sharp chamfers or edges on the inside flanges of steel reels?

D) Because you are doing an AC voltage test, I imagine there is little evidence left to examine at the failure point. Would it be possible to do DC testing instead in order to get a better samples of the failures?

E) Have you discussed the insulation problem with the polymer manufacturer (Dow Europe or Borealis) Are they happy with the use of 2.5% carbon black as the color masterbatch?

Note - Carbon black has been used in semi-conducting shield compositions in the amount of about 20 to about 60 percent by weight based on the weight of the composition, but preferably it is used in an amount of about 25 to about 45 percent by weight. You on the other hand are looking for UV protection and not semi-conducting properties.

Regards
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com
519 641-3212

Good day sir!

Sorry for the late reply. Its just that Ive been busy because of the Eid holiday. Well anyway sir we are doing the Process 2.

The resulting pellets are then extruded after being accurately mixed by a gravimetric mixer at the extruder with both the catalyst and a color masterbatch. (Process 2)

As recommended by the supplier of our XLPE masterbatch for Carbon Black, we must use 11% to make 2.5% CB but sometimes we use only 8-9%. But as per our technical information, this has nothing to do with the shorts.

I have not checked the eyelets of our drum twister or bow twister. I will check them all after the vacation.

As per the drums, we use wooden drums for the finished products. We also put some cartons on the flanges of the drums. We use stapler to fix the cartons. I saw before that some of the staples are not inserted properly. If the staples wire and the bare conductor touch each other would it make a shorted cable???

I have discussed the insulation problem with our supplier but their answer is complicated that I have sent their reply to the higher management of our company.

We had one order where in all of the conductors are insulated and we did not have any shorted cables. With this I was thinking maybe we have shorted because one of the conductors is bare.

For the DC testing I will discuss this one to our QA Department.

Thank you very much again for your reply sir!

Hello again,

A staple wire touching the bare center conductor would not short the cable.

A staple wire coming off after penetrating the insulation on a phase conductor and then touching the bare center conductor would certainly produce a short.

The staples must be properly put into the inside flange surfaces. Likewise it is also critical for the drum surfaces.

You now should have all the information to sleuth out your problem.

Regards
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com
519 641- 3212