Static wire holder arcing to ground ??? 1

[sparky1976]

Recently we've trip on our 33 kV feeder due to static wire dropped or fell on to overhead line.
The problem was the holder/stud dedicated for static wire is broken.
The temporary result of investigation, the stud is eaten by arcing to pole rebar (it's a concrete pole). My temporary conclusion is the rebar on the pole is not a good ground,so its eaten by arc and in long time the tension of wire snap the stud that already thin. The pole position is in the curve of the line.
Is it possible the induction voltage on static wire do such a thing ? (the pole is beetwen two grounding point of static wire)
Please advice.

 

[rhatcher]

Does the term "static wire" refer to a lightning shield conductor on your system? If so, it may be that the intentional grounding for your static wire (the ground conductor running down the pole from the shield conductor to a good ground) is poorer than that provided by the concrete pole. Please provide more info if this does not help.

 

[sparky1976]

Dear rhatcher,
Tanks for your prompt response,
Yes it is lightning shield conductor on the top of the pole,(above the 33kV line) with fiber optic in it for transfer trip from end user XFMR to our breaker.
My question is,
Is it possible for this wire bleeding its induction voltage bleeding trough pole rebar before its bleeding trough dedicated ground ? (the pole that had this problem is in between two dedicated ground point).

What is the standard for grounding this conductor, the distance I mean ?

 

[SMLD]

Yamin,

I had a similar problem on my 13.8kV system about 5 years ago. We had a pole top guy wire, guying a pole, burn off and detach from the pole. Upon investigation, I concluded that the guy had a much lower impedance than the vertical down ground, 20 ohms verses 450 ohms, respectfully. However this was not the root cause. With further investigation, it was observed that during inclement weather, the insulating deadend on C phase was indeed tracking to groung.( pole crossarm) We observed this first with a infared camera, then confirmed it with a HV test across the deadend. We changed the deadend, drove 2 ground rods, and installed a #2 cu vertical grd.

You might have something breaking down on that pole, which is searching for ground. You mentioned that it is inbetween two ground points. If something is breaking down it seems possible that it is using the static wire to get to the next low inpedance ground point. The clamps that hold the static wire to the pole are not usually designed with electrical conductivity in mind, thus a higher impedance. This is where resistive heat will be present and possible failure.

Chris

 

[sparky1976]

Tanks Chris,
Yes we have to change 2 insulator on phase A (pole position is on curve of the line). We'll check the ground resistance on that two points.
On your post I conclude you end up installed #2 cu vertical ground in every pole ???

 

[SMLD]

Yes, ideally we like to have a #2 cu vertical down ground on every pole, due to the lightning season. Also, having a good gound on every pole, ( 25 ohms or less)ensures that if there is an electrical fault on a pole, the feeder breaker will trip much faster due to the higher fault current. In the past, ( 10 years ago) we didn't have our system grounded correctly. On occation, we would get calls from farmers, homeowners, etc...that people would be getting shocks off of water faucets within the home. One farmer's cows stopped producing milk because they were getting shocked in the stalls. 90 percent of the time we tracked down the root cause as a primary high voltage leak, compounded by a high impedance ground. These trouble calls are now few and far between, because of all the grounds we installed in a 10 year period. Now if a insulator on the primary circuits breaks down electrically, it will blow a fuse or operate a recloser. We would rather have a power outage due to a insulator fault, than have a pole and its hardware become energized, waiting for someone to complete the path to ground.

It sounds to me you found the root cause of your problem..now if you install a better ground on that pole next time it would hopefully trip the breaker quicker, causing less damage. Then you face the challenge of locating the cause of the feeder operation or lockout. Thats where the infrared camera really helps us out. It can pick up a differnce of 5 degrees F.

 

[sparky1976]

SMLD,
What is the standard of pole grounding resistance ? (anyone can help me)


tanks
yamin

 

[SMLD]

The NEC implies that the ground resistance should be measured at the time of installing the ground and that the ground resistance must not exceed 25 ohms for artificial (buried or driven) grounds, (AEH) The NEC also goes on to state acceptable ways to reduce the resistance if it exceeds 25 ohms.

1. Use larger diameter ground rods.
2. Use longer ground rods or ones that screw together.
3. Putting two, three or more ground rods in parallel.
4. Chemical treatment of the soil.

Check out the National Electrical Code or the American Electrician's Handbook for further details.

Chris