Earthing/Grounding of Structures beneath Transmission Lines

[JezNZ]

Hi,

I am looking for advice on the grounding / earthing of structures beneath transmission lines. The scenario is a 220kV line which crosses a waterway, and a temporary steel bridge structure also crossing the waterway is being installed, directly underneath the lines.

Does the NESC Handbook have suitable application guidelines for this scenario? Or is there another suitable application guide for grounding to minimise electrostatic / coupled risks?

Thanks

 

[davidbeach]

That's just a bad place for the bridge...

 

[JezNZ]

That's just a bad place for the bridge...

Unfortunately they tend to put bridges where the road is!

 

[waross]

From first hand experience under a 500 kV line and for a couple of hundred feet on either side of the line.

Use a length of #4 or #6 rough service single conductor cable (We used light welding cable, but the current is very low)
Connect one end of the cable to the frame of the crane.
Install a ground clamp on the other end of the cable.
When the crane or picker truck stops, connect the cable to anything grounded before elevating the boom.
Lifts should be grounded before landing.
Any ungrounded, conductive object in the air below the power lines will take a capacitive charge at a voltage determined by the relative distance between the line and effective ground.
In some soil conditions, effective ground may be below the actual ground level.
Touching an ungrounded object under the line may result in a shock similar to a static shock.
The larger the object the greater the charge.
Swinging steel sections into place under the line, it is recommended to use steel slings so that the section will be grounded by conduction back to the crane or picker truck.
If a large section is swung into place without being grounded, the workers may be unable to touch it to guide it into place.
This is experience talking. I witnessed a large section being swung in that developed so much charge because of its size that it had to be swung out and landed.
A grounding jumper was installed across the non-metalic lifting sling and it was swung back into place with no issue.
Yes, I did touch it, but I could not hold it.
Small sections would shock the men when they touched the metal but if they were able to hold on, there was no further shock after the initial jolt.
Large sections the initial jolt was too powerful to hold through.
Once the crew gets used to the grounding and grounding jumpers are close to hand, the extra time to ground is negligible.
You will find that the time to ground is less than the time to endure ungrounded work.

 

[ema157]

As I believe you are in NZ, you could use the transmission company's service specification 07.23 (Work site and Mobile Plant temporary earthing and equipotential bonding of transmission lines) as a starting point.

 

[JezNZ]

As I believe you are in NZ, you could use the transmission company's service specification 07.23 (Work site and Mobile Plant temporary earthing and equipotential bonding of transmission lines) as a starting point.

Thanks for response, however fyi I found DL 01.04 is much more relevant, specifically section 6.5.1 pretty much covers off the question.