Touch Potential Between Substation and Customer Fence 3

[wesles24]

I understand that there can be some touch potential between a substation fence and a customer's metal fence near by, but how do you determine how close it can be before it becomes a problem? I have seen 6 Feet online. Are there any regulations for this? I know that you could connect the customer's fence to the substation ground, but how about an isolating fence? What type of material should the isolating fence be between the two fences?

Thanks

 

[cuky2000]

The touch potential are case-by case specific. It is primarily a function of the short circuit current injected into the earth, the clearing time, the soil resistivity, the surfacing material among other factors.

There are several standard such as the IEEE Std 80, NESC, and others. However, may not be a direct regulation that address your concerns with fence directly.

There are a few option to isolate fence such as isolation metallic chain link section (using MV insulator) or non conductive fence section (search for Amico Fence)

I hope this could help

 

[stevenal]

Six feet or greater is used so that a person is unlikely span the isolation section. Insulators would be used on both sides of the section, so that the insulators support the isolation section above the ground.

 

[Power0020]

you need to find out the actual transferred voltage to the fence using CDEGS or CymGrd or even by hand calculations. it may not need any isolation.

 

[wesles24]

If we simply were to run a copper ground bar from the substation grounding to the customer fence along the side that faces the substation fence would this be sufficient?

Thank you for all the responses.

 

[stevenal]

Grounding a fence you don't own sounds risky to me. You will have no control over any fence expansion or removal, and the workers will likely be untrained in the electrical risk involved. Substation fence touch potential does just involve grounding the fence, it requires buried conductors, a resistive surface layer, and continuous weed control. Will you provide this for the customer fence?

Another thing to watch: The foreign fence may not be up to the substation required height. A low fence attached in any way to the perimeter fence will be a climbing aid nullifying the primary purpose of the perimeter fence.

 

[wesles24]

Those are all good points, we would definately burry the conductors, but are you saying a resistive surface layer such as crushed rock would have to also be extended to the fence? I guess the big question is how close can the customer fence be before anything should be done and this may be found in the CDEGS or CymGrd programs that I do not have access to. An isolation fence may be the best way to go, I cannot find any rules or regulations on this. Does OSHA, NESC, or RUS not have any regulations for this?

Thanks for the info

 

[krisys]

Connecting the outer fence to the inner fence might prove disastrous, unless you spread the earth mat along the outer fence for the reason given below:

The inner and outer fence will be jumpered and hence for the earth faults, the potential of outer fence would also rise along with the inner fence. But, inner fence is safe, as the earth mat is supposed to encompass the inner fence. So the potential difference between the ground and the structure are within the acceptable value.

In the mean time if you are not spreading the ground mat along the outer fence, there could be a higher potential difference between the metal structure (here in this case the outer fence) and the found. This could be dangerous.

 

[stevenal]

See IEEE 80. The distance is called "maximum reach distance" in 3.21, but I don't see a number provided. How far can you reach? Probably close to your height. Pretty unlikely a basketball player will be straining to contact his fingertips during a fault, but an average size person casually contacting both metal surfaces as shown on the left in Fig. 12 is much more likely.

 

[wesles24]

Thanks for all the information. What I was able to find:

NESC 384 C: Bonding should be provided between all aboveground metallic supply and communications enclosures that are separated by a distance of 6 feet or less. For the purpose of this rule, pole grounds are not required to be bonded to the communication enclosure.

IEEE 80: Substantial metal-to-metal touch voltages may be present when a person standing on or touching a grounded object or structure comes into contact with a metallic object or structure within the substation site that is not bonded to the ground grid. Calculations of the actual metal-to-metal touch voltage are complex. In practice, hazards resulting from metal-to-metal contact may best be avoided by bonding potential danger points to the substation grid.

It is impractical, and often impossible, to design a ground grid based on the touch voltage caused by the external transferred voltages. Hazards from these external transferred voltages are best avoided by using isolating or neutralizing devices and by treating and clearly labeling these circuits, pipes, etc., as being equivalent to energized lines.

OSHA: The employer can use a grounding grid to equalize the voltage within the grid or bond conductive objects in the immediate work area to minimize the potential between the objects and between each object and ground. (Bonding an object outside the work area can increase touch potential to that object, however.)

OSHA also referenced ANSI C2 which is NESC as above.