Spacing of perimeter fence grounding rods 1

[danschwind]

I’m dealing with a problem regarding the required spacing for grounding rods used to ground perimeter fences for an industrial installation. The installation(s) itself is a fuel distribution facility, with vertical tanks storing gasoline, diesel and ethanol, plus all the required ancillaries for truck loading and eventually barge and rail loading. The facilities have existing grounding grids.

Now, for the perimeter fences, I’m being told of the need to insert grounding rods every 50m to ground it, on top of electrically sectioning it every 200m. There is no electricity generation in the facility so it is basically to protect human and animal lives from lightning electrifying the fences, I reckon. The fences will not be connected to the existing grounding grids.

I’m not an electric engineer (and honestly, I’m very limited in everything regarding electricity) but my gut feeling tells me the 50m of spacing between grounding rods to be a little conservative. I’m ok with the 200m sectioning though. I’ve tried – and failed – to find any solid reference for the grounding of industrial perimeter fences, in particular this spacing between grounding rods.

NFPA 780 Annex H states 45m/92m if the ground is dry/wet, but this annex is more applicable to rural fences AFAIK. IEEE-80 wasn’t of much help in this particular subject.

The fences vary from facility to facility (there are over 40 in the country), but bellow there are some pictures of some of them for reference:

I don’t know what extra information I need to define this really, so I would appreciate any guidance you guys can give me. In other words, I’m lost.

thread238-69334 and thread238-432617 were found during my searches and they are close to what I’m soliciting, but most of the comments suggests that some calculations should be performed according to IEEE-80…is this really necessary for something like this? And for something that is related mainly to lightning protection?

By the way, just to clarify why I’m doing such a thing that is very unrelated to my formation: I work as a consultant of R&D for a company, and most of the subjects fall outside my expertise. It is usually not a problem as the “research” aspect is usually linked to standards and whatnot. In this case though, I’m failing to find any palpable resource to define this maximum distance.


Hope you all stay healthy and safe!


Daniel
Rio de Janeiro - Brazil

 

[7anoter4]

IEEE 665/1995 IEEE Guide for Generating Station Grounding
5.4.5 Metallic fences:
Grounding of perimeter fences is important because it is an area where the general public can be exposed to dangerous touch voltages. Perimeter fences included within the ground grid area should have a conductor parallel to the fence on the outside at a distance of 0.5-1.5 m (1.5- 5 ft), with 1 m (3 ft) generally recommended.
The fence and conductor should be bonded together and to the ground grid at frequent intervals.
Where a perimeter fence is to be located at the property border, the ground conductor should be located on the property line with the fence set back from the grid conductor in order to minimize touch voltage exposure to the public.
Where a perimeter fence is not included in the ground grid area, reference should be made to the precautions in IEEE Std 80-1986[Now 2013]. Precautions should be taken, such as an isolated or insulating fence section, to prevent coupling dangerous grid voltages during ground faults to the fence outside the grid.
The practice of providing a driven ground rod at each fence post for approximately 7.5 m (25 ft) on either side of the outer conductors of each transmission line, with a grounding conductor parallel to the fence and connected to the rods, may be employed at each fence to provide an immediate and direct path to earth in the event of a fallen line.

 

[danschwind]

Hi 7anoter4, thanks!

Isn't this excerpt aimed towards generating stations? Is this same philosophy applied to general industrial facilities?

Daniel
Rio de Janeiro - Brazil

 

[7anoter4]

If you have not grounding return current from supplying line-overhead line or cables-you have not GPR [Ground Potential Rise] that means no touch or step voltage on the grounded equipment so the fence does not present a life danger.
If you have high [or medium] voltage substation in the fence limits you may have GPR and then this arrangement will be the same. See -for instance IEEE 142/2007 chpt.2.4.2 Design of paths for power frequency ground-fault current flow.
You may also calculate the minimum distance between grounding rods following IEEE-80/2013 cpt.17.3 Grounding of substation fence.

 

[danschwind]

7anoter4,

Shouldn't precautions be taken regarding atmospheric discharges (lightning)? This is the main point of contention here, being the lack of information regarding this spacing when electrification due to lightning is the only concern.

Daniel
Rio de Janeiro - Brazil

 

[7anoter4]

See NFPA 780/2011
Annex G Protection for Picnic Grounds,Playgrounds, Ball Parks, and Other Open Places
H.2.2 Iron Posts. Ground connections can be made by inserting
galvanized-iron posts, such as are ordinarily used for farm
fencing, at intervals and attaching in electrical contact all the
wires of the fence. Grounding can also be achieved by driving a
length of not less than 12.7 mm (1⁄2 in.) in diameter galvanizediron
pipe beside the fence and attaching the wires by ties of
galvanized-iron wire. If the ground is normally dry, the intervals
between metal posts should not exceed 46 m (150 ft). If the
ground is normally damp, the metal posts can be placed up to
92 m (300 ft) apart.
H.2.3 Depth of Grounds. Pipes should be extended into the
ground at least 0.6 m (2 ft).
H.3 Breaking Continuity of Fence.
H.3.1 In addition to grounding the fence, its electrical continuity
should be broken by inserting insulating material in breaks in
the wires at intervals of about 150m(500 ft). These insertions can
be in the form of fence panels of wood or lengths of insulating
material to the ends of which the wires can be attached. Such
lengths of insulating material can consist of strips of wood about
50mm× 50mm× 600mm(2 in. × 2 in. × 24 in.), or their equivalent
as far as insulating properties and mechanical strength are
concerned.
H.3.2 In areas where herds can congregate along fences, the
continuity should be broken at more frequent intervals than
described in H.3.1.