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 Post subject: Recommended clear zone around substations?
PostPosted: Sat Dec 14, 2013 10:25 am 

Joined: Mon Oct 19, 2009 7:42 am
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What are the requirements, recommendations or guidelines for the mimimum clear zone on the outside of substation perimeter fencing?

Is the clear zone for maintenance, [url='http://arcflashforum.brainfiller.com/#']securityImage[/url], ground grid protection, or other?

Where can I find these requirements or recommendations? Looked in NESC, IEEE, without much luck. Have found some references to a mimimum recommended 20 to 30 foot clear zone for security, but not much else.


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PostPosted: Sat Dec 14, 2013 2:31 pm 
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That is all that you will probably find. IEEE std. 80 is on a lot of aspects of substations. Older outdoor overhead types require a significant amount of clearance for shock protection reasons so fences are usually established at 10 foot or longer boundaries. These days though with vault mounted types for instance there is no clearance requirement at all. Due to relatively high costs of construction and engineering required, overhead style substations have all but completely fallen out of favor for new installations. As long as you maintain required clearances and "secure" it from access by unqualified personnel (NEC talks about this in Chapter 1), that is all that is required on the outside. Inside, generally most of the space will be access for bucket trucks and crane/derricks as well as ground grids. Following Factory Mutual requirements, mineral oil transformers need 25 feet clearance between each one and other equipment, and less flammable 5 feet. Following these design requirementsa common modern design puts a gate at each end with a 25 foot ooen corridor down the middle for truck access with transformers spaced down one side and switchgear down the other. That being said, portable substations frequently used by surface and underground mines have clearances of about 12-24 inches used as a buffer to prevent heavy equipment damage which is the deck around the actual equipment. They are otherwise quite compact and the only extensions from the sleds are ground grids. See the pictures for instance on Line Power, MCI, SMC, or Intermountain Electronics web sites. Note that the clearances essentially become zero for metal enclosed or metal clad designs where the outer enclosure is grounded and essentially everything is not exposed. This just leaves a few exposed bushings which can be eliminated using elbow connectors like pad mount residential utility transformers use, or else by placing bushings out of reach, genefally 12 feet off the ground. This probably explains why you are not seeing what you seek...depending on design, clearance is zero.


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PostPosted: Sat Dec 14, 2013 2:37 pm 
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See for instance [url="http://www.sandc.com"]www.sandc.com[/url] and look at their underground and padmount switchgear for examples of prebuilt semi-custom and stock bolt together gear that has zero clearances. Also Ruehl, Powell, Square D, Cooper, Siemens,, and Metalfab are just a few builders that I know of. Most are just sheet metal shops that use someone else's gear, and none build 100% of their own gear, no matter what they tell you. Some are more standardized in their designs while others basically engineer and build to order. I have not found significant cost differences between the two, although there is some lead time and cost associated with the very first one, especially if it is radically different from previous designs.


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PostPosted: Mon Dec 16, 2013 8:56 am 

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I have also found a recommendation to maintain at least a 6' clear zone to prevent encroachment upon the ground ring usually routed on the outside perimeter of the substation fence in addition to a visible clear zone of 20' for intrusion detection. Still only a recommendation, not a hard and fast requirement. The recommendation indicates that if a clear zone is not practical, other compensatory measures may be necessary, such as increased fence height, increased lightng, or surveillance cameras or intrusion detection sensors. Makes it hard to argue against a planner who wants to locate new construction right up against your primary substation.


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PostPosted: Tue Dec 24, 2013 9:25 am 

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First of all, contact your power provider. Unless you generate your own power, the high side of the switch gear and transformer probably belong to them and their construction rules will apply. All conductive perimeter fencing needs to be tied into the substation ground grid, including the gates and any pedestals used for key-operated functions. The outside of the substation is kept clear of vegetation for fire control, pest control and security. There are some instances where you may choose to extend the ground grid outside the substation fence at the vehicle entrance roadway in order to provide constant ground potential for the back axels of mobile substation assemblies when accessing energized subs.
There are also a few subs where the ground grid was extended 3-6 ft beyond the fence in order to reduce the chance of the public or high priced livestock being shocked by step potential differentials.
The ground within the sub is usually covered with coarse riprap (roadbase) to discourage rodent habitats and provide non-corrosive contact with the ground grid.
If your substation is within an urban environment, local fire codes and possibly public siting concerns will come into play.


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PostPosted: Fri Dec 27, 2013 8:51 am 
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We have a 54 MW generator where I work (cogen not standby) and most of the locations I have worked owned the sub. There is a substantial amount of cost savings by owning the sub. If you go there the ljne of demarcation is the last pole, but due to interoperation requirements especially if you have makn-tie-main, the utility usually wants some agreement and usually jt is best to use IEEE C2/OSHA 1910.269 rules. Your sub, your rules but that does not mean you can abuse the utility or vice versa. The coarse gravel is actually there for step potentials. See IEEE standard 80. The huge concern for ground grids, etc., stems from outdoor overhead substation designs with either ungrounded, multigrounded (most common), or solidly grounded systems. Resistance grounding, even low resistance grounding (400 A resistor is typical) were almost unheard of in the US until about 20 years ago. Where I am at now even the 23 kv internal distribution system has a 2000 A resistor (with 0.5 second tripping), with about 70 miles of distribution pole lines. Except the utility 230 kv incomkng that they maintain (overhead design) all subs are metal clad or metal enclosed. Ground grids are usually 3 eight foot rods spaced 8 feet apart. By controlling ground fault current, Earth resistance becomes more of a lightnjng concern than power faults. This design can work up to around 35 kv. Certainly under 10 kv high resistance grounds (under 100 A) are highly recommended. Above 35 kv it becomes pretty impractical to use resistance grounding as far as I know. A couple years ago, a contractor 'tested' the protection in a 4160 sub. He walked away with only minor injuries even after directly faulting the ground to line across his arm with a 25 A resistor. There are a number of technical papers on [url="http://www.msha.gov"]www.msha.gov[/url] on this as well as IEEE PCIC documents for the chemical industry. With lower (under 10 kv) voltages it becomes possible to totally prevent fibrillation due to ground faults. Even the traditional (ground grid well under 1 ohm everywhere, coarse gravel cover) fibrillation due to step potentials can be eliminated. Touch potential however js still a concern, thus metal enclose and ground everything oossible to minimize it. Exposed energized components are at 10 feet up to 50 kv, or following MAD in IEEE 516 above 50 kv. This has been the standard for decades, with some variation due to improved grounding strategjes in mining, chemical industries, and some countries outside the US. You can't for jnstance live with a 10 foot buffer in a 36 inch Kentucky coal seam running a 4160 volt continuous miner and when the crew may be 10 ler shjft with 1 electrician on days/on call, it is definitely not practical to mandate only qualified personnel can plug things in or operate disconnects. As I said, it depends on design. Buffers can be anywhere from 0 to 10's of feet. Use OSHA 1910.269 rules (or IEEE 516 or 70E since they all refer back to 516) as a general rule for anythjng exposed or where step/touch potentials may exist. Outside of substations, cable sway and stretch become concerns so with freely swinging cables follow NESC C2 for clearance recommendations and/or calculations whjch may also involve software such as SAG10 or several others. IEEE 80 is very helpful in understanding where thjs occurs. Transformer oil fires are very nasty and may necessitate longer clearances. See FM Global for good publicly available guidance on this as they spent the most time researching it, but it is generally 25 feet for mineral oil or 5 feet for less flammable fluids.


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PostPosted: Tue Dec 31, 2013 10:54 am 
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Great response, at least until I got to this line
"it is definitely not practical to mandate only qualified personnel can plug things in or operate disconnects. - "

Really??, In that industry, none of the other saftey advances in use today, were considered "practical" either.
If it is "impractical" to have the electrician there, then you need to give the person you EXPECTt to "operate the disconnect", or " plug something in", sufficient training, so as to be qualified for those tasks. I expect that this is actually the case.

OTOH if you (the corpoarate you) are expecting some one you consider to be unqualified for a task, to routinely perform that task, you set your self up for a massive failure.


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PostPosted: Sun Jan 12, 2014 6:44 am 
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Miners are required not only to be qualified but the federal inspectors who inspect 2-4 times per year at minimum by law specifically approve training plans for some types of mines and always check training records. Everyone must be task trained. Where this gets into hair splitting is that for instance 70E requires a lot of things to be "qualified" that would be required for working on presumed energized and exposed equipment. The inherent problem with the term qualified is that it has both a task and voltage range. For instance I am qualified to do just about anything up to 35 kV. I am not qualified for bare hands work of any kind nor working above 35 kV. Our linemen are not qualified on 5 kV shielded cables, and mine electricians are not qualified on open wire circuits, and many "plant" electricians are not qualified above 1 kV. Operators are all trained to use plugs and operate enclosed breakers and disconnects, and recognize and avoid possibly energized equipment (call electrician). Mine operators are trained to glove up to move up to 8 kv cable and use medium voltage plugs where used. As I said, "qualified" is both task and voltage specific. The old union card is not enough anymore.


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