Gravel in Switchyards

Good question. My first response would be to ruin my pants when I am working in a substation. My second would be that you really don't want a muddy bog or a lawn mower driving around inside a substation (insulators get a bit fussy when they are covered in grass).

Gravel is less costly than pavement.

Mainly to provide a low maintenance surface that grass and weeds cannot grown in but be able to be easily removed for excavation and installing of new cables and foundations. Switch yards are subject to constant changes in cables and wiring as electric loads grow. Conduits are installed where they think future needs will be but it doesn't work out that way. They still have to spray it with a herbicide. They are licensed to use a herbicide product that is not available to the public.

Railroads use the same licensed product.

The rock (where available) is crushed dolomite.

Transformers contain oil. The gravel is often considered adequate secondary containment when evaluating sites for spill prevention and containment.

kulbrz

well said.

Check into grounding or earthing resistance. When there is water in the yard a person could have their feet in water or be in a damp surface. That person would be of a lower impedance. Whenever a fault occurs in a system, high currents will be flowing. Your design is to make everything grounded with a uniform potential (no ground voltage drop) so that a person will be safe. Check into the terms Touch & Step Potential. That should explain this better.

Here is a link

http://en.wikipedia.org/wiki/Earth_Potential_Rise

A properly installed switch yard has a steel mesh grid buried beneath the surface. that extends over the complete fenced in area and bonded to the surrounding fence.

Relying on the stone for step potential protection is a totally unacceptable practice with US Insurance companies and OSHA.

The fences are also bonded with cad welds at each post and also the system neutral.

Each swinging gate has flexible bonds (two) to the grid.

Overhead (manually operated ) switches generally have an additional surface mounted grid for the operator to stand on that is bonded to the switch handle. The operator also wears rubber gloves.

As I have stated in a previous comment, the primry purpose for using the stone is for ease of maintenance.

Generally the substation plan is prepared before the grid design . The earthing grid should cover as much ground area as possible . Soil resistivity can be determined using the four probe method . A thin surface layer of crushed rock helps in limiting the body current . A value of 3000 ohm-m is assumed for resistivity of crushed rock . However , it is preferable to measure the resistivity of the material to be put on the surface .

At 345,000 volts and 15,000 amps fault current capability, typical in bulk power substations, most US utility companies are more concerned with meeting the system grounding requirements for the distance relaying functions. At these voltage levels, reliance on the surface dolomite rock resistance for step voltage protection would be a futile effort.

As previously stated, US substation design, cannot depend on this form of operator protection. Practice is to install (the equivalent to) a bonded metallic 6" square mesh grid under and around the entire area with the intent of keeping the step potential of the entire area within safe limits. Reliance on the resistance of the surface rock, for protection of human life would be an action difficult to support and might be considered a negligent criminal action in a US court of litigation.

And when it rains, what happens to the surface rock resistance?

I question your source of information, You frequently use the term earthing that indicates to me that you may be relating to engineering practice outside the US.

Perhaps, we are relating to entirely different world philosophies, What Country or public utility depends on the rock resistance for employee shock protection?

At the company I worked for, we considered cost and availability for the crushed rock the major criteria for purchase.

I have not recently reviewed any IEEE papers on this subject. but if you can quote any accepted IEEE paper recommending this practice ? I will gladly concede my argument.

I no longer serve on any of these IEEE committees. (retired)

Read IEEE 80 2000?

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2. a person who receives the hospitality of a club, a city, or the like.

3. a person who patronizes a hotel, restaurant, etc., for the lodging, food, or entertainment it provides.

4. an unknown person unwilling to be identified that posts an answer to a question on CR4 pretending to be an expert in spite of the fact that they "guessed" at the answer.

Weed suppression.

Quick drying surface, won't 'pool` water.

I am not in electricity but I think gravel is used because of practical and economic reasons. If it however adds to the safety it is a bonus.

It is relatively cheap,

Can be repaired and maintained cheaply.

It can accommodate heavy equipment like a crane etc without the possibility of permanent damage.

Drainage is good.

It will have less puddles after rains than a slab.

Can accommodate some spills and affected areas can be cleaned cheaply.

It should be or can be made as safe as anything else.

Access to yards are usually restricted and the operators with access must obey prescribed safety rules.

If somebody feels that gravel poses any safety risc the matter should be taken up with the correct authority and not jaythanki.

Designers must follow prescribed regulations.

My - this is quite a debate with a fair amount of common sense thrown in. However I have never heard of the resistivity of crushed rock being discussed in UK substation compounds

We call all our open air substations with Live HV parts - compounds.

Here we also use gravel on the base of our open air substations - no particular type just whatever is in the nearest quarry usually between 20mm - 25mm grade.

We use this for two reasons - one drainage. HV substations with exposed conductors and insulators have the access controlled to authorised persons only who have been examined on their competence an knowledge of the HV rules, which mainly involves safe distances - a combination of both vertical and horizontal from the Live part.

Obviously the safe distance may not be quite so safe if somebody was standing in a puddler of water, and in this country our utilities specialise in building pavements with puddles included - in fact I think its mandatory for all public footpaths.

So we prefer to stay away from artificially finished surfaces.

The second is ease of maintenance - with gravel we only need to employ an authorised person who can just spray some weedkiller on twice a year and that is that.

Grass would be nice I suppose, but it can get very muddy and slippery because we get a lot of rain here, then the authorised person would need a lawn mower and a strimmer. Then there is the grass cuttings to get rid of, our councils will not empty the bins if they are too heavy you know.

We also have earth mats both HV and LV, but again nothing to do with step voltage and any type of rock is bad news because it results in bad earth path readings, we pray that there is some good clay or black soil down there.

But most of our HV compounds have automated switching either from the central control or the compound switchroom. Our manual HV switching is normally limited to rural overhead lines on wood poles where if you are lucky you will have some nice heather under your feet, but more likely a dirty great big thistle up your backside!!

Here is another of the "guessed" answers: I suspect that gravel is used because it IS better than grass which could burn when dry and does live in earth which is messy to work in when wet. It makes a reasonalby sturdy working area for heavy machinery and can be removed much easier than a paved area when underground utilities are to me modified or added.

I think most of you guys provide a misleading information, as it is clearly specifyed on the IEEE Green book for grounding

"4.2.5 - Substation Grounding ... Coarse crushed rock (example:granite), is normally spread all over the surface of the soil within such a substation grid area, not for housekeeping reasons, but to provide a high resistance surface treatement to reduce the hazard from step potential to persons within this area during the severe fault. Granite rock even when wet from rain, high a high insulation resistance. (4.5x10^5 ohm.cm wet, 1.3x10^8 ohm.cm dry). It is not unreasonable to be unable to use good quality crushed stone due to the location the substation may be in, so other means to increase the safety of personel may be required. Asphalt can be used because of its high wet resistivity (>1, 000,000 ohm.cm) "

No more no less.

Good report. You did your homework.

My IEEE membership ran out and since I am retired, I do not have the funds to renew it, hence I no longer have free access to IEEE papers .. Good job.

Snakers

Quite a few reasons actually...

1. Prime reason - The gravel with voids forms a good insulating layer above the soil ( earthing ) so that step potential is easily achieved.

2. Water retention in soil - During rains, water seeps in through the gravel. After rains, the gravel does not allow the soil to dry as sunlight cannot penetrate it.

3. It also serves to slow the weeding in the soil. Some weeds attract birds which then tend to nest in the structures.

4. Some types of gravel act as a flame retardant in case of flaming oil being dropped from a CT/PT.

5. Very important - The corona sound from the disc insulators attract male snakes which tend to climb the structures and go to the acsr conductors, thereby causing line failures. Snakes find it difficult to traverse across gravel ( pointed ).

Hope this answers your question...

James (jamesferrao@gmail.com)

very good answer

James: Quite a few reasons actually... 1. Prime reason - The gravel with voids forms a good insulating layer above the soil ( earthing ) so that step potential is easily achieved. 2. Water retention in soil - During rains, water seeps in through the gravel. After rains, the gravel does not allow the soil to dry as sunlight cannot penetrate it. 3. It also serves to slow the weeding in the soil. Some weeds attract birds which then tend to nest in the structures. 4. Some types of gravel act as a flame retardant in case of flaming oil being dropped from a CT/PT. 5. Very important - The corona sound from the disc insulators attract male snakes which tend to climb the structures and go to the acsr conductors, thereby causing line failures. Snakes find it difficult to traverse across gravel ( pointed ). Hope this answers your question... James (jamesferrao@gmail.com)

WTG James! My uncle, who has operated an industrial rock crusher for 15 years, had all the answers except #5! That's a new one on him and he's really impressed as he was sure he knew better than anyone. Thanks for that awesome reply.

(1) Get proper insulation for human . (2)During any fire hazard due to leakage of oil from transformer or any equipment which contain transformer oil,the oil will go inside the gravel and so fire will not spread rapidly to other equipment gravel prevent from furher damage to near equipment (3) Snakes are not very much prefored to move on gravel, so any maintenace come in night time, the maintenance person will feel safe, from poisionus snake.

Fortunately for my uncle, he never encountered any poisonous snakes in his years operating the jaw crusher. I hadn't even considered that hazard. Thanks.

Amazingly, after 15+ years operating a jaw crusher my uncle finally encountered a snake!

It tried to bite him but fortunately he has very quick reflexes and he managed to evade it. I hate snakes, I'm sure glad it wasn't me!

IEEE 80-2000 Defines the AC grounding requirements for substations and specifically states, "covering the surface with a material of high resistivity is very valuable in reducing shock currents." It also states that thought must be given to the probability of degradation of insulation due to compression and the filling of the voids by debris.