steps to ensure effective substation grounding

7/27/2019 Steps to Ensure Effective Substation Grounding

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Steps to Ensure Effective Substation Grounding

Substation Grounding

An electrical substation is a critical resource in a power system. Safe operation of a substation

calls for a properly designed and installed grounding system. A well-designed groundingsystem will ensure reliable performance of the substation over its entire service life.

How does good grounding improve substation reli abil i ty?

Good grounding path of sufficiently low impedance ensures fast clearing of faults. A fault

remaining in the system for long may cause several problems including those of power

system stability. Faster clearing thus improves overall reliability.

I t also ensures safety.

Aground faultin equipment causes the metallic enclosure potential to rise above the true

ground potential. An improper grounding results in a higher potential and also results in

delayed clearing of the fault (due to insufficient current flow).

This combination is essenti all y unsafebecause any person coming into contact with the

enclosure is exposed to higher potentials for a longer duration.

Therefore, substation reliability and safety must be as built-in as possible by good

grounding scheme, which in turn will ensure faster fault clearing and low enclosure potential

rise.

Ensuring Proper Grounding

The following steps, when put into practice, will ensure a reliable, safeand trouble-free

substation grounding system:

1. Size conductors for anticipated faults2. Use the right connections3. Ground rod selection4. Soil preparation5. Attention to step and touch potentials6. Grounding using building foundations **7. Grounding the substation fence **8. Special attention to operating points **9. Surge arrestors must be grounded properly **10.Grounding of cable trays **11.Temporary grounding of normally energized parts **

** Will be published in next part of this technical article
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1. Size Conductors For Anticipated Faults

Conductors must be large enough to handle any anticipated faults without fusing

(melting).

Failure to use proper fault time in design calculations creates a high risk of meltedconductors. Two aspects govern the choice of conductor size: the first is the fault current that

will flow through the conductor and the second is the time for which it can flow.

The fault current depends on the impedance of the ground faul t loop. The time of current

flow is decided by the setting of the protective relays/circuit-breaking devices, which will

operate to clear the fault.

The I EEE 80suggests using a time of3.0 sfor the design of small substations. This time is

also equal to the short-time rating of most switchgear.

2. Use the Right Connections

Grounding Connections, Resistance Test and Bonding Test

It is very evident that the connections between conductors and the main grid and between the

grid and ground rods are as important as the conductors themselves in maintaining a

permanent low-resistance path to ground.

The basic issues here are:

1. The type of bond used for the connection of the conductor in its run, with theground grid and with the ground rod

2. The temperature limits, which a joint can withstand.The most frequently used grounding connections are mechanical pressure type (which will

include bolted, compression and wedge-type construction) and exothermically welded type.

Pressure-type connections produce a mechanical bond between conductor and connector bymeans of a tightened bolt-nut or by crimping using hydraulic or mechanical pressure. This


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connection either holds the conductors in place or squeezes them together, providing surface-

to-surface contact with the exposed conductor strands.

On the other hand, the exothermic process fuses the conductor ends together to form a

molecular bond between all strands of the conductor.

Temperature limits are stated in standards such as I EEE 80and I EEE 837for different types

of joints based on the joint resistance normally obtainable with each type. Exceeding these

temperatures during flow of fault currents may result in damage to the joint and cause the

joint resistance to increase, which will result in further overheating.

The joint will ultimately fail and result in grounding system degradation or total loss of

ground reference with disastrous results.

3. Ground Rod Selection

Substation grounding rod

In MV and HV substations, where the source and load are connected through long overhead

lines, it often happens that the ground fault current has no metallic path and has to flow

through the groundmass (earth). This means that the ground rods of both source and load side

substations have to carry this current to or from the groundmass.

The ground rod system should be adequate to carry this current and ground resistance of

the grounding system assumes importance.

The length, number and placement of ground rods affect the resistance of the path to earth.

Doubl ing of ground rod length r educes resistance by a value of 45%, under unif orm soil

conditions. Usually, soil conditions are not uniform and it is vital to obtain accurate data by

measuring ground rod resistance with appropriate instruments.

For maximum efficiency, grounding rods should be placed no closer together than the length

of the rod. Normally, this is 10 ft (3 m). Each rod forms an electromagnetic shell around it,

and when the rods are too close, the ground currents of the shells interfere with each other.
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It should be noted that as the number of rods is increased, the reduction of ground resistance

is not in inverse proportion. Twenty rods do not result in 1/20th of the resistance of a single

rod but only reduce it by a factor of 1/10th.

For economic reasons, there is a limit to the maximum distance between r ods.

Normally, this limit is taken as 6 m. At more than 6 m, the cost of additional conductor

needed to connect the rods makes the design economically attractive.

In certain cases, the substation layout may not have the required space and acquiring the

needed space may involve substantial expense. Four interconnected rods on 30 m centers will

reduce resistivity 94% over one rod but require at least 120 m of conductor.

On the other hand, four rods placed 6 m apart will reduce resistivity 81% over one rod and

use only 24 m of conductor.

4. Soil Preparation

Soil resistivity is an important consideration in substation grounding system design. The

lower the resistivity, the easier it is to get a good ground resistance.

Areas of high soil resistivity and those with ground frost (which inturn causes the soil

resistivity to increase by orders of magnitude) need special consideration. The highest ground

resistivity during the annual weather cycle should form the basis of the design since the same

soil will have much higher resistivity during dry weather when percentage of moisture in the

ground becomes very low.


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Grounding Tests: Earth Potential and Grounding Mesh Effectiveness.

One approachto take care of this problem is to use deep driven ground rods so that they are

in contact with the soil zone deep enough to remain unaffected by surface climate.

The other approachis to treat the soil around the ground rod with chemical substances that

have the capacity to absorb atmospheric/soil moisture.

Use of chemical rods is one such solution.

5. Attention to Step and Touch Potentials

Limiting step and touch potential to safe values in a substation is vital to personnel safety.

Step potentialis the voltage difference between a persons feet and is caused by the voltage

gradient in the soil at the point where a fault enters the earth. The potential gradient is

steepest near the fault location and thereafter reduces gradually. Just 75 cm away from the

entry point, voltage usually will have been reduced by 50%.

Thus at a point of75 cm f rom the fault(which is less than the distance of a normal step), afatal potential of a few kilovolts can exist.


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Touch potentialrepresents the same basic hazard, except the potential exists between the

persons hand and his or her feet. This happens when a person standing on the ground

touches a structure of the substation, which is conducting the fault current into ground (for

example, when an insulator fixed on a gantry flashes over, the gantry dissipates the current

to earth).

Since the likely current path within the human body runs through the arm and heart region

instead of through the lower extremities, the danger of injury or death is far greater in this

case. For this reason, the safe limit of touch potential is usually much lower than that of step

potential.

In both situations, the potential can essentially be greatly reduced by an equipotential wire

mesh safety matinstalled just below ground level.

This mesh will have to be installed in the immediate vicinity of any switches or equipment a

worker might touch, and connected to the main ground grid. Such an equipotential mesh willequalize the voltage along the workers path and between the equipment and his or her feet.

With the voltage difference (potential) thus essentially eliminated, the safety of personnel is

virtually guaranteed.

An equipotenti al wi re mesh safety mat is usuall y fabricated from #6 or #8 AWG copper or

copper-clad wire to form a 0.5 0.5 m or 0.5 1 m mesh. M any other mesh sizes are

available.

To ensure continuity across the mesh, all wire crossings are brazed with a 35% silver alloy.

Interconnections between sections of mesh and between the mesh and the main grounding

grid should be made so as to provide a permanent low-resistance high-integrity connection.

6. Grounding Using Building Foundations

The concrete operation to Building control foundation

Concrete foundations below ground level provide an excellent means of obtaining a low-

resistance ground electrode system. Since concrete has a resistivity ofabout 30 m at 20 C,

a rod embedded within a concrete encasement gives a very low electrode resistance compared

to most rods buried in the ground directly.


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Since buildings are usually constructed using steel-reinforced concrete, it is possible to use

the reinforcement rod as the conductor of the electrode by ensuring that an electrical

connection can be established with the main rebar of each foundation.

The size of the rebar as well as the bonding between the bars of different concrete members

must be done so as to ensure that ground fault currents can be handled without excessiveheating.

Such heating may cause weakening and eventual failure of the concrete member itself.

Alternatively, copper rods embedded within concrete can also be used.

The use of Ufer grounds (named after the person who was instrumental in the development

of this type ofgrounding practice) has significantly increased in recent years. Ufer grounds

utilize the concrete foundation of a structure plus building steel as a grounding electrode.

Even if the anchor bolts are not directly connected to the reinforcing bars (rebar), their closeproximity and the conductive nature of concrete will provide an electrical path.

There are a couple of issues to be considered while planning for grounding using the

foundations as electrodes. A high fault current (lightning surge or heavy ground fault) can

cause moisture in the concrete to evaporate suddenly to steam.

This steam, whose volume is about 1800 times of its original volume when existing as liquid,

produces forces that may crack or otherwise damage the concrete. The other factor has to do

with ground leakage currents. The presence of even a small amount of DC current will cause

corrosion of the rebar. Because corroded steel swells to about twice its original volume, it can

cause extremely large forces within the concrete.

Although AC leakage will not cause corrosion, the earth will rectify a small percentage of the

AC to DC. In situations where the anchor bolts are not bonded to the rebar, concrete can

disintegrate in the current path.

Damage to concrete can be minimized either by limiting the duration of fault current flow

(by suitable sensitive and fast acting protective devices) or by providing a metallic path from

the rebar through the concrete to an external electrode.

That external electrode must be sized and connected to protect the concretes integrity. Properdesign ofUfer groundsprovides for connections between all steel members in the foundation

and one or more metallic paths to an external ground rod or main ground grid.

Excellent joining products are available in the market, which are especially designed for

joining rebars throughout the construction. By proper joining of the rebars, exceptionally

good performance can be achieved.

An extremely low resistance path to earth for lightning and earth fault currents is ensured as

the mass of the building keeps the foundation in good contact with the soil.
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7. Grounding the Substation Fence

The second most common substation hazard is lack of grounding (photo from IAEI

Magazine)

Metallic fences of substations should be consideredjust as other substati on structures.

The reason for this is that the overhead HV lines entering or leaving a substation may snap

and fall on the fence. Unless the fence is integrated with the rest of the substation grounding

system, a dangerous situation may develop. Persons or livestock in contact with the fence

may receive dangerous electric shocks.

Utilities vary in their fence-grounding specifications, with most specifying that each gate post

and corner post, plus every second or third line post, be grounded. All gates should be bondedto the gate posts using flexible jumpers. All gate posts should be interconnected. In the gate

swing area, an equipotential wire mesh safety mat can further reduce hazards from step and

touch potentials when opening or closing the gate.

It is recommended that the fence ground should be tied into the main ground grid, as it will

reduce both grid resistance and grid voltage rise. Internal and perimeter gradients must be

kept within safe limits because the fence is also atfull potential rise.

This can be accomplished by extending the mesh with a buried perimeter conductor that is

about 1 m outside the fence and bonding the fence and the conductor together at closeintervals (so that a person or grazing animal touching the fence will stand on the

equipotential surface so created).

8. Special Attention to Operating Points

To protect the operator in case of a fault, it should be ensured that he is not subjected to high

touchorstep potenti alswhen a fault happens in the equipment he is operating.

This calls for use of a safety mesh close to these operating points on which the operator willstand and operate the equipment.


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There are four types of safety mats.

1.A steel grate or plate on supporting insulators. This works only if the operator can be kept

completely isolated on the grate. Therefore, insulators must be kept clean.

Any vegetation in the vicinity should be cut or eliminated completely (this approach issimilar to the insulating rubber mats placed in front of most indoor electrical equipment).

Safety is ensured by increasing the resistance of current path, so that the current flowing

through the operators body into the ground does not exceed safe values.

2. A steel grate on the sur face, permanently attached to the grounded structure. This

arrangement has the operator standing directly on the grate.

3. Bare conductor buried (in a coil or zig-zag pattern) under the handle area and bonded to the

grounded structure.

4. Prefabri cated equipotenti al wi re mesh safety mat buried under the handle area andbonded to the grounded structure. This is likely to be the least expensive choice.

In all but the first arrangement, both the switch operating handle and the personnel safety

grate (or mat) should be exothermically weldedto structural steel, thus ensuring nearly zero

voltage drop.

9.Surge ArrestorsMust be Grounded Properly!

When there is a surge in the electrical system (by indirect lightning strikes or due to

switching) surge arrestors placed near all critical equipment divert surge energy to groundand protect the equipment from being subjected to the surges.

Usually, surges involve a very fast rise timeduring which the current changes from zero to

extremely high values of several kiloamperes. It is therefore necessary that the conducting

path from the grounding terminal of the surge arrestor to the earth must have minimum

impedance.

Even a small amount of self-inductance offered by a grounding conductor will mean very

high impedance because of the steep wavefront of the surge and very high voltages from

appearing in the grounding system (albeit briefly). To dissipate the surge current with

minimum voltage drop, each surge arrestor ground lead should have a short direct path toearth and should be free of sharp bends (bends act like a coil and increase the inductance).

Often surge arrestors are mounted directly on the tank of transformers, close to the HV

terminal bushings. In these cases, the transformer tanks and related structures act as the

grounding path.

I t must be ensured that mul tiple and secure paths to ground are available (this includes

making effective connections).

Whenever there is any question about the adequacy of these paths, it is recommended to use aseparate copper conductor between the arrestor and the ground terminal (or main grounding
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grid). Since steel structures (due to their multiple members) have lower impedance than a

single copper conductor, the grounding conductors should preferably be interconnected to the

structure near the arrestor.

10. Grounding ofCable Trays

Overhead cable trays and ladder racks are jumpered and grounded with AWG #2 bare copper.These conductors, along with the cable bus that collects ground leads from individual

cabinets, are connected to the nearest wall-mounted collector bar.

The NEC vide Ar t. 318specifies the requirements for cable trays and their use in reducingthe induced voltages during a ground fault. All metallic tray sections must be bonded together

with proper conducting interconnections. The mechanical splice plates by themselves may

not provide an adequate and a reliable ground path for fault currents.

Therefore, the bonding jumpers (either the welded type used on steel trays or the lug type)must be placed across each spliced tray joint.

If a metallic tray comes with a continuous grounding conductor, the conductor can be bonded

inside or outside the tray.

When cable tray covers are used, they should be bonded to the tray with a flexible conductor.

The trays should also be bonded to the building steel (usually at every other column).
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11. Temporary Grounding of Normally Energized Parts

Temporary grounding of normally energized parts with ground rod and earth wire clamp

When personnel work on high-voltage electric structures or equipment, any conductivebodies should be groundedas a measure of safety.

This is done so that in the event of the circuit becoming live due to inadvertent switching,

the safety of personnel (in contact with the parts, which would become live) is ensured.

The usual grounding method is to attach a f lexible insulated copper cable with a ground

clamp or lug on each end. These flexible jumpers require periodic inspection and

maintenance. For cable connections to clamps, welded terminations (either a welded plain

stud or a threaded silicon bronze stud welded to the conductor end) will provide a secure,

permanent connection.

The clamp or lug is soli dly connected to ground, then the other clamp is attached to the cable

being grounded.


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What Is Step and Touch Potential and ReducingResistance To Ground?

Step PotentialStep potential is the step voltage between the feet of a person standing near an energized grounded

object. It is equal to the difference in voltage, given by the voltage distribution curve, between twopoints at different distances from the electrode. A person could be at risk of injury during a fault simply

by standing near the grounding point.

Touch PotentialTouch potential is the touch voltage between the energized object and the feet of a person in contact

with the object. It is equal to the difference in voltage between the object and a point some distance

away. The touch potential or touch voltage could be nearly the full voltage across the grounded object if

that object is grounded at a point remote from the place where the person is in contact with it. For

example, a crane that was grounded to the system neutral and that contacted an energized line would

expose any person in contact with the crane or its uninsulated load line to a touch potential nearly equal

to the full fault voltage.

Step PotentialWhen a fault occurs at a tower or substation, the current will enter the earth. Based on the distribution

of varying resistivity in the soil (typically, a horizontally layered soil is assumed) a corresponding voltage

distribution will occur. The voltage drop in the soil surrounding the grounding system can present

hazards for personnel standing in the vicinity of the grounding system. Personnel stepping in the

direction of the voltage gradient could be subjected to hazardous voltages.

In the case of Step Potentials or step voltage, electricity will flow if a difference in potential exists

between the two legs of a person. Calculations must be performed that determine how great the

tolerable step potentials are and then compare those results to the step voltages expected to occur at

the site.

Hazardous Step Potentials or step voltage can occur a significant distance away from any given site. The

more current that is pumped into the ground, the greater the hazard. Soil resistivity and layering plays a

major role in how hazardous a fault occurring on a specific site may be. High soil resistivities tend to

increase Step Potentials. A high resistivity top layer and low resistivity bottom layer tends to result in

the highest step voltages close to the ground electrode: the low resistivity bottom layer draws more

current out of the electrode through the high resistivity layer, resulting in large voltage drops near the

electrode. Further from the ground electrode, the worst case scenario occurs when the soil hasconductive top layers and resistive bottom layers: in this case, the fault current remains in the

conductive top layer for much greater distances away from the electrode.


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Fault clearing time is an important factor to consider as well. The more time it takes the electric utility

company to clear the fault, the more likely it is for a given level of current to cause the human heart to

fibrillate.

An important note to remember is that most power companies use automated re-closers. In the event of

a fault, the power is shut off and then automatically turned back on. This is done in case the faults

occurred due to an unfortunate bird that made a poor choice in where to rest, or dust that may havebeen burned off during the original fault. A few engineers believe that Fibrillation Current for Step

Potentials must be far greater than Touch Potentials, as current will not pass through any vital organs in

the former case. This is not always true as personnel that receive a shock due to Step Potentials may fall

to the ground, only to be hit again, before they can get up, when the automatic re-closers activate.

Touch PotentialWhen a fault occurs at a tower or substation, the current will pass through any metallic object and enter

the earth. Those personnel touching an object in the vicinity of the GPR will be subjected to these

touch voltages which may be hazardous.

For example if a person happens to be touching a high-voltage tower leg when a fault occurs, the

electricity would travel down the tower leg into the persons hand and through vital organs of the body.

It would then continue on its path and exit out through the feet and into the earth. Careful analysis is

required to determine the acceptable Fibrillation Currents that can be withstood by the body if a fault

were to occur.

Engineering standards use a one-meter (3.28 ft) reach distance for calculating Touch Potentials. A two-

meter (6.54 ft) reach distance is used when two or more objects are inside the GPR event area. For

example, a person could be outstretching both arms and touching two objects at once such as a tower

leg and a metal cabinet. Occasionally, engineers will use a three-meter distance to be particularly

cautious, as they assume someone may be using a power tool with a power cord 3 meters in length.

The selection of where to place the reference points used in the Touch Potential or touch voltage

calculations are critical in getting an accurate understanding of the level of hazard at a given site. The

actual calculation of Touch Potentials uses a specified object (such as a tower leg) as the first reference

point. This means that the further away from the tower the other reference point is located, the greater

the difference in potential. If you can imagine a person with incredibly long arms touching the tower leg

and yet standing many dozens of feet away, you would have a huge difference in potential between their

feet and the tower. Obviously, this example is not possible: this is why setting where and how far away

the reference points used in the touch calculation is so important and why the one-meter rule has been

established.


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Mitigating Step and Touch Potential hazards is usually accomplished through one or more of the

following three (3) main techniques:

1. Reduction in the Resistance to Ground of the grounding system2. Proper placement of ground conductors3. The addition of resistive surface layers

Understanding the proper application of these techniques is the key to reducing and eliminating any

Ground Potential Rise hazards. Only through the use of highly sophisticated 3-dimensional electrical

simulation software that can model soil structures with multiple layers and finite volumes of different

materials, can the engineer accurately model and design a grounding system that will safely handle

high-voltage electrical faults.

Reducing Resistance to GroundReducing resistance to ground (RTG) of the site is often the best way to reduce the negative effects of

any Ground Potential Rise event, where practical. The Ground Potential Rise is the product of the fault

current flowing into the grounding system times the resistance to ground of the grounding system. Thus,

reducing the Ground Potential Rise will reduce the Ground Potential Rise to the degree that the fault

current flowing into the grounding system does increase in response to the reduced Ground Potential

Rise. For example, if the fault current for a high-voltage tower is 5,000 amps and the resistance toground of the grounding system is 10-ohms, the Ground Potential Rise will be 50,000 volts. If we reduce

the resistance to ground of the grounding system down to 5-ohms and the fault current increases to

7,000 amps as a result, then the Ground Potential Rise will become 35,000 volts.

As seen in the example above, the reducing resistance to ground can have the effect of allowing more

current to flow into the earth at the site of the fault, but will always result in lower Ground Potential Rise

values and step and touch voltages at the fault location. On the other hand, further away from the fault

location, at adjacent facilities not connected to the faulted structure, the increase in current into the

earth will result in greater current flow near these adjacent facilities and therefore an increase in the

Ground Potential Rise, touch voltages and step voltages at these facilities. Of course, if these are low to

begin with, an increase may not represent a problem, but there are cases in which a concern may exist.

Reducing the resistance to ground can be achieved by any number of means as discussed earlier in this

chapter.

Proper Placement of Ground ConductorsA typical specification for ground conductors at high-voltage towers or substations is to install a ground

loop around all metallic objects, connected to the objects; keep in mind that it may be necessary to vary

the depth and/or distance that ground loops are buried from the structure in order to provide the

necessary protection. Typically these ground loops require a minimum size of 2/0 AWG bare copper

conductor buried in direct contact with the earth and 3-ft from the perimeter of the object, 18 inches

below grade. The purpose of the loop is to minimize the voltage between the object and the earth

surface where a person might be standing while touching the object: i.e., to minimize Touch Potentials.

It is important that all metallic objects in a GPR environment be bonded to the ground system to

eliminate any difference in potentials. It is also important that the resistivity of the soil as a function of

depth be considered in computed touch and step voltages and in determining at what depth to placeconductors. For example, in a soil with a dry, high resistivity surface layer, conductors in this layer will

be ineffective; a low resistivity layer beneath that one would be the best location for grounding

conductors. On the other hand, if another high resistivity layer exists further down, long grounding rods

or deep wells extending into this layer will be ineffectual.

It is sometimes believed that placing horizontal grounding loop conductors very close to the surface

results in the greatest reduction in Touch Potentials. This is not necessarily so, as conductors close to

the surface are likely to be in drier soil, with a higher resistivity, thus reducing the effectiveness of these

conductors. Furthermore, while Touch Potentials immediately over the loop may be reduced, Touch

Potentials a short distance away may actually increase, due to the decreased zone of influence of these

conductors. Finally, Step Potentials are likely to increase at these locations: indeed, Step Potentials can

be a concern near conductors that are close to the surface, particularly at the perimeter of a grounding

system. It is common to see perimeter conductors around small grounding systems buried to a depth of3-ft below grade, in order to address this problem.
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Reducing Step and Touch Potential HazardsOne of the simplest methods of reducing Step and Touch Potential hazards is to wear Electric Hazard

Shoes. When dry, properly rated electric hazard shoes have millions of ohms of resistance in the soles

and are an excellent tool for personnel safety. On the other hand, when these boots are wet and dirty,

current may bypass the soles of the boots in the film of material that has accumulated on the sides of

the boot. A wet leather boot can have a resistance on the order of 100 ohms. Furthermore, it cannot be

assumed that the general public, who may have access to the outside perimeter of some sites, will wearsuch protective gear.

Another technique used in mitigating Step and Touch Potential hazards is the addition of more resistive

surface layers. Often a layer of crushed rock is added to a tower or substation to provide a layer of

insulation between personnel and the earth. This layer reduces the amount of current that can flow

through a given person and into the earth. Weed control is another important factor, as plants become

energized during a fault and can conduct hazardous voltages into a person. Asphalt is an excellent

alternative, as it is far more resistive than crushed rock, and weed growth is not a problem. The addition

of resistive surface layers always improves personnel safety during a GPR event.

Telecommunications in High-Voltage EnvironmentsWhen telecommunications lines are needed at a high-voltage site, special precautions are required to

protect switching stations from unwanted voltages. Running any copper wire into a substation or toweris going to expose the other end of the wire to hazardous voltages and certain precautions are required.

Industry standards regarding these precautions and protective requirements are covered in IEEE

Standard 387, IEEE Standard 487, and IEEE Standard 1590.These standards require that a Ground

Potential Rise study be conducted so that the 300-volt peak line can be properly calculated.

To protect the cell site and communication towers, telecommunication standards require that fiber-optic

cables be used instead of copper wires within the 300-volt peak line. A copper-to-fiber conversion box

must be located outside the GPR event area at a distance in excess of the 300-Volt Peak or 212-Volt

RMS line. This is known in the industry as the 300-volt line. This means that based on the calculation

results, copper wire from the telecommunications company may not come any closer than the 300-volt

peak distance. This is the distance where copper wire must converted over to fiber-optic cable. This can

help prevent any unwanted voltages from entering the phone companies telecommunications network.

The current formulae for calculating the 300-volt line, as listed in the standards, has led to

misinterpretation and divergences of opinion, resulting in order-of-magnitude variations in calculated

distances for virtually identical design input data. Furthermore, operating experience has shown that a

rigorous application of theory results in unnecessarily large distances. This has caused many

compromises within the telecommunications industry. The most noted one is a newer standard, IEEE

Standard 1590-2003, that lists a 150-meter (~500-foot) mark as a default distance, if a ground

potential rise study has not been conducted at a given location.
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steps to ensure effective substation grounding

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