Grounding 10

[Engcan]

Hi,
I was given a report which came from recommendation from a electrical contractor, which has Professional Engg. with them about maintenance of a switchgear.

One of the recommendations was put a "Rubber-Mat" in front so that a person could stand while a person is operating a breaker.

Now here is the question:
A few months back, the local safety authority asked us to put a ground mat made of cooper cage in front of a wooden pole so that person could stand when he is operating the load break switch. It was on 15 KV. And the handle of load break switch should be connected through a 2/0 AWG wire to this mat and this mat should be grounded with a 10' road.

OK Now, I understand if there is a ground fault then the switch and person standing on mat will be at the same potential. I hope that is the reason just to have same potential.

Now in case of switchgear, if a ground fault happens, for example a breaker get shorted to ground that what will happen to person operating the breaker and standing on rubber mat. If nothing happens to him, why cannot we put a rubber mat in front of pole holding a load break switch connected to over head distribution line.

This thing is confusing me, please explain what I am missing here.
Thanks

 

[Skogsgurra]

A rubber mat is the better choice - at least in my thinking.

The copper mat and the different leads will produce voltage drops due to high fault currents. It would - still in my mind - be better if the ground rod weren't there. Then there wouldn't be any fault current and no voltage drop.

Gunnar Englund

http://www.gke.org

 

[waross]

These are two different methods of affording personal safety.
Don't doubt the reasons for or wisdom of the ground mat in front of the high voltage switch.
In the instance of the high voltage switch, The copper mat connected to the switch handle assures that the operators feet are at the same potential as his hand, thus no current will flow through his body. This is protection by maintaining an equal potential.

In the case of the proposed mat for the switch gear, the purpose is to prevent current from flowing in the workers body by insulating him.
If for some reason, a fault in the breaker causes the breaker handle to become energised, the insulating mat will prevent a current flow to ground through the workers body.

Given the different voltages and currents likely to be present during a fault on either type of system, each method is appropriate. Also considering the environment each will be placed in and maintainance or possible lack of maintainance, both types of protection are appropriate as generally used.
A warning, Some grades of rubber are conductive. If you purchase a protective mat, please assure yourself that it is approved for life safety protection. Don't send someone to pick up the cheapest rubber mat at the local hardware.

Your specific question, why can't we use a rubber mat in front of a high voltage switch? The mat would have to be to withstand at least 15KV. It may be able to do this when it is new but when it is wet or dirty or sun baked and cracking it is doubtfull that it could be depended upon to provide protection. The copper grid will last for decades without attention. A rubber mat outside in the weather just could not be depended upon.
yours

 

[itsmoked]

Nice summation waross!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

- kcress@<solve this puzzle>

 

[dpc]

The high voltage switch operating handle is grounded via a flexible strap. In the event of a local fault while operating the switch, the grounded metal mat (we normally use aluminum grating) and the switch handle will be kept near the same potential, minimizing current flowing through the operator.

In the case of the rubber mat, the intent is prevent a flow of current if the breaker became energized due to a fault in the breaker during operation.

 

[waross]

Hello skogsgurra
Actually the ground grids are common in North America. Some one will probably post the specifications, but typically the grid may be on 6" centers and large enough that the worker must be standing on it with both feet when operating the switch. Maybe 1.5m x 1.5m. Some are on the surface,- some are buried in loose crushed rock. It depends on the local authority.
"It would - still in my mind - be better if the ground rod weren't there."
I agree. I would rather see the ground rod connected to the switch handle than to the grid, but I don't get to say. That's decreed by the UHJ.
respectfully

 

[Engcan]

Thanks Waross,
So from your explanation, can I presume that had there been a rubber material that could sustain this weather thing, we might had rubber grounding grids instead of copper. Suppose inside a substation,near a generating station, where normally gen voltage is around 13.8 kV we , use a ground mat on the whole floor, we need not worry about step and touch potentials. what is your opinion.
Will this work?
Thanks

 

[waross]

I think that the answer to your question is more in the real world than in the theoretical world. It's a question of two different types of protection and which one is appropriate.
With the outdoor pole mounted switch, there is typicaly one point of contact with the device to be operated. It is quite simple and dependable to provide equipotential protection. Also, the currents are typically less on primary circuits. With primary currents there is not much chance that current flow will cause enough IR (Current x resistance) voltage drop in the 2/0 wire to injure the operator.

The following thoughts come to mind regarding a rubber mat for protection when operating a pole mounted switch.
Personally I would consider the mat to be eqivalent to a hot stick. That is, it would be part of the equipment set of the operator or crew. It must be large enough that there was no chance that the worker could step off of it while operating the switch.
The crews would have to be trained in its use. It would be rolled up and stored in a protective container in a dry environment when not in use. It would be deployed before the switch was operated and then cleaned, dried if necessary, and returned to its protective storage. A mat should be issued to each crew that would ever be required to operate the switch. I doubt if the insulating mat would be accepted by OSHA or other authorities. In the event of death or injury of an operator you would most likely be in a liability positon. And when all was done and documented, I still wouldn't want to use it on a wet stormy night, the kind of night when the weather is conspiring to cause all sorts of faults and surges.
Speaking of surges, the grounded grid is better protection for an operator in the event of nearby lightning strikes than a rubber mat.
Nothing can be designed to protect against a direct lightning strike, but we can provide protection against nearby strikes on the lines. To make it more interesting and posibly more survivable; A lightning strike may be a dierct strike on a line conductor or a strike on the neutral or grounded "Sky wire". A strike on a line wire can flash over the switch to the operating mechanism. A strike on the grounded conductor can cause direct injury, or induce a high voltage on line conductors.
Rubber mat;
In the event of a lightning strike nearby, the voltage will almost certainly exceed the ultimate dielectric strenghth of the mat with terminal results for the operator.
Grounded grid;
The voltage between the switch handle and the ground grid will be a very small percentage of the voltage at the point of lightning strike. The actual voltage that the operator will be exposed to will be the result of the current surge times the impedance of the 3 or 4 feet of #2/0 jumper. This current will be limited by the impedance of the primary line from the point of lightning strike to the ground mat. The lightning discharge current will be further limited by Multiple Protective Earthing on the primary line.
The bottom line is that a nearby lightning strike that would be terminal for an operator standing on a rubber mat may be survivable for an operator on a grounded grid.
Remember, we are not talking about a hypothetical "What if", and "My guess is as good as your guess." We are talking about protecting the life of a fellow human being if and when things go bad.
If the device does not work (whether a ground grid or a rubber mat), the details of what went wrong will most likely be part of a coroners report.
The grounded grid is dependable and relatively immune to the weather for decades.
I hope that accurately explains the reason for a ground grid instead of an insulating mat in front of a pole mounted high voltage switch.
I'll address the insulating mats in a later post.
yours

 

[Skogsgurra]

Agree, I wasn't thinking about weather. We seldom have pole mounted breakers. Most MV industrial switchgear are indoors. Our climate wouldn't allow a rubber mat oudoors.

Gunnar Englund

http://www.gke.org

 

[waross]

I haven't seen insulating rubber mats for many years. When I do see rubber mats in front of switchgear I usually assume that the purpose is to provide better footing. I can't find a reference to rubber mats in the NEC, but I'm open to correction and the mats may be required by other codes. That said, I'm cringing and waiting to see if I get shot down.
I'm not actually a fan of the rubber mat approach. I think that it may promote a false sence of security.
My concerns are two-fold.
First the integrity of the rubber mat. What steps must be considered to keep it clean and free from damage. This depends to some extent on the type of work normally done in the facility and whether it is feasible to de-energize the equipment and remove the rubber mats before heavy maintenance is performed.
Second, in regards to placing rubber mats in front of switchgear, I have no objection to the improved footing, however let's consider the nature of faults in switchgear. Switchgear is designed and constructed to avoid any hazardous voltages on 'Non-current-carrying-parts' or the parts that an operator will normally come in contact with. In the event of catastrophic failure I would be more concerned with the force of the explosion and the arc flash than with the shock hazard.
The rubber mat may afford some protection to electricians working in and around open, energized MCCs as they may be in a position to contact energised parts. However they are just as likely to have the other hand resting on grounded metal, nullifying to a great extent the protection afforded by the rubber mat.
I was checking some prints in an MCC room years ago. There was a violent explosion about 30 feet away. I looked up to see an electrician streched out full length on the floor with his coveralls smoldering and two others standing rigidly nearby. I ran over to the the man on the floor who I thought was dead. I thought that cremation was premature so I extinguished the embers on his coveralls with my hands. As I was slapping his legs to extinguish the embers he started to moan and move. About that time, the other two men who had been out on their feet started to ask what had happened.
The cause of the explosion was a bare #14 ground wire that was being installed and got away and bridged two hot bus bars. The arc was cleared by the instantaneous trips on an 800 amp breaker. (High setting, 8000 amps minimum). The force of the explosion blew off several covers which were properly fastened as per design with 1/4-20 screws into spring clips. We found the covers across the room. Surprisingly the electrician suffered only arc flashes to his eyes. He got drunk that night and was back at work the next morning.
The point is; rubber mats would have been of no use. No-one got a shock.
I worked at a mill in the 60's where we had three events in less than a year as the result of someone doing something foolish in an opened MCC. One man had severe burns to his hand, another was blinded for an hour or so by the flash, and the third had a combination of less serious burns and less serious eye flash. The MCCs were mounted on a dry wooden floor. The only shocks sustained were phase to phase which wouldn't have been avoided by a rubber mat.
Another concern I have is the responsibility someone must assume for the condition of the rubber mats. From the point of view of liability, the mats should be tested at regular intervals to assure that they have not deteriorated, been penetrated or cut and have no dirt or other substance that would lead to surface tracking. Of course records should be kept also.
It's possible but it's not a responsibility that I would want.
I'm sure there are exceptions and differing opinions, but personally I consider rubber mats nice to stand on, but not something that I depend on for protection.
respectfully

 

[waross]

An interesting further note on grids and mats.
According to the Canadian Electrical Code, 1990, here can be problems with comunication equipment between interconnected substations. In the event of a fault, the ground potential between the stations may be in the order of thousands of volts. This difference may show up in interconnected communication equipment and pose a hazard to operators.
In the 1990 Canadian Electrical code the notes in appendix B suggest that the touch and step potential may be limited to safe values by the use of gradient control mats (Grounded grids) and rubber mats.
In the 1998 Canadian Electrical Code, (I have been on a tropical vacation for a while), gradient control mats are still required but there is no mention of rubber mats.
If there are any NEC historians following this thread please let us know of any similar rules and or changes. Thanks.
yours

 

[waross]

A further clarification. The Canadian code requires that the ground grid, or ground rod be connected to the switch handle, not the grounded mat (or gradient control mat). The gradient control mat is then connected to the switch handle. There may still be some current flowing through the connection to the gradient control mat, but it will be less than the main current to ground. This arrangement will further reduce any voltage between the switch handle and the gradient control mat. Canadian code also requires a layer of 150 mm of crushed stone under the mat. You don't want the mat to have a good connection to ground Except through the cable connections. The less current that flows to ground through the gradient control mat, the lower the voltage between the switch handle and the mat.
yours

 

[cuky2000]

Although insulated mats is sometime specified in MV systems, conductive mats are used extensively in industrial, commercial and utility environment and exclusively used in high voltage applications.

As a site note, an electronic search in several IEEE standards including #80, 141 and 142 do not provide any reference associated with insulated grounding mat.

Here are a few illustration and excerpt of typical grounding practice in the US .

http://cuky2000.250free.com/Gnd_Practice_US.jpg

 

[waross]

Thanks for the input cucky2000
I spent a little time without success looking for an illustration of the "Equi-potential grid". Your picture of the switch handle and the interconnections of the various cables is just what I was trying to describe. I note with interest that the OHSHA mat is described as a conductive mat, rather than an insulating mat and complies with equi-potential requirements.
respectfully

 

[cuky2000]

This is a portable conductive mat
See the enclose catalog for reference

http://www.hubbellpowersystems.com/powertest/literature_library/pdfs4lib/Chance-Hotline/3023.pdf

 

[Engcan]

Hi Waross,
Thanks for such an effort of explanation. In your second last post you mentioned "You don't want the mat to have a good connection to ground Except through the cable connections. The less current that flows to ground through the gradient control mat, the lower the voltage between the switch handle and the mat".

The pole is gounded and switch is grounded through the pole and the handle of the switch is again grounded through gradient control mat.
The S&C electric guys asked us that it is advisable to have one ground rod connected to mat and that to of 8 ft length buried. Then the local safety inspector asked for four
rods and then I requested him and he agreed to one.
I told him that we dont have enough space around poles.

Please advise, what is going with this reason.

Best Regards,

 

[waross]

Hello Engcan
The theory of equi-potential protection is just that, equal potentials.
I am going to repeat some stuff we all know so that we're all on the same page.
Rule one;- When a current flows through a resistance, a voltage or potential is developed.
Rule 2;- When a potential is applied to a resistance a current flows.
Rule 3;- The higher the current the higher the voltage.
Rule 4;- The higher the voltage the higher the current.
Rule 5;- The higher the resistance the higher the voltage.
Rule 6;- Voltage doesn't kill you. Current kills you.
The higher the voltage, the higher the current.

Okay, back to the Equi-Potential mats.
Let's consider a hypothetical situation. As you're operating the pole mounted disconnect switch a jumper breaks and drops down onto the gang operating mechanism.
If the switch is grounded, there will be a flow of current, down the operating pipe, through the grounding conductor to the ground rod and from the ground rod into the ground.
All of these circuit components have resistance including the ground. The ground resistance may be fairly high. Fairly high resistance means fairly high voltage drop. (Amps times resistance still works here)
This current flowing through the ground is responsible for "Step Voltages", and at the switch handle "Touch Voltages".
The line voltage will be divided up in proportion to the resistances and impedances of the various circuit components. These include the resistance of the ground, the resistance of the ground rod, the resistance of the grounding conductor, the resistance of the operating pipe, and the impedance of the line back to the source. With dry soil conditions, the major resistances and/or impedances are probably the line impedance and the ground resistance. Compared to these values, the resistance of the grounding conductor and the operating pipe is probably negligible.
This means that the line voltage will divide between the line impedance and the ground resistance.
The voltage drop in the ground resistance can easily develop lethal step voltages and lethal touch voltages on the switch.
Now we will install our copper grid and connect it to the switch. We have two options for the connection to the ground rod. We will examine the least desirable first.
We will run a grounding conductor from the switch to the grid and from there to the ground rod.
Now the voltage between the switch handle and the grid and anyone standing on it will be the product of the current and the resistance of the grounding conductor. There will be a voltage difference between the grid and the switch handle but it will be negligible in proportion to the negligible resistance of the grounding conductor. This will provide fairly good protection for the operator as long as everything is in good condition.
However let's consider the possible and probable failure modes and the implications and results thereof.
Over time the connections may corrode. The connection to the switch handle in particular may fray and break strands because of the movement associated with the switch operation. Corrosion and broken strands result in higher resistance.
See Rule 5;- The higher the resistance the higher the voltage.
Deterioration, corrosion and/or breakage in the connection between the Grid and the switch can result in lethal voltages between the switch and the grid (And the poor operator standing on the grid).
The second option for connection of the various components.
We will connect a ground wire the switch handle and run it directly to the ground rod. We will now place the grid on a bed of crushed rock as per the Canadian Electrical Code. We will then connect the grid to the switch handle with two separate grounding conductors as per the Canadian Electrical Code.
Now very little current will flow to ground from the grid. With the combination of the high resistance of the crushed rock and the low resistance of the two grounding conductors our grid will stay at virtually the same potential as the switch handle.
What was a Grounded Grid is now an Equi-Potential Grid.
Even with frayed or corroded connections the protection will be much better than with the grounded grid. In the case of the grounded grid, the touch voltage was a product of the current and the resistance of the bad connection. The current was relatively high so the voltage would be proportionately higher.
In the case of the Equi-Potential Grid, the voltage is still the product of the current and the resistance but because of the relatively high resistance of the bed of crushed rock, the current will be much less. The two separate connections give redundant protection and it is expected that if one connection breaks off it will be noticed and repaired. The second connection will still be providing protection.

Summary.
A Grounded Grid will provide good protection under good conditions and limit the touch voltage on the switch handle to a negligible amount. Under conditions of corrosion and/or broken or frayed wires, a lethal touch voltage can be developed at the switch handle.
An Equi-Potential Grid will limit the touch voltage on the switch handle to zero volts. Under conditions of corrosion and/or broken or frayed wires, the protection will be much better than with the Grounded Grid. There is also the redundant protection of the second connection wire.

Equi-Potential Protection is a cousin to the Faraday Cage or Shield and works on the same principle.
Hope this helps.
yours

 

[stevenal]

Here's how it was initially explained to me:

The switchman's body is a resistor. How do you limit the voltage across and the current through a resistor? Provide a parallel short circuit path with heavy conductor. Rather than stringing 4/0 through the switchman's sleeves and trousers, it is simpler to bond the handle to a conductive mat below the switchman's feet. Add some more resistance in series with the switchman to represent gloves and shoes and you have further reduced the voltage across and current through the resistance of the switchman. Note that no mention has yet been made of the grounding. This handle to mat bond keeps the switchman's hands and feet at the nearly the same potential regardless of ground path. This bond should be visible. I've seen too many installations where two wires go into the dirt or rock with no assurance to the switchman of any continuity below. Grounding of course must be provided in order for overcurrent protection to operate. Consider the case where handle bond connects to one side of the mat while the ground electrode connects to another. Now you have needlessly exposed the switchman to step potential. The mat will probably handle a great deal of current without much voltage drop, but there is no reason to chance it. Connect the handle bond and electrode both to the same point on the mat and any possible voltage gradient from current through the mat is eliminated.

We have told our people to always keep both feet on the mat. We have in turn been told that when the sparks are flying and the glass is falling that they are out of there. I guess if both feet are airborne its okay.

 

[waross]

Hi stevenal
The explanation you were given is accurate. One thing I would point out again, and this is related to your observation of the step potential that can be developed if the ground current flows through the mat.
My preference and I think a growing trend in the industry, is to GROUND EVERYTHING EXCEPT THE EQUI-POTENTIAL MAT.
Now DON'T GROUND THE EQUI-POTENTIAL MAT BUT CONNECT IT TO THE SWITCH HANDLE.
There is a subtle difference that may someday same a life.
You don't want ground current to flow in the Equi-Potential mat or the connection to it. Its purpose is to maintain the worker at the same potential as the switch handle. If there is a problem with the grounding and the voltage on the switch handle rises to several thousand volts, you want the Equi-Potential mat to rise with it so that there is no voltage difference between the mat and the switch handle. You don't want this action inhibited by a connection to a ground rod.
I know it's scary but it is actually safer.
If the connection is from the switch to the mat and then to ground and a fault current blows off one of the connections between the switch and the mat, then the linemans body is left as the connection to ground and the path for the ground current.
If the Equi-Potential mat is not in the path of the ground current it will still afford protection if a ground connection blows off due to the ground current. (There should be no heavy current in the connection to the Equi-Potential mat. The line man standing on the mat, still connected to the switch, is still at the same potential as the switch whatever that may be. I know there are exceptions and stuff happens, and when the sparks are flying it is quite likely that several things are going bad at the same time.
Nothing is perfect but I believe that both by circuit analysis and field experience the ungrounded Equi-Potential Mat, connected to the switch handle with two cables (For a redundancy faCTOR) is the safest method of protection.
Further to your first sentence, stevenal, Going one step further, the way to further limit the voltage difference across the linemans body is to reduce the current. This is acomplished by teeing off to the Equi-Potential mat rather than making it part of the ground path.
Be aware that the voltage of the switch handle may be considerably above the surrounding ground potential, but the Equi-Potential mat will be at the same voltage as the switch and no current will pass through the linemans body.
Even though the mat is electrically safe, the area around it may have very lethal step voltages.
" I guess if both feet are airborne its okay. " It's not just okay, it may be a matter of life or death. Don't put your left foot down until you have picked your right foot up. Sort of hopping from foot to foot, very quickly!
And yes, if the sky's lighting up and sparks are rainind down I understand the lineman wanting to leave the vicinity. I just hope he remembers to keep one foot in the air at all times for the first 15 or 20 feet, or more.
respectfully

 

[stevenal]

Good points, all. On the last one, though, I doubt he's thinking about anything other than leaving quickly. If he's moving fast, only one foot is on the ground at any time anyway.