ground switching on transmission poles 4

[Fire17]

Hello all, I'm a new Engineer for a utility power, and I'm trying to figure out what other alternatives is out there for ground mats or equi-potential mats, these suggestions come from the linemen when they find difficult to install these mats on transmission switching poles generally, one suggestion the made was to put ground butt plates in a perimeter as if there was a ground mat, if that makes sense.

Thanks.

 

[waross]

When a task is difficult, the professional tries harder.
Without knowing the reason for the difficulty it is difficult to suggest a solution.
The operator MUST be held at the same potential as the handle that he is operating.
That is the purpose of an equi-potential mat.
The equi-potential mat must be large enough that the operator is not able to operate the switch with either foot off of the mat.
Are there space limitations?
A possible solution may be a permanently installed custom sized mat that fills all of the available space, but without galvanic contact with any grounded object save the ground that it rests on.
BE aware of the possible liability in the event of an injury or fatality occurring should the custom mat not be installed correctly, not used correctly or not maintained correctly.
Your safety department should sign off on any solution.
I would include the following procedure:
Before use, the equi-potential mat must be checked with the following procedure:
1. The mat must be disconnected from the operating handle and the resistance to ground must be checked.
The resistance to ground must be above xxx Ohms.
2. The mat must be reconnected to the operating handle and the resistance between the mat and the operating handle must be checked.
The resistance between the operating handle and the equi-potential mat must be zero Ohms.
If these conditions can not be met, than the mat must not be used as is.

1. The mat may be removed and re-installed and rechecked.
If the mat does not pass the recheck, it must not be used.
If the mat now passes the test it may be used.
2. A portable mat may be installed and used.

Note #1, The check procedure is a good idea for every equi-potential mat before every use.
That is for both portable equi-potential mats and permanent equi-potential mats.
Note #2 When a task is difficult, the professional tries harder.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[bacon4life]

I am not picturing quite what you are thinking of installing. Pole butt plates may not be large enough for the fault currents on a transmission switch, and may be too thin to avoid excessive corrosion losses.

Some options of variable quality:
1)Mount the switch higher on the pole, requiring operation by bucket truck or climbing.
2)Install motor operators, which reduces how often a worker must operate the handle.
3)Install permanent mats, with means of verifying the grounding connection.
4)Install copper conductors underground to make an equipotential zone. Unfortunately, there is no way for workers to verify that a buried mat still exists when they go to do switching.
5)Move the location of the switch to a pole where installing a portable mat is easier.

 

[waross]

I am not picturing quite what you are thinking of installing. Pole butt plates may not be large enough for the fault currents on a transmission switch, and may be too thin to avoid excessive corrosion losses.

The theory and operation of equi-potential mats is to prevent and avoid ground current passing.
An equi-potentialmat mat must keep the mat and the switch handle at the same potential. If a small ground current does inadvertently pass the equi-potential mat wiring, the connection to the switch handle must be a low enough impedance to hold the voltage difference between the switch handle and the mat at close to zero Volts.
The operation of an equi-potentail mat is related to Faraday cage theory. The mat keeps the worker and the switch handle at the same potential even though the switch handle (and the mat and the worker) may become energised at line to ground voltage due to failed or broken equipment.
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Fire17]

Let me re-state my question, how would you properly ground a transmission switching pole in granite or basalt when you can't even drive a ground rod ? the normal procedure is to install a ground mat...

 

[waross]

You completely misunderstand the principle of an equi-potential mat.
AN EQUI-POTENTIAL MAT IS NOT A GROUNDING DEVICE.
A PROPERLY USED EQUI-POTENTIAL MAT DOES NOT GROUND AND SHOULD NOT GROUND.
IN THE EVENT OF A SWITCH FAULT DURING SWITCHING, AN INADVERTENTLY GROUNDED EQUI-POTENTIAL MAT MAY LEAD TO INJURY OR DEATH OF THE OPERATOR.
THE ONLY CONNECTION OF AN EQUI-POTENTIAL MAT IS TO THE HANDLE OF THE SWITCH BEING PROTECTED.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[FacEngrPE]

Granite and basalt are both poor conductors. Setting a equi-potential mats on an insulating surface when connected only to the handle the lineman will be touching should reduce the step potential between the mat/handle and the protected lineman as Long As the Lineman only touches the handle while standing on the mat. If there is a fault and the lineman steps off of the mat,

I have attached the instructions for the Hubble - Chance Equi-mat. The instructions clearly state the points waross makes.

See also training material
Grounding Video – Chance® (Temporary Protective Grounding)

Understanding an Equipotential Protective Grounding Zone
My employer lost an apprentice lineman due a line fault on a de-energized 10kV circuit. The post event examination was not able to determine anything about the fault other than it was a low current event, and that the circuit was disconnected and no power was present when measured later. Grounding / bonding practice during deenergized work was implicated and changed. This event was about 45 years ago.

 

[waross]

Thanks for the support FacEngrPE.
Near where I was working, I saw a crew connect a permanent equi-potential mat and then to be extra safe (They thought) they ran a jumper from the mat to the structure grounding electrode.
They didn't believe me, but I made such a fuss that they checked with their engineering department, (I guess so that they could put me down).
The next day they removed the extra jumper and apologized to me.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[FacEngrPE]

Your are welcome. I was trying to be a bit scary, for I think good reason, this subject deserves it.

 

[jghrist]

How does the additional connection of the mat to the structure grounding mat significantly increase voltage between the operator's feet and the handle? There would now be a parallel path for fault current from the handle to ground. One down the pole ground to earth and one down the handle-mat connection to earth. If half of a 10kA ground fault followed the handle-mat connection with 10 ft of 2/0 copper (resistance = 0.000811 x 10), the voltage would only be 4 volts.

The extra connection provides safety in the event of a bad connection in the handle-mat path.

 

[waross]

In the event that the fault current path is down the switch actuator mechanism, all of the fault current may pass through the mat if the mat has a connection to ground.

the voltage would only be 4 volts.

And in the event that someone is injured or dies due to an improperly installed mat, you may have the opportunity to re-evaluate your calculations.
(Well I didn't count on that factor.)


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[jghrist]

In the event that the fault current path is down the switch actuator mechanism, all of the fault current may pass through the mat if the mat has a connection to ground.

The switch handle will be connected to the pole grounding wire (or structure if it is steel), so the current will flow through the pole grounding wire to the pole grounding electrode. If the switch handle is not connected to the pole grounding wire, then the only path to ground is through the connection to the ground mat which is sitting on the ground but not bonded to the grounding electrode. That would be a very dangerous condition.

 

[waross]

If the switch handle is not connected to the pole grounding wire, then the only path to ground is through the connection to the ground mat which is sitting on the ground but not bonded to the grounding electrode.

The fairly high resistance of the mat to ground will limit the current.
Yes, the switch handle and the mat will both be at a high potential, but both will be at the same potential.
Just in case the switch handle was connected to the pole grounding wire years ago and the pole grounding wire is no longer dependable.
Industry standard:
Connect the mat to the switch handle.
Make no other connections to the mat.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[jghrist]

Can you cite an industry standard? I've been specifying two connections to substation switching platforms for a long time. Even if all the fault current flowed through the mat, the voltage drop in the handle-mat wire would only be a few volts. I wouldn't want to be standing on a ground mat holding on to an energized switch handle. Without a solid ground, the mat would be like a downed conductor.

 

[jghrist]

See RUS 25_14 kV Construction Bulletin 1728F-803 Dwg H4.1

https://www.rd.usda.gov/sites/default/files/UEP_Bulletin_1728F-803.pdf

 

[cranky108]

Note, the application in a substation, and outside the substation may be different. That is, the substation should have a ground grid, where outside the substation you do not. The step and touch potential should be small, because of the ground grid, below the substation rock.
Outside the substation, no clue what you may have.

 

[waross]

If half of a 10kA ground fault followed the handle-mat connection with 10 ft of 2/0 copper (resistance = 0.000811 x 10), the voltage would only be 4 volts.

Would you care to re-cslculate with the addition of combinations of contact resistance included in the calculations.
After years of service, it may be dangerous to assume that none of the connections has developed increased resistance due to corrosion or damage.
Over time, connections between similar metals are particularly susceptible to increased contact resistance.
Long story but I was asked, (unofficially), for advice by an engineer investigating a fire in a building caused by a high voltage, (13.8 kV) superimposed on the 120:240 Volt distribution system.
The high voltage punctured the insulation in the 120;240 Volt wiring system in a building and the resulting arc started a fire.
The power was eventually cleared manually.
How could this happen and not trip something immediately?
All of the distributed grounds or pole grounds in the had become open or high resistance due to age neglect and lack of inspection/maintenance.
I much prefer portable equi-potential mats, that are installed and connected for each use.
There is only one connection to be made, and the lineman should be inspecting the mat and making the connection as if his life depended on it.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[crshears]

Yet it is so easy to slip back into calling equi-potential mats "ground mats"; if only the latter didn't flow much more thoughtlessly and effortlessly from the tongue . . .

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[crshears]

How about "switching mat"?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]