Working in a Substation (132 kV)

[Sempronio1960]

A question about a 132 kV substation working.
For photo, link to

http://img849.imageshack.us/slideshow/webplayer.php?id=22131448.jpg

please.
I would want to ask you what you think about a wood barrier, done in it to separate "Vicinity zone" from a "Live working zone" (as in EN 50110-1, "Restricted Approach Boundary" and "Prohibited Approach Boundary" I think in NFPA 70E), photo 1. On the wood barrier there is a electrowelded net. The barrier is tall around 4 meters. The electrowelded net is connected to earth, photo 2. If I touch a nail I feel a light current (I believe of electrostatic origin), greater if I touch the nail and the electrowelded net together, photo 3. Excuse for my poor english.

 

[cuky2000]

I believe that your question is regarding if this improvised barrier is safe operating in the vicinity of a live 132 kV.

What is the purpose of this barrier?. There is plan to work in the deenergized bay in front of the barrier?

Here are a few comments to consider:
1- What you detecting in the nail is a sizable indication of induced voltage at steady state.
2- During assymmetrical short circuit conditions, it is expected that induced voltage should be higher in the nail and more severe in larger metallic elements.
3- The wood could be considered a semiconducting plane variable with the moisture content and the level of induced voltage in the metallic rebar/screen.
4- The grounding on the rebar appear to be poor quality.
5- Clearance from the improvised barrier to live parts is unknown. Should be checked again applicable local safety code/standard.
6- The allowable step and touch potentials standing in native grassy soil, could be improved with a layer of crushed rock and local ground grid or working in a equipotential plane.

 

[electricpete]

If I touch a nail I feel a light current (I believe of electrostatic origin), greater if I touch the nail and the electrowelded net together, photo 3.

I think that might be a result of capacitive electric field coupling, similar to what I mentioned in my post 1 Apr 11 12:26 (not an April fool's joke) here: thread238-295720

I doubt it would be dangerous. I think it acts like a high impedance source which can create relatively high open-circuit voltage, but limits the current to very low values. I don't know much about regulatory requirements.

Just my thoughts fwiw. I could be wrong.




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(2B)+(2B)' ?

 

[cuky2000]

Hi Electricpete,

Safety is a relative term that could be defined with respect to a thrershold value(s)

For example, the NESC limit the induced current to 5 mA. Other recognized codes and guidelines limit the induced voltage to 15 V.

 

[jghrist]

I agree with electricpete, but it would indicate that the fence is not well grounded. Grounding should dissipate the capacitive coupled voltage.

 

[electricpete]

To follow on my earlier comments, here is how I envision the situation:

Vs===Xc1===Vnail===Xc2===Fence/Ground.

Vs is an energized conductor (138/sqrt3 kv to ground)
I assume the nail is floating at voltage Vnail and the fence is well grounded.

Physically the nail is within 1 or 2" of the Fence/ground, but maybe 40" (1 meter) from the energized conductor, Vs.

Xc1 is capacitive impedance between Vs and the nail. It is huge... usually considered an open circuit.

Xc2 is capacitive impedance between nail and Fence ground. It is also fairly large, but much smaller than Xc2 since the nail is much closer to Fence/Ground than to 138kv system.
Xc2 << Xc1

Under open circuit conditions shown above (measure Vnail to ground using an ideal infinite-input-impedance meter), we can find Vnail as:
Vnail = Vs* Xc2 / (Xc2+Xc1)~ Vs* Xc2 / Xc1

If Xc1 is 100 times Xc2, then we can have Vnail = 1/100 * 138kvac/sqrt(3) = 800 vac... still pretty darned high.

What is the short circuit current? I will claim if you are at a safe distance like 1 meter, then the short circuit current is very small. It is the same capacitive current you would see if you connected a superconducting wire from that nail to ground and measured resulting current, which I hope you agree is tiny since that capacitive current is still flowing through 1 meter of open air. I think it is safe to say we are not getting anywhere near 5 microamps, much less 5 milliamps.

As long as that 138kv ac voltage doesn't arc over to the nail/fence (and I assume it is far enough away to avoid that), then it is not a dangerous nail to touch imo (fwiw *)..... by virtue of the fact that the very high impedance Xc1 will limit current to miniscule levels (even though voltage is appreciable).

If dimensions are known we can begin to put numbers to those capacitances.

* By the way, I am not telling anyone what to do. You are responsible for safety of your installation. I'm just talking through the electrical circuit question as best I know how.


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(2B)+(2B)' ?

 

[electricpete]

I think it is safe to say we are not getting anywhere near 5 microamps, much less 5 milliamps.

Hmmm. Maybe not. If it is above threshold of perception, then it is higher. There is also the human impedance to put into the equation. I still think it's safe. I know we talked through this back when we were looking at compressor cabinet door and decided it was safe... right before we attached a groundwire to the door.

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(2B)+(2B)' ?

 

[electricpete]

To followup some more…. we are interested in capacitance and shoft-circuit current coming from a 138kv L-G conductor and a nail located some distance away, let’s say d = 1 m.

As a first cut, let’s compare to parallel plate capacitor with conservatively large area…. Let’s say 1 m^2 and fringing neglected.

C = Eps * A / d = Eps0 * 1m^2 / 1m = 8.854E-12 F.

I = V *(2*pi*F*C) = 138,000/sqrt(3)*2*pi*60*8.854E-12 = 0.00025A = 0.25 milliAmps.

This assumes a very large area. I’m pretty sure we could come up with a closer approximation if we look for a more applicable geometry, but I am also pretty sure an actual nail and conductor separated by 1 meter would give lower short circuit current than the 1m^2 parallel plate capacitor.


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(2B)+(2B)' ?

 

[Sempronio1960]

Thanks to all for you answers. Also for you the touch potentials on the nail is by elctrostatic source.

I believe that your question is regarding if this improvised barrier is safe operating in the vicinity of a live 132 kV.

For photos, link to

http://img827.imageshack.us/slideshow/webplayer.php?id=87014157.jpg

please
We are operating in "vicinity zone", as in EN 50110-1:2005-02 (see photos 1 and 2) or in "resctricted space", as in NFPA 70E (see photo 3). We are operating in the green arrow side ("non in tensione", see previous photos, at right of the wood wall).

What is the purpose of this barrier?. There is plan to work in the deenergized bay in front of the barrier?

The purpose of the barrier is to prevent the entry in the "live working zone", EN 50110-1:2005-02, or in the "Prohibited space", NFPA 70E. There is a plan to work in in the green arrow side (deenergized). I don't like the electrowelded net becouse, imho, as it is connected at ground grid at an end point, it can cause touch potential.
What do you think about?

 

[rasevskii]

It would probably be safer with only the wooden fence and without the electrowelded screen. Putting up any kind of temporary metal lashup inside an energized substation is asking for an accident.
Suppose there is a windstorm and it tips over into the energized section...

The metal screen shown is not well grounded it would appear.

Why is there concern about nailheads? There is only capacitive coupling there and no one would get hurt by touching one of them.

I suppose this is some kind of legal question rather than technical, and some inspection requirements have to be satisfied. "Oversafe" but more dangerous...

rasevskii

 

[rasevskii]

A simple idea would be to just put insulating tape over each nailhead. That would keep someone busy for awhile, and calm down the safety inspectors...

rasevskii