Touch shock at 345kV breaker control panel 1

[pwrengrds]

I was at a wind park substation (345 kV / 34.5 kV) in the central US (Oklahoma) working on other projects and was asked to look at the main 345 kV breaker control panel. It is grounded to the ground mat (4/0 bare copper), both at the control panel and the breaker frame. When touching the padlock to open the door it had slight visual arcing and shocks when touched, both the padlock, and door handle. It's fairly minor, almost like a static charge. Once the door was opened it could be felt on the edge of the door. The interior was grounded at the ground pad inside the panel.

I tried to measure it using a fluke to ground on both AC and DC, didn't see any voltage. It seemed to decrease when we turned off the 120 VAC, but it could have been in my head. I measured the resistance to ground, from the grounded grate we were standing on, it is grounded.

Any ideas as to the cause and how to fix it? The weather was dry and clear. It has been a noted problem since start up two years ago.

 

[itsmoked]

It's likely shrimp. (skogs LOL)

I'm very surprised you can continually feel something but not measure anything. I'd expect you to measure more than about 30V if you're feeling it on your hands. Or anything more than 12V on your forearms.

Are you talking about a modern Digital MultiMeter Fluke or an older meter-movement type?

Is there any high frequency source present in the facility? Powerful transmitter(s) perhaps?


Be interesting to look at that panel in pitch darkness.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[pwrengrds]

It was measured using a Fluke 170 series, not sure exactly which one.

 

[waross]

Electro-capacitive discharge??
Does the shock feel like a static shock?
Nuisance shocks were a continual nuisance when we were building a 500 kV capacitor station. Any ungrounded object between the high voltage lines and the ground becomes a common plate of two capacitors in series. One plate is the ground, the common plate is the ungrounded object and the third plate is the high potential line.
The discharge shocks are similar in feel to a static discharge shock. Once an object is touched, there is no more shock, but if contact with the object is broken for a moment, there is another shock when contact is restored.
If the earth is dry and has a very high resistance the electrical ground may be some distance below the surface, making the effect worse.
Intuitively, series capacitors formed by by an object in the electrical field developed by an extremely high potential field will be very low, in Farads or micro Farads. The stored charge is likely too small to be measured by a normal meter; the meter will act as a shunt resistor and discharge the stored charge. A non-contact device such as a Volt-Tic may give an indication.
It may be that the switch is at ground potential and your body is acting as the third capacitor plate and charging and then discharging when you touch a grounded object. From what I understand of your post, such a hypothesis would fit with our experience at the capacitor station. We found many interesting and unexpected ways to get nuisance discharge shocks.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ausphil]

Is there an earth strap across from the cabinet to the door? If not, this could explain the feeling when touching the door and the padlock. Also, if there is one, check that it is continuous.

The issue of not measuring any voltage may be because of the input impedance of the multimeter. If it is a low input impedance meter, it may measure voltages lower than the open circuit potential present, but it should be representative of what you are actually feeling.

We have had issues where a person received a shock (tingles) from a painted cabinet. The metalwork of the cabinet was earthed, but a charge built up on the non-conductive paintwork. A tingle was felt between points on the paintwork and earth. When measured with a low impedance voltmeter (high impedance voltmeter with 2 kOhm across the input terminals), there was essentially no voltage measured there. When measured with a high impedance voltmeter (10 MOhm), a voltage of up to 40 Volts was measured.

Interestingly, where the paint had worn off, any exposed metalwork was solidly earthed and there was voltage present, so if the cabinet had been unpainted, there would have been no issue.


Ausphil

 

[Skogsgurra]

That tingling sensation (Smoked) is rarely felt in dry locations. LOL++

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Jebb]

We've had the same problem in some of our 345 & 500 stations. It's exactly what waross and ausphil explained. The problem we had was that the handle and latching bars were not bonded very well. Once we fixed the bonding (i.e. scraped off some paint at the bonding screw) there was no issues.

 

[crshears]

My first impulse would be to agree with Bill; your footwear may be too good...

Before our employer stipulated the use of dielectric footwear for all field employees, I never personally experienced this to be a problem; I could open mechanism box doors anywhere, all day, and never once get a shock.

As soon as dielectric footwear became de rigueur, hoever, it became a huge problem, especially in one 230 kV switchyard where the buswork was not nearly as far from the ground as in most other locations; I did not appreciate becoming a living, breathing application of Kirchoff's laws as applied to electric fields...

I soon developed the habit of holding a key in my left hand and using it to establish continuous galvanic contact with an unpainted portion of the mechanism box under consideration immediately before opening the door with my right hand. A spark would jump across the gap when key contact was made, but since the electrostatic current flow was distributed across slightly less than one square inch of skin, I never felt a thing.

Once I had the door open, but before letting go of the handle with my right hand, I would put my keys away, then grasp and hold on to some convenient bonded/grounded metal part within the box with my left hand before reaching with the other for whatever within the box required the laying on of hands.

This quickly became routine...mostly because if I was careless about it, I would not remain unfeeling for long!

CR

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