Meduim Voltage (4160V) Epoxy versus Porcelin Insulators

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Electric-Light

Senior Member
I've played around with older bottle style ignition coil. Hooked it up something like this:

http://wiki.4hv.org/index.php/Ignition_coil

Well, I got the voltage to get high enough to arc over from center, over the edge to one of the terminals. The material is phenolic resin, however the arcing burned the surface and created a carbon track and insulation was done.

If you have an arc over over anything that can char, it will leave a carbon track and draw a conductive path. Since porcelain is inorganic, it is immune from carbon tracking.
 

natfuelbill

Senior Member
Can you re word that? Less prone compared to what? How do escalating problems reduce maintnence? I don;t understand the question(s).

Yeah sorry about that. Too much Jack n coke.

Is 4160v equipment prone to flashover due to low maintenance?
 

zog

Senior Member
Location
Charlotte, NC
Had a too much Jack and Coke moment myself last week at the beach.

ANSI/NETA recommend MV metal enclosed switches are inspected and cleaned every 12 months and electrically tested every 24 months. Those intervals are typically adjusted for equipment condition and relibility requiremtns to give you a multiplier from 0.25 to 2.5.

The NFPA 70B also has recommended intervals, they are the same I think, if not they are close (Don;t have my copy handy)

Lastly you can check the opeation and maintnence manual for your switch that Squade D provides, you will find similar recommendations there. And it discusses dirt, moisture, tracking, and flashovers.
http://static.schneider-electric.us...Switchgear-Metal Enclosed/9840/6040DB0201.pdf
 
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CONTROL FREQ

Member
Location
OHIO
What challange? First you agreed with me, then you posted some part of a study that had little to do with the subject of tracking in enclosed equipment. Sure porcelin might be better in a wet environment, and hard to argue that dirt may not stick to it as easily either, but they also are fragile and when you throw a HVL switch there is a pretty serious shockwave that could shatter a porcelin insulator. Which is why, just maybe, that Square D used the material they did for that application (They do know what they are doing sometimes).

But my point is, and has been throughout this entire thread, that maintaining the heaters working and cleaning the switches every couple of years is a lot more important than the materials used.

WE INTERUPT THIS ?LOVE-FEST? To say ?An ounce of PREVENTION is worth a TON of cure!?
PS, ZOG first time I saw that pic, I thought somebody was taking a PEE on Greg Biffle?s back TIRE! You shoulda put your face shield up!!! :cool: Greg Biffle is cool.
 
I've played around with older bottle style ignition coil. Hooked it up something like this:

http://wiki.4hv.org/index.php/Ignition_coil

Well, I got the voltage to get high enough to arc over from center, over the edge to one of the terminals. The material is phenolic resin, however the arcing burned the surface and created a carbon track and insulation was done.

If you have an arc over over anything that can char, it will leave a carbon track and draw a conductive path. Since porcelain is inorganic, it is immune from carbon tracking.

Had the same happened in a 480V switchboard.
The area was dusty AND humid - wet milling operations, and steam drying ovens - and the enclosure was rather old. The main breaker feeding the panel would trip. Maintenance people would re-set it ? mostly without testing - and it would hold. (It was a HRG system and there was no ground fault indication. It was rather difficult to troubleshoot as it was a large system, single, several hundred feet long feeder with multiple taps on it that had transformers. A real nightmarish configuration, I don?t think the Code ever allowed some of the things that were done there over a 100 year period. Production wanted the service back and was unwilling to shut down the entire operation for some random fault. They went through this several times before they called me.)
The panel was just re-energized and the breaker was 'holding'. They had the panel open and it was so old that the guts were exposed. The area was dimly lit and I was about to turn on my flashlight when I noticed something strange. I thought I am seeing a faint glow inside the panel. Sure enough, leaning closer I see that the Bakelite insulation bus brackets had a glowing track between two phase buses. So what I concluded that it probably started with the moist surface dust providing a high resistance conductive path, which would slowly heat up and carbonize the material. As the material carbonizes, it becomes a better conductor than the dust. Eventually it let thorough enough current for a long enough time to trip the breaker. As this process repeated, eventually the Bakelite itself developed a carbonized groove. After the trip the current would begin to flow but was not high enough to immediately trip, but went into the LT range of the breaker and the next trip would occur days later.

Indeed there was no evidence of arc-over.
 

zog

Senior Member
Location
Charlotte, NC
Indeed there was no evidence of arc-over.

You usually won't see tracking in LV equipment, tracking and corona occur somewhere between 2kV and 4kV, depending on the environment.

One of the newer predicitve testing methods we use it Partial Discharge surveys, hand help detector that you just hold up to the outside of the metal enclosure that detects TEV via capacitive coupling. If there is any tracking, even in the early stages, the detector will pick it up, then you do a more detailed inspection when you can schedule a shutdown. Only takes a few minutes to survey a switchgeaer line up. !
 

Electric-Light

Senior Member
You usually won't see tracking in LV equipment, tracking and corona occur somewhere between 2kV and 4kV, depending on the environment.

It can supposedly happen even at 120v in very humid climate. Accumulated dust at the outlet can get wet from dew or condensate on the wall dripping on it and resistance becomes low enough to make it a heating element.

http://www.jniosh.go.jp/icpro/jicosh-old/english/topics/Tracking.html

This is something that was done in a lab to duplicate the condition in connection with a fire investigation which corroborates what weressl have observed

2j46m9l.jpg


Though not the same situation, high frequency AC does not stay off long enough in zero crossing to extinguish the arc, so it behaves very similar to DC. You can draw a pretty impressive arc on the output leads of high frequency electronic lighting ballast by shorting it, then slowly pulling apart.

There are some ballasts that limits ignition voltage once the lamp has been ignited, called UL "CC" rated.
 
It can supposedly happen even at 120v in very humid climate. Accumulated dust at the outlet can get wet from dew or condensate on the wall dripping on it and resistance becomes low enough to make it a heating element.

http://www.jniosh.go.jp/icpro/jicosh-old/english/topics/Tracking.html

This is something that was done in a lab to duplicate the condition in connection with a fire investigation which corroborates what weressl have observed

2j46m9l.jpg

Well, Zog's disclaimer would have stood had the statement been more precise.

First he stated: "You usually won't see tracking in LV equipment,..." , but then he went on to saying: "tracking and corona occur somewhere between 2kV and 4kV," is where he made the mistake as tracking can occur at lower voltages, but then he adds: "depending on the environment." for which he would be loved by the plaintiff attorneys and hated by the defense.

To be clearer; tracking depends on the distance, surface contaminants and humidity affecting those, it is largely independent of voltage. Tracking provides a low current leakage along the surface that may result in an arc-over. Corona is voltage dependent and facilitates the shortening of arc-over distance in air. Acting along with airborne contaminants and humidity, it produces a low level leakage that can result in an arc-over. Most of the time the excess voltage discharges in a mode what is called partial discharge. The stress ionizes the open air which produces free oxygen, or ozone, that is highly oxidizing. The oxidized contaminants both in the case of airborne and surface contaminants will reduce the current path resistance and facilitate better conductivity, resulting in the escalation of the leakage current.
 
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