Secondary Coil Failure - INSIDE the tube

[beckerrs]

I slowly increased the power to the tesla coil and was rewarded by 4 inch sparks and loud music. But as I increased the power, the spark stayed at 4 inches with no increase in sound volume. After about 3 minutes I thought I saw some yellow flashes of light from the main board and shut down the power. Inspection showed no obvious failures with no burning smell. I repowered the coil. This time I noticed the yellow flashes appeared to be coming from the top of the heat sink. Then I realized I was seeing the reflection of arcing INSIDE the secondary bouncing off the top of the heat sink. After removing the secondary from the box and peering inside I could see lots of charcoal! No smell or smoke was visible while the coil was operating because the tube is sealed. When I pried the end caps off the coil, the familiar smell of burning plastic was evident.

What happened? When I soldered the ends of the coil to the ring terminals, I left about an inch and a half of slack so I wouldn't stress the wires if the ring terminal rotated while tightening the wing nuts to the bolt. This extra slack was pushed up inside the coil. If you figure 200,000 volts across this 10" coil, that is 20,000 volts per inch. the wire loop was about 1 inch up inside the coil against the inner wall of the tube. Right about at the 20,000 volt point. The arcing took place through the wall of the tube to the loop of wire. Since I couldn't see or smell anything amiss the problem just got worse. Look at the attached photo. Eventually the arcing progressed up the tube, creating the long black line you see in the photo.

After winding another secondary, I took pains to leave no slack in the wires going to the ring terminals. I was rewarded with 18 inch sparks and LOUD music, with the power control only half way up! Beautiful!

[DJKOR]

There are a couple of these cases posted here already. It tends to happen when you have poor grounding and the spark finds it easier to track down the inside of the coil instead.

[Aaron Ravensdale]

I read this in another post before and change my coil in this way.
For the toroid I use a screw to connect the secondary on the other rim. I also guide the cable inside a rubber tube.

2013-10-25 18.46.08.jpg

Toroid connection

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For the ground wire I drill another hole in the primary coil holder and the base lid.
I also use a additionally connector.

AND I use nylon screws for secondary mounting... some users got a short between the two bolts!

2013-10-25 18.48.02.jpg

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2013-10-25 18.48.40.jpg

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[Tetsula]

Looks like very similar to these posts:
viewtopic.php?f=24&t=1117
viewtopic.php?f=25&t=1080

[stegu]

That screw connection to the rim of the toroid is a potential breakout point. Have you had problems with a spark breaking out from there? That would be bad news, because a spark emanating from the underside of the toroid would almost certainly hit the secondary coil instead of breaking out towards open space.
There is a good reason to have the connection to the toroid at its center. If you do this modification, at least keep the screw short, or top it off with a cap nut or a small metal sphere.