Come and Meet Extreme Electronics and the Twin PI tesla coils (and some other stuff 🙂 at Derby Mini Maker faire. We will be by the jet engine just off the main hall.
Ok so it started as one of those phone calls that asks if I can make safe sparks. Quite a regular ask for me and I always start with no. High voltage isn’t safe.
Anyway after a number of discussions. I was asked to make a “relatively” safe way of allowing a BBC presenter to create lichtenburg figures.
The method I came up with was to use a 15KV OBIT (Oil Burner Ignition Transformer). I prefer this type of transformer to a MOT (Microwave Oven Transformer) for purely safety reasons. One touch from an MOT and you are either Burnt, Dead or Burnt and Dead, Not good odds. This is due to them giving up to 100mA at 2KV, plenty to get 1-2mA across the heart of the unwary. The other issue with MOT’s is they are low enough voltage not to cause corona, which means they are silent, a dangerous mix. OBITS on the other hand give out 10-15Kv at 5-10mA. Not safe, but you are much more likely to survive an accidental contact. Also due to the OBITs high voltage, you can hear when they are on.
The boxes were split in to two, the Transformer and high voltage at the “HOT” end and the lower voltage (mains) at the other.
The control box had a key lockout which needed to be in place for the HV to be turned on. This key was attached to me with a short lead so I could ensure the HV area was safe before I allowed any power to the box. The Go button was momentary, so that if any danger was spotted the power could be quickly removed.
The HV end was in a roped off area (out of shot)
The mains was supplied via an earth leakage trip for added safety.
The HV end was supplied via 30KV insulated cable to two crock clips that attached to the screws on the board.
The board was dampened with water and washing up liquid, (required to properly soak more than just the surface of the board)
This really is a don’t try it at home experiment, when the power is on, you can’t touch ANYTHING, damp wood, water and high voltage not a good mix.
Luckily the presenter was Marty Jopson a HV fan so the explanation and safety briefing was fairly easy as he understood the risks. (He has his own collection of HV equipment including a one Tesla.)
This was all done in the most glamorous of locations 🙂
The result. No one died… and I think they got a good film of the Lichtenburg figures being produced. I will find out 19:00 BBC1 4th July 2016.
Other Making Lichtenburg links
Test of transformer choice and wood type for reliable Lichtenberg figures.
Obit – 20Kv 20mA
Both pieces of wood lightly soaked in water (only) just before test.
wood – 4mm MDF
Wood length 18″ – each pattern ~6″ across.
Course grained 3 ply
Burns only continued in line with the grain. Test abandoned.
by Paul E. Schoen of P S Technology, Inc.
There is some more information [regarding the resistance of a human body] at http://van.physics.uiuc.edu/qa/listing.php?id=6793 , where it states that the external human body resistance is about 1k to 100k Ohms, and the internal resistance is 300 to 1000 ohms. Only a thin layer of dry skin separates the internal resistance from an external object.
The human body capacitance to a far ground is 100-200 pF, which is really a minimum value. This correlates to an impedance of about 13 megohms at 60 Hz, which corresponds to a minimum of 9 uA at 120 VAC to ground. This is enough to be sensed and used for capacitively operated light dimmers.
Here is a way to measure your body capacitance: http://web.mit.edu/Edgerton/www/Capacitance.html
The inside of your body can be considered a conductor, and thus if you place your hand flat on a metal plate, you will form a capacitor with an area of perhaps 15 square inches, with a thin (maybe 0.005”) insulating layer of dry skin, which will form a capacitor much higher in value than the 200 pF stated above. According to a formula in http://www.sayedsaad.com/fundmental/11_Capacitance.htm , this would be C = 0.2249 * k * A / d = 1350 pF, (assuming k for skin is 2, about like dry paper). This will be an impedance of about 2 megohms , and current of 60 uA. This is still below the normal threshold of sensation, and still far below the usual safe current levels of 1 to 5 mA.
The actual thickness of the epidermis (per http://dermatology.about.com/cs/skinanatomy/a/anatomy.htm ) varies from 0.05 mm (0.002”) for eyelids to 1.5 mm (0.06”) for palms and soles, but the actual outer layer of the epidermis that is a good insulator is composed of flat, dead cells, which is much thinner. So the capacitance could be much higher than the quick estimate above.
Probably the main reason for electrical current to reach levels high enough for electrocution to occur (6 to 200 mA for 3 seconds, according to http://www.codecheck.com/ecution.htm ), is when skin becomes sweaty or otherwise loses its dry protective layer, which quickly exposes the underlying 1000 ohms or less, which will conduct 120 mA at 120 VAC.
There are safe ways to measure the body’s resistance and capacitance using realistic higher voltages, skin conditions, and contact surfaces, but I’m not going to suggest anyone try it. Suffice it to say that ohmmeter readings are misleading, and any carelessness around any kind of voltage source can be dangerous.
For very high voltages, there are standard minimum distances that must be maintained between a worker and an energized line: http://www.dir.ca.gov/oshsb/rubberglove.html . I found this on a search for rubber glove testing.
The field intensity near high voltage lines is so great that it might be fatal to touch them even if you were suspended in free air. You may notice that birds can sit on lower voltage transmission lines which are 5kV to 50 kV or so, but not on 200kV+ lines.