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Faraday Suit and Electric Fence

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Finn Hammer

Tue Aug 16 2016, 07:49PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

All,

I have made a Faraday suit for a TV spot, and this suit is supposed to shield the host against a Tesla Coil Discharge, which it does admirably. (I regret that the video is tilted)

The producer also wanted to test if it shields against the shock that an electric fence dishes out, and this is not the case if I have my hands bare in the gloves. If I wear double polyurethane examination gloves, however, the shock goes unnoticed.

I am a bit at a loss trying to explain this, I frankly assumed that I could have touched the fence without gloves under the chainmail, and the suit would have shorted out the jolt.

I would appreciate anyone able to explain why the shock from the electric fence is not part of the Faraday Suits protection.

The suit is a combination of Chainmail gloves, the shirt, trousers and socks are sown from knitted metal, and the helmet is the usual tin shoulder yoke with a birds cage surrounding the head.

Cheers, Finn Hammer

Sigurthr

Tue Aug 16 2016, 08:42PM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

The low frequency of the voltage source means that distributed capacitances present a higher impedance, this allows for a larger voltage gradient between nodes; large enough that a shock is felt.

You could try bonding all of the armor pieces together and grounding the suit to shunt the current.

DerAlbi

Tue Aug 16 2016, 09:12PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Interesting problem. I am also in the dark here. I am brainstorming every *stupid* idea i have.
a) Whats the frequency of the TC? (i am thinking skineffect here). A sufficient high frequency will allways be transported within the metal gear even if the clothing "malfunctions".
b) "malfunction"? Have you tried to close a circuit (100mA constant current, 10V under no load) with your clothing? (hand to grounded foot). When you have attached an oscilloscope to your hand, are there "glitches" when you wiggle around? What i am thinking here: such clothing relies on a statistical contact between every element which are oxidized and what not. Those problems become smaller with higher voltage because breaks in the chainmail (or islolated spots) will arc over (on a microscopic level) removing the breaks.
c) given the statistical nature of the conduction, have you ever measured the resistance of your clothes? With a TC, besides the skineffect, you have a really high impedance source with the arc in series and everything. If you would draw an equivalent circuit of all the resistors in the current path, would there be much voltage drop across your amor? (I dont think so). However an electric fence output, might have a lower overall output impedance. (i can be completely wrong here)
d) I wonder if there is an inductive component in your suit given its size. If you are near the fence driver the closed loop you represent a significant part of the circumference of the area you create while short circuit the driver. Since the fence output should have a pretty steep output pulse (conmpared to the TC), inductance can matter here. A problem with the theory is, that you shouldnt feel anything from such a high frequency effect due to skineffect again. So this is d) for dumb.

Mystery unsolved. Sry

So far i concentrated on the difference of the two cases. TC is high frequency, a shorted fence driver should deliver a constant current for a "long" time. Besides the initial rising edge its basicaly DC.
You will allways have a current divider between your body and the conducting shield. hmmh. Measure resistance (while wiggeling around). Record the worst case and this should give you a hint

Edit: Sigthur was faster: are you telling me the individual parts of the armor rely on capacitive coupling?
(Well ok. basicaly thats what i mean by the statistical nature of the current conduction within the armor, in case of interruptions, its only a capacititive current flow - doesnt work for DC)

Finn Hammer

Wed Aug 17 2016, 07:21AM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

This is interesting.

I guess it is not too obvious, exactly what waveform the fence is able to produce, across the human body resistance, but if I can keep focus after the recording, which is tomorrow, I will go out and measure it on the fence. I personally believe that the loop area of the fence, which is immense, has a great role to play in the waveform.
Perhaps it is time to measure the current waveform from a grounded Tesla Coil Arc too?
Probably by comparing these two waveforms, an obvious answer will emerge. Until then,

Cheers, Finn Hammer

Mads Barnkob

Wed Aug 17 2016, 08:17AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Electric fenses are basically a capacitor discharging into a large inductor, giving it a very slow ring up. Same as seen in defibrillators.

Did you consider isolators on the shoes, like Terry Fritz, I guess it is to avoid getting welded to metallic surfaces :)

2Spoons

Wed Aug 17 2016, 11:13PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

Mads Barnkob wrote ...

Electric fenses are basically a capacitor discharging into a large inductor, giving it a very slow ring up.

I had a job designing electric fence energisers, and I have to disagree with the "slow ring up". On many of our competitors products the leading edge of the pulse was very fast, and often we only saw the true magnitude of the leading edge with a very fast scope. Usually the only inductance seen by the capacitor is the leakage in the output transformer - which was quite small, given the small number of primary turns and the typically low mu core.

On the other hand, the design I worked on had a tuned tank on the primary side, with a clamp diode, all designed to produce a half sinusoid pulse. Interestingly a lot of the farmers who tested these things (usually by holding the output terminals - Southland farmers in New Zealand are a tough bunch) thought our fence energisers were 'hotter' for the same energy rating.

Blackcurrant

Wed Aug 17 2016, 11:46PM

Registered Member #2989 Joined: Sun Jul 11 2010, 12:01AM
Location: UK
Posts: 94

Some other stuff that came to mind
Electrosurgery as in nerves don't feel pain due to fast AC
As for skin effect some current will still flow through the inner
and Faraday cage, if your in a lift mobile phones still work due to cut outs in the metal
Also I would compare your suit with the type they use on HV power lines
If you can feel it maybe the current is higher than a Tesla coil?
Just a load of mad ramblings but I hope you figure it out.

Hydron

Thu Aug 18 2016, 06:31AM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

2Spoons wrote ...

Interestingly a lot of the farmers who tested these things (usually by holding the output terminals - Southland farmers in New Zealand are a tough bunch) thought our fence energisers were 'hotter' for the same energy rating.

Ahahaha that sounds about right. We were wussy city kids and used a blade of grass to test the fence.

Interesting comments about the tuned tank - I spent a summer working at Tru-Test and saw that the energisers had more than just the cap + SCR + output transformer, but didn't get into any of the details of that part of the design.

Reminded me that this:

is probably of interest here too - patent on using IGBTs for building an energiser with greater pulse shape control and without the output transformer

Finn Hammer

Thu Aug 18 2016, 05:56PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

All,

Thank you all for your input.
I put the suit in series with a 800W halogen flood lamp, and measured a voltage drop of 30 volts across it from collar to waist. Alltogether, this translates to around 10 ohms of resistance, the same resistance in the arm and glove, plus some in the knitted mesh trousers, say all in all 25 ohms of resistance from ground to glove.
If the fence energizer puts out 2.5kV, then the glove sees 100V, and I guess that explains why it was an unpleasent experience.

Cheers, Finn Hammer

2Spoons

Fri Aug 19 2016, 01:59AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

And 2.5 kV is low for an e fence. Most of our product put out 8kV, except for the big bastard I designed, which would do 10kV. And that's into a 500 Ohm load too.

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