# Grounding A Faraday Cage On The Seventh Floor

I am working on a project for my physics class to test the resistance of various flames.

This will involve using a spark gap with high voltage and I am in the process of building a small Faraday Cage to protect the people outside from any sparks that are a higher voltage than expected, also just to keep them from having the urge to touch it. (The logic being that people are a lot more respectful of radioactive sources when there's a Geiger counter clicking away in the background.)

The problem is this: I need to earth the cage so that the cage itself doesn't become charged but I'm on the 7th floor and its not using a mains power source and I'm not sure what I could use as an earth.

simulate this circuit – Schematic created using CircuitLab

I had thought of earthing it to a radiator pipe, since they're grounded anyway, but I'd be worried about maybe building up charge there that passes through somebody on a higher floor or something and most other ideas I had, had similar problems.

I had also thought about connecting it to the ground of the spark generator assembly, which is not connected to the earth, but I have some strange gut instinct telling me that would be damaging to something or stop the cage working as intended.

Its very important the safety precautions I take are as good as possible because my lecturer is liable for my safety and I don't want to make any mistakes that could hurt somebody due to my ignorance.

I have done some research on grounding and Faraday Cages but I can't seem to find anything other than "Warm, wet dirt is better than cold, dry dirt" and I don't think carrying a bucket of warm moist soil up to class would be appreciated, enjoyable to do, or even work that well as a ground because I can't find anything on exactly how much soil is needed to qualify it as a good earth.

NOTES: There are earthed outlets. I am from Britain if that's important. I wasn't sure if I could just remove the live and earth cables from the plug and use it solely for its ground pin as I am more familiar with electronics than building wiring.

The cage will be approx. 42x30x23 cm

The cage is totally bare steel wire, not painted.

The frequency of the AC spark is approximately 70Hz with a rise time of approx. 50us

This assembly and the experiment will be on an insulated base raised from the table by bricks.

The length of arc will be approx 3cm but it will be passing mostly through the flame, not air.

The voltage will be on the order of 3kV and I cannot tell you what the current limit of the source will be until I can have a look at the specs for the source, which is approx 30 miles away right now.

• Is there no earth grounded outlet nearby? How much energy is released , what is the arc length, rise time and cage size? Arc welders work outside all the time. Is it HVDC or HVAC? Is the cage painted. There may be a simple solution with these answers. Commented Apr 19, 2018 at 14:30
• Editing the question now. Commented Apr 19, 2018 at 15:05
• That should be everything. Commented Apr 19, 2018 at 15:18
• Wha arc length in air ? Voltage ? DC? ? current limit? inductance ? Resistance? L/R? Commented Apr 19, 2018 at 15:22
• How do you verify impulse with 50 Ohm AC load to current sense? Commented Apr 19, 2018 at 15:26

Your power outlet has an earth contact you can use for your purpose (even if you don't use the power outlet as your power source).

It's one of those contacts that are exposed and can be touched. E.g. in this picture the two contacts at the top and bottom of each of the two Schuko outlets.

• If I am using a lab pack as a power source, would it be safe to connect the cage to its ground pin? (By which I mean on the socket that powers the lab pack, not the common pin from its output.) Commented Apr 19, 2018 at 15:28
• Or should the ground of the cage be isolated from the power source? Commented Apr 19, 2018 at 15:28
• @Douglas you can get plug in blocks specifically for earthing - e.g.. Commented Apr 19, 2018 at 15:32
• @Douglas: Ok, in case of a British socket I suggest to prepare a plug that gives you just the earth connection
– Curd
Commented Apr 19, 2018 at 15:32
• @Douglas just easier. They're also useful for plugging in ESD wrist bands. But if you have a plug to hand, you might as well use it. Commented Apr 19, 2018 at 15:51

Considering your low stored energy and low rep rate, a plastic safety enclosure would probably be adequate.

Since you are driving your ignition coil with a 10uF cap at 12V charge at 70Hz, your average power dissipation is: $$\frac{1}{2}(10\mu F)(12V)^2(70Hz)=50mW$$

With a 50us rise-time (seems kind of slow for a spark gap), the RF noise should be around 5kHz. Your peak power is high, but considering the low stored energy, your pulses will be very short and shouldn't put out much EMF.

If you do use a Faraday cage, the electrical ground is the best option (as Curd mentioned), but using a building ground like a radiator pipe will work just as well as far as safety is concerned.

• The reason for using a cage rather than plastic is so that the flames have a readily available supply of oxygen. But your answer was still very informative thank you. Commented Apr 20, 2018 at 17:36