Arc Demo

[AbeZ]

It's fairly common to see arc demonstration trailers at many utilities for putting on presentations that warn the public of the hazards of electricity.

These typically appear to use distribution, step-up transformers that charge a capacitor at 4160-7200 kV from a 120 to 240 VAC wall supply. I have a few questions regarding the design of this:

What is a typical value of capacitance used for this type of demonstration that will provide an impressive arc? Is a 10 kVAR sufficient to allow running off of a 240 VAC, 50A supply?

What is used to limit the inrush current? Is the small distribution transformer impedance the only device that limits the capacitor inrush? What are good options for limiting this inrush that will reduce the 120/240 VAC supply sizing requirements? Or, does this not really matter if the supply has a continuous rating sized for the capacitor kVAR rating?

Would it be possible to use DC to slowly charge the capacitor and prevent the need for large AC source requirements? Once the capacitor is charged, the arc demo can be performed using the stored energy in the capacitor?

Thanks for any info you can provide.

 

[itsmoked]

The distribution transformers I've screwed with were very hard to energize backwards. They blew all sorts of breakers. I'd skip that.

Get your hands on a large neon sign transformer. They're typically about 2 to 4 hundred watts and will do the trick for you. I've seen them in 5, 7.5, 10, 15, 20kV versions. You can put in a series resistance to limit the cap charging to something reasonable for the transformer.

As for capacitance I've used what results in about 1 Joule to feak-out college open house guests via exploding wires. (well tin foil shavings)

I'm not sure what you're precisely aiming to do but anything around 10 joules should be attention getting.

An actual arc flash likely has a longer duration and may need more stored energy. Part of what makes arc-flash events so bad is the inclusion of vaporized metal to provide a low impedance metallic plasma capable of conducting and sustaining the arc. This speaks to probably using some sort of exploding wire in your demonstrator to simulate a metallic short of the hapless bus.

Perhaps look at distribution pole capacitors for something robust.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

A neon transformer. That takes me back a long way. I used a neon transformer to build a Jacobs ladder or traveling arc for a science demonstration in high school.
Another option may be an ignition transformer from an oil fired furnace gun burner.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Zogzog]

We use this