Tuesday, September 11, 2018

Steam-punk lightning choke

An induction coil or lightning choke should be installed in the fence lead-out wire near the energizer. The induction coil is made by coiling 6 to 8 loops of heavily insulated 12 gauge wire in an 8 to 10 inch diameter circle and taping the loops together. A lightning choke is made from a loose coil of hi-tensile wire with 8 to 12 loops spaced about 2 inches apart on a mounting board.   -Gallagher Electric Fence Company

Inductance is a form of impedance.  That is, it impedes the flow of electricity.  It is not "resistance".  Resistance is proportional to current.  Inductance is similar to inertia, it is proportional to increases or decreases in the rate that current flows through a wire or path.

Inductance is proportional to the area of the loop so eight loops with a diameter of 10 inches has an area of about 600 square inches.

Inductance also depends on the material within the loop.  Iron inside the loop increases the inductance a LOT!  This is one reason why automotive companies transitioned to DC resistance spot welds, especially for guns that welded the center of the floor pan.

Beyond the basics, it is my understanding that only the material that is very close to the wires comes into play when considering very fast, very transient events.

A 3/4" by 8", Grade 8 bolt inside a PVC sleeve with two wraps of wire around it.  The two wraps of wire is a SWAG, nothing more.
Hardened steel is considered magnetically "hard" because it retains magnetism.  Mild steel is considered magnetically "soft" because it does not retain magnetism.
The fact that the wire is not closer to the steel bolt concerns me a little bit but I want the insulating effect and don't want the wire, rated for 600V, to short to the bolt.
I am probably a bit "under choked" but figure I will be OK since I am very tightly fused with the air-gap arrestor.  I just need to make sure that:
A.) I have a fabulous ground for the arrestor
B.) I have very good connections to the rest of the fence to guarantee that the voltage gets dragged down enough to not shunt through the arrestor.

In words, the way this system works is that lightning hitting the feed wire raises the voltage of the line between the charger and where the lightning hits.  The "choke" restricts the current attempting to enter the box.  The voltage between the lightning strike and the choke rises enough to "pop the cork" on the air-gap arrestor.  The resistance of the arrestor drops to zero and the voltage dumps to ground.

Why not do it the way Gallagher said to do it?
For one thing, I don't like having large amounts of hot wire hanging out where folks can bump into it.

The other way to do it, with insulated wire, is kind of pricey.  The wire I have is rated for 600V and I am not sure that would be good enough.  That and the fact that I would need almost thirty feet of wire...it just seemed more economical from a space and material standpoint to use much less wire and a bolt that I had laying around.

1 comment: