Sunday, December 09, 2012

DIY Electric Predator Fence

A grey fox killed and ate one our hens recently, and was able to enter our yard where they free-range. Originally we had found where he was digging under the fence to access our yard, so we reinforced the bottom portion to prevent that. However, we found out the hard way that grey foxes can climb. As a result we decided to retrofit our fence by electrifying it.

We used a low-impedance 75 mile charger for our fence even though it encloses less than an acre. Its important to use a low-impedance type that can do at least 5,000 volts when trying to keep out predators because they have thick fur which high impedance and lower voltage types cannot shock through reliably.  Modern fencers usually shock on a 1-second interval for safety reasons. This unit is no different.

Since we added high voltage wire to the top of the fence, and because the animal that we are defending against is a fox which climbs the fence, we can not expect to use the ground to complete the loop. This is because the fox will not be touching the ground and the fence at the same time when he climbs, and will not complete the circuit to receive a shock.. As a result the welded wire fence itself it grounded, and there are alternating energized/grounded wires on the top. A ground system is still important to discourage deer from entering, since they will likely stand on the ground and touch the fence with their body or nose. It also is required to prevent interfering with nearby electronic equipment, since electric fences are not much different than early radio transmitters.

The Insulators

Electric wires on fences need to be isolated and secured to the fence by an insulator, if they are to work. Iron T-posts and regular wood posts have commercially available insulators that allow simple attachment. However, the posts in my fence are made from U-shaped stamped steel and none of the insulators fit on them readily. Also, the posts are only 4 feet high, and I would like to attach the electric wires above the fence, effectively making it taller. So, I built my own insulators out of PVC pipe. I have found that bare PVC pipe lasts a very long time and is not readily destroyed by UV rays. Initially I thought about painting these green so they would not stand out so much, but because they will be energized at over 10,000 volts, is probably better if they are more visible. Also, I added warning signs to the fence wire:

To make the posts, I cut 2-foot sections of 3/4" PVC. I used the 480 PSI rated pipe because it has a thick wall and is very strong. There is cheaper 200 PSI pipe sold but its much thinner and I was concerned about it being too flimsy or cracking. There are  6 inches of overlap with the fence post to make it easy to attach, so each insulator extends 18" above the mesh. The fence itself is grounded by a wire intertwined between the mesh along the entire length.

The insulators are attached easily with exterior deck screws. Galvanized would work too and would be cheaper - but this is what I had laying around. Each peice fits into the "U" of the post and is screwed in place. I did not pre-drill these holes, which saves time and allows for a perfectly tight and strong bond.

The wire I used is 14 gauge galvanized for the grounding, and 14 gauge aluminum for the hot wires. Most guides for electric fences tell you to use the same type of wiring throughout, since it will otherwise corrode connections between the two types of wire. However, since the hot and ground wires never connect in my system, it shouldn't be a problem. I decided against polywire/tape because it degrades more readily in UV than these metals do, does not carry as much current, and is not as smooth for my homemade insulators.

To keep tension on the wire through seasonal variations I used galvanized springs on the corner posts. These can be attached on a corner post and then secured to put tension on the wires. I put a small bend in each wire and attached the spring to each - this allows the current to flow between the wires without breaking the connection, and the springs themselves are energized as well. For the aluminum wires this represents a dissimilar metal, so will see if they corrode or not at the junctions.

The fence is up and running, I purchased a 10kV tester and it measures reliably at just over 10,000 volt at the far end of the fence. So far so good!

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