Underground wires are buried to protect them from exposure. While it is rare for underground wires to suffer damage, from time to time activities such as digging for a new fence, planting a tree, gardening, or even the activity of pets and animals can cause tears and ruptures in buried wires. Locating the broken section of wire isn't an exact science, but there are newer tools available which can help you narrow down the search and avoid digging up huge portions of your yard.
Call your local power company and schedule a time for one of their employees to come to your yard with a wire finder and clearly mark the ground above the wires you suspect. Visually inspect the area along this line and look for areas where maybe the wire has been dug up by an animal, or see if there has been some sort of disturbance, such as someone digging for a fence and accidentally cutting the wire with a shovel or other tool.
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Test your transmitter to see if the problem is actually in the wire, or if it is in the transmitter. Turn the transmitter off and detach any outside wiring running to it (or hire a qualified electrician to do this for you). Cut a short length of unused wire between four and six feet in length and attach that to each end of the transmitter to form a loop and check that the machine is indicating a full connection with no breaks in the line. Disconnect your test wire once you see whether the transmitter is working or not.
Find the break in the wire using a wire break locator if you determine the problem is not with the transmitter or if you cannot find any visual damage or sections where the wire has been dug up. Follow the manufacturer's instructions on how to hook up your brand of wire break locator. Keep it on the most sensitive setting and walk along the marked area indicated by your electric company. Follow the sound of the device, as it emits a low hum when it is over a live wire. When it comes across a broken section the humming stops, indicating you are over the break.