Most wiring (especially for HVAC) is not rated to be in constant sunlight. The sun will degrade the wiring, dry it out, and harden it to the point where the jacket breaks apart and the copper is exposed. This can cause shorts and/or breaks in connection. For instance, what you see in the picture is a thermostat wire. The outer coating of the thermostat wire is UV-rated and can withstand the harsh conditions of constant sun light. However, the wires within the outer coating are not UV-rated. If there is a break in the outer coating the exposed wires underneath will begin to fall apart as time goes on. This is why splices and breaks in connections between wires are covered with electrical tape because the electrical tape is also UV-rated and creates a weather-proof seal around the wires.
What you see in these pictures is a break in the outer coating that exposed the inner wires. The sun eventually degraded these wires to cause a short and complete break in our reversing valve connection. I cut this bad area away and ran a new wire with a new outer coating from the break in the ground to the outdoor unit. I sealed the wires where I made my splice connection with electrical tape, and then insulated them with black insultube (what you see around refrigerant lines) for extra protection.
