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Disconnects are on the edge of being HVAC or electrical, but most places the HVAC technician is allowed to repair and replace disconnects. Even if you work at a place where it isn't allowed the ability to find problems is a diagnostic skill you will want to have. Always work safely with proper safety gear and on de-energized equipment whenever possible.
Check Pull Condition
On a pull-out disconnect, the pull should fit tightly with little to no carbon buildup or signs of arcing. In some cases, you can replace the pull alone but most of the time if it is damaged you will need to replace the entire disconnect.
Check for Proper Connections
Poor connection and improper wire sizing are the two biggest causes of disconnect damage. Confirm the wire matches the MCA (Minimum Circuit Ampacity) of the connected unit and then check the connection lugs for a snug fit while unenergized. It's best to follow the torque recommendations and use torque screwdriver when possible.
When in Doubt, Check for Voltage Drop
Whenever you suspect an issue with a disconnect you can simply check for voltage drop across the line and load sides of the disconnect on each leg. You should read little to no voltage between line and load as shown above. If you have a thermal imaging camera or an infrared thermometer you can also check for hot spots on the disconnect lugs.
Seal Behind Disconnects w/ Rear Penetrations
In many cases, a disconnect will be fed with the wire coming from the wall behind. In these cases, it can be difficult to properly seal the wall and the penetration into the disconnect. This is why the top of the disconnect should be sealed to the wall to prevent rainwater from running down behind and into the wall or disconnect.
It is a good practice to replace any damaged connectors and make sure there are no signs of moisture intrusion or corrosion in the disconnect and inspect the whip for proper fittings and strapping at the same. Look at the ground connections in the disconnect and where they connect to the system and make sure they are properly connected under a dedicated grounding screw.
There have been many breakdowns and even fires over the years caused by issues with disconnects, make inspecting them a regular part of your maintenance and service call process and you can save some issues.