Electrical Troubleshooting Quiz

One of the plant's four plant air compressors blew a 100A fuse. The other two fuses are OK. After a few incidents where "blind fuse replacement" resulted in equipment damage, the plant engineer issued a memo stating that no fuse will be replaced until a determination has been made as to why it blew. Unfortunately, the memo didn't include instructions, and the maintenance department hasn't developed a checklist. Your DMM doesn't show a direct short. If the fuse blew due to a random event, you could be here all day running tests.

What should you check before replacing that fuse?

Identifying the cause of a blown fuse can be time-consuming and difficult. The maintenance department needs to develop equipment-specific checklists for the critical equipment and a general procedure for all other equipment.

The service manual for the air compressor may have recommendations, so start there. Some things to make sure you do:

• Inspect the fuse clips and connections for mechanical integrity. If you have a high-ohms tester, use it to check for electrical continuity.
• Perform insulation resistance tests on the motor windings.
• Measure voltage phase to phase. If there's more than a 2% difference between any of these measurements, you have a phase imbalance. If voltage is low on the phase that lost the fuse, you may have insulation problems; hi-pot the circuit conductors.
• Look in the power monitor logs to see if a transient event occurred coincident to the loss of that fuse.
• Before starting the compressor, install a power analyzer and vibration analyzer. Monitor operation for 15 minutes.