Electrical Troubleshooting Quiz

Feb. 25, 2013

Last month, a 200 hp motor started blowing fuses every few days. It's on its own branch circuit with no other loads. This motor runs only occasionally. It turns the compressor of a supplemental plant air system, charging a supply tank to 400 psi. It starts when pressure drops to 50 PSI. If plant air pressure drops below the main compressor's ability to keep up, the tank supplies 40 psi to the plant air system.

The fuses are the correct size and the same size used prior to this problem. There's no evidence of a fault, so why do those fuses keep blowing?

Even if they're the right size, fuses of the wrong type might not handle the inrush current. Don't just check to see that you're using dual element fuses instead of single element. Check to see that the rating is sufficient for this application. Make sure you review the fuse selection against the Section 430.52(C) of the 2011 NEC.

The problem may not be the fuses. How many restarts/hour is this motor getting? On the first start of the motor, when the windings are cool, inrush current may be below the threshold for blowing those fuses. But if the motor restarts several times in a short period, the windings will build up heat and the motor will require more current to start.

Look on the load side of the motor; problems with the compressor or load coupling may be increasing inrush current just enough to exceed the fuse rating. If you don't find the problem there, examine the input (power supply) and then the motor system (alignment, bearings, windings).

Voice your opinion!

To join the conversation, and become an exclusive member of EC&M, create an account today!

Sponsored Recommendations

Electrical Conduit Comparison Chart

CHAMPION FIBERGLASS electrical conduit is a lightweight, durable option that provides lasting savings when compared to other materials. Compare electrical conduit types including...

Fiberglass Electrical Conduit Chemical Resistance Chart

This information is provided solely as a guide since it is impossible to anticipate all individual site conditions. For specific applications which are not covered in this guide...

Considerations for Direct Burial Conduit

Installation type plays a key role in the type of conduit selected for electrical systems in industrial construction projects. Above ground, below ground, direct buried, encased...

How to Calculate Labor Costs

Most important to accurately estimating labor costs is knowing the approximate hours required for project completion. Learn how to calculate electrical labor cost.