When Circuit Boards are Toast, Part 12

Oct. 11, 2011
Now that you've spent considerable time on the neutral and ground issues, you find that your power quality analyzer still shows flat-topping

Now that you've spent considerable time on the neutral and ground issues, you find that your power quality analyzer still shows flat-topping. What's going on?

To answer this, we'll assume a 4-wire system in this scenario. The typical transformer supplying single-phase loads is of dry-type construction, and it has a 4-wire wye secondary stepping down from 480V. We'll assume that, too. Not every facility has a 4-wire system, and sometimes loads are on a transformer with a delta secondary for power quality reasons.

A common source of flat-topping is transformer overloading. Typically, it occurs through the addition of loads over time. Fortunately, overloading is easy to confirm or disprove.

Here's how:

  1. Use a current probe to measure the load on each phase.
  2. Total the readings to determine actual kVA.
  3. Compare actual kVA to your transformer's rated kVA.
If actual kVA is less than 80% of rated kVA, this transformer is probably (but not necessarily) loaded to its most efficient point of operation. Evaluate per ANSI/IEEE Standard 241 (Red Book). If the branch circuits have high harmonic loads, derate the nameplate kVA (Red Book or Emerald Book), and then reassess. Now, compare phase readings to each other. The phases should be equally loaded. If they differ by more than a few percent, consult the Red Book.

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.