When Circuit Boards are Toast, Part 12

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.