A few weeks ago, a 400A feeder breaker started exhibiting nuisance trips. DMM measurements, taken randomly, show the breaker is nowhere close to its maximum load. During shift changes over the weekend, another electrician performed the recommended maintenance on it. Yet, it still trips for no apparent reason.
Your boss has asked you to replace the breaker. What are some things you should do before carrying out that request?
The problem may not be in the breaker. Although DMMs are amazingly useful, they have limitations. You need a power analyzer or power monitor to show you what that breaker is actually dealing with. For example:
- What is the power factor?
- What is the harmonic content?
- Are there voltage sags (48-hr sampling)?
- Are there momentary current surges caused by motor starts or other large loads (48-hr sampling)?
The fact that you have nuisance tripping doesn’t mean the breaker is the problem. The breaker probably is tripping because it is supposed to, so you’ll have to look closely at the loads on that breaker.
- Equipment modifications have increased the amount of inrush current on the branch circuits that feeder supplies.
- Cable insulation deterioration has resulted in leakage that is subsequently causing trips (an early indicator of an impending cable fault). Symptoms usually follow temperature swings that leave condensation in the raceway.
- Operator error is producing new conditions for the equipment. In a plastics plant, a new operator was overloading the scrap grinders. This caused excessive current draw. Due to poor circuit coordination, the feeder breaker tripped but the branch circuit breaker stayed closed.