One high-value section of a mid-sized manufacturing plant is supplied by a single feeder. Branch circuits include the overhead lights, convenience receptacles, two conveyor systems, a powder application/finishing booth, and some smaller production machines.
Every few days, the breaker supplying that feeder trips. Nobody knows why, so the plant engineer contacted your employer, an electrical services firm. Your boss subsequently sent you to investigate. What are some questions you need to ask, information you need to look at, and data you need to collect? What is a potential problem unrelated to any electrical anomalies, and how can you provide a means of detecting it?
Ask the right questions
Any time a feeder breaker is tripping but none of the branch circuits supplied by it trip their breakers, suspect a need for a breaker coordination study. But don’t just jump in with that recommendation. It may be that one or more branch circuit breakers are failing to operate.
Questions to ask:
- When did this start?
- Does it occur at roughly the same time of day?
- Does it occur only on one of the plant’s two shifts and not on the other?
- Has anybody reported minor shocks at a water fountain, a tingling sensation when touching equipment, a burning smell, or anything else like that?
Information to look at:
- Ask to see the maintenance records for the feeder breaker and for the branch circuit breakers. Sometimes, there are no such records. If that is the case here, schedule breaker testing to rule out any inoperable breakers as the cause of the feeder trip (this won’t rule out the cause of what should trip the branch circuit breaker).
- Ask to see the insulation resistance testing records for that feeder. It, too, might not exist. Make a note that this deficiency needs to be corrected, and testing on all feeders should be performed per the applicable power distribution system maintenance standards.
- Visually inspect the utilization equipment for grounding and bonding errors. If you see a ground rod connected to any piece of such equipment, that’s an error. Keep a written record as you go of any bonding that needs to be done per Article 250, Part V (basically, all non-current-carrying metallic objects in the system should be bonded together and to the equipment grounding conductor).
- Does the feeder breaker have a test button? Using a bright light and magnifier, carefully examine the surface of that button. If it has an oily pattern instead of the same dust that’s on other surfaces near it, then probably someone is pressing that button and thus causing the trip. The real danger here is the breaker is opening under load, which could trigger an arc blast and kill the button presser and any innocent bystanders.
Data to collect:
- Voltage readings on all three phases of the feeder to check for voltage imbalance, low voltage, or high voltage.
- A power factor reading on the feeder.
- Recorded voltage monitoring from now until after the next trip, catching the highs and lows.
- Recorded current monitoring on the feeder from now until after the next trip, catching the highs and lows.
- Harmonic analyzer readings.
Surveillance
And what about that potential problem unrelated to any electrical anomalies? To detect it, use at least two hidden cameras (four would be better if you have time and suitable locations for installing them) to watch that breaker and whoever is looking right at it (you need a camera to capture the face of someone facing the breaker).
Now the trick here is to install them without the button pusher knowing they are there and without raising any suspicion that any surveillance might be installed. An effective way is to meet the plant manager on the non-existent third shift when nobody is there. Install the cameras and test to ensure they are capturing the footage. It is critical the plant manager doesn’t tell anybody about the cameras, else word will leak back to the button pusher, who will then find some other way to endanger the plant’s employees.
Install the app on your phone and the plant manager’s phone. Wait for the button pusher to return, and hope that breaker doesn’t blow up. It might be good to remove unessential loads to reduce the danger.
What if the next nuisance trip occurs, and there was no button pusher? Then proceed by performing all of the unperformed maintenance and testing on that feeder system.
