A local manufacturing plant with limited in-house electrical expertise has contacted the electrical services firm you work for. Its complaint is that a 400A molded case circuit breaker is nuisance-tripping.
After each of several instances in which the breaker tripped, an electrician duly checked for any direct faults, found none, and reset the breaker. After the equipment was all back online, the electrician would check the loads with a digital multimeter (DMM). Nobody performing this check ever found more than about 285A to 325A.
After some discussion, the team decided to replace the breaker. To their astonishment, the new breaker is doing the same thing. They want someone from your firm to use a breaker tester to prove both breakers are bad. Your boss handed you the work order and told you to handle the project. What approach might you take?
To trip or not to trip
It’s a sad fact of life that where a plant has limited in-house electrical expertise, a decision-maker (e.g., plant engineer) will often decide a breaker is bad if it trips and good if it doesn’t. In fact, a common failure mode (arguably the most common) for a breaker is that it fails to trip at its rated load. That is why we do breaker testing and breaker maintenance on a scheduled basis. A breaker that is opening in response to something (as opposed to not being able to close) is probably good.
It is unlikely that both breakers are defective, but it could be that both breakers had some adjustment made to them. For example, in a stamping plant in Tennessee, the electrician replaced a feeder breaker and “adjusted” a mechanism so it wouldn’t stick. That adjustment was actually a physical modification, and it significantly reduced the load at which the breaker would trip. The recommendation there was not to undo the adjustment but to replace the breaker.
It might be that the breaker is one that allows you to adjust the time curve. If so, note the settings. Then obtain the manufacturer’s tech guide for the series of breaker. Determine the correct setting and then compare it to the actual setting. If the first beaker was set wrong and the replacement was set to match the first one, then you have likely found the problem.
To determine the correct setting, you will need to identify and characterize the loads supplied by the breaker. You should perform this step whether you have an adjustable breaker, because performing this step may reveal the problem. For example, suppose the main load is a motor that starts across the line. There’s inrush current; does this breaker correctly allow for it?
If the breaker adjustment is not the issue, keep in mind this is the breaker opening. It is not the motor overload protection devices opening. That means the problem is something big such as a fault or overcurrent rather than something small such as an overload. We know it’s not a bolted fault, else the breaker would not close (thus, no nuisance-tripping, since it has to be closed to trip).
So, you need to look at the environment the breaker is in. You need to see what the breaker sees. To do that, you need to monitor the power on both sides of the breaker (but especially on the load side). While a recording DMM will certainly capture the peak current events, a power analyzer will capture current events in detail and with a time stamp. Once the power analyzer is connected, de-energize all the loads on the circuit, then bring them back on one at a time. Make sure the power analyzer captures the power events at each step-change in load.
It is almost certain you will see the answer in the results. But if you don’t, remember two things:
- You have not had control of the “chain of custody” of either breaker since they arrived at the plant, so you don’t know if they have been damaged or modified.
- You have been assuming the breaker is correctly selected for the application. Maybe it was at one time or maybe it never was. Maybe somebody took their best guess, without properly characterizing the loads.
Now that you have the usage data and the load characterization, you can fix the second problem and determine the correct breaker. If the plant will then authorize your firm to also purchase and install the replacement, you also will solve the first problem. These two solutions together will solve the original complaint.
