Determining the cause of breaker tripping with Ap light indication on the Micrologic trip unit
How do you determine the cause of Powerpact P and R or Masterpact NW and NT circuit breakers with Micrologic trip units tripping with an Ap light indication?
Powerpact and Masterpact circuit breakers
There are many reasons why a circuit breaker may trip with Ap indication. It could signal tripping due to a programmed advanced protection function in the trip unit, or for a number of other reasons. Reasons include but are not limited to wiring errors, a bad neutral CT, a trip unit failure, circuit breaker sensor problems, a bad performer plug, trip unit overheating, or a fault above the instantaneous override level.
See below for some troubleshooting guidelines.
Remove front cover from Powerpact breaker to access the trip unit connections. On Masterpact, connections can be accessed from the secondary disconnect blocks.
Ensure jumpers are on terminals Z3-Z4-Z5.
If ZSI is used, external wires will be terminated in these terminals instead of jumpers.
Ensure there is a jumper on T1 and T2, or wires coming from NCT landed on these terminals. If there are wires coming into T1 and T2, there should also be a wire (grounded shield) landed on T4. Note that sometimes the wires that should be on T1 and T2 are on T2 and T3 or other terminals. If wires are in T1 T2 T3 and T4, the breaker is older and it’s OK to leave these wires in place. NCT wiring changed around 3rd Q 2007.
Measure resistance on NCT wires (Terminals T1-T2). This should be approx. 50 ohms; any number between 10 and 100 is OK. If zero or above 100 ohms, check out the wires to see if they are shorted or open.
Whether or not problems are found with NCT wiring, for a test remove wires and place jumper in trip unit terminals T1-T2, and turn breaker ON. If it carries load where it would have tripped before, you have isolated the problem to the NCT or NCT wiring. If wiring is good, replace the NCT as it may be bad and still pass our limited testing.
If no problems are found so far, swap in a known-good trip unit if possible. A trip unit power supply failure can cause an Ap trip and we have no means to test for it other than replacement.