Depending on the model, Square D Micrologic trip units may feature self-protection/advanced-protection (AP) functions in addition to the typical over current functions found on most low voltage power circuit breakers. The AP LED is designed to indicate the following:
- Internal over temperature of the trip unit exceeding 125°C
- Loss of internal power supply regulation to the Application Specific Integrated Circuit (ASIC)
- Micrologic P/H advanced protection functions (over voltage, under voltage, phase imbalance, etc.)
- Close and latch rating of the circuit breaker exceeded. This feature is designed to protect the breaker in the event the breaker is closed onto a high level fault condition*
- Short circuit withstand rating of the circuit breaker exceeded
If you are experiencing nuisance trips with an AP LED indication you can take the following steps to troubleshoot any problems with the external wiring or external power supply.
Note: If the circuit breaker is closed and the Ap light remains on after pressing the trip unit reset button, open the circuit breaker and call the manufacturer. An internal error has occurred that cannot be repaired in the field.
Warning: Before beginning any work, be sure to apply appropriate personal protective equipment (PPE) and follow safe electrical work practices. See NFPA 70E. Turn off all power supplying equipment before working on or inside equipment and always use a properly rated voltage sensing device to confirm power is off. Be sure to replace all devices, doors and covers before turning on power to the equipment.
Troubleshooting steps for circuit breakers NOT equipped with a neutral CT:
- Verify accessory connecter UC2 inside of the breaker cell has a jumper installed between T1 and T2, and that it has a resistance of zero ohms
- The output terminals (- and +) of the 24Vdc power supply must not be grounded. Also verify that the F2+ and F1- terminals on the trip unit are not connected to earth ground. Use only one 24Vdc for each trip unit
- Micrologic trip units built after December of 2005 were improved to meet the ANSI and IEC EMC/EFI testing standards after complaints of nuisance tripping caused by EMC/EFI events during switching operations. If the trip unit in question was built prior to December of 2005 it should be replaced
- If no issues are found, the circuit breaker may have a damaged internal CT and/or wiring and the circuit breaker should be replaced
Troubleshooting steps for circuit breakers with a neutral CT installed:
- Measure the resistance between accessory connector UC2 terminals T1 and T2 from the Neutral CT. Any reading less than 100 Ohms should be considered acceptable
- Check resistance between accessory connector UC2 terminals T3 and T4 from the Neutral CT, these readings should measure in MegOhm ranges or Open. Next, check T1, T2, T3 and T4 connections to Ground. T1, T2 and T3 should be open and T4 should be grounded. If readings are not correct, check Neutral CT wiring and Neutral CT. Replace Neutral CT if necessary. If wiring is damaged, reference Square D Instruction Bulletin 48049-082-01 and rewire the Neutral CT per the wiring guidelines
- The output terminals (- and +) of the 24Vdc power supply must not be connected to earth ground. Also verify that the F2+ and F1- terminals on the trip unit are not grounded and be sure to use only one 24Vdc for each trip unit
- Square D added increased EMC/EFI immunity to Micrologic trip units built after December of 2005 following reports of nuisance trips during switching operations. For this reason, the trip unit should be replaced if it was manufactured prior to December 2005
- If problem is not found, the breaker may have a damaged internal CT and/or wiring and it should be replaced
References: Square D RC 71
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