Applicable Systems
Okken
Symptoms
The Masterpact MTZ breaker trips on short-circuit with the I indicator active. The Micrologic trip unit logs the fault current magnitude and waveform. The downstream bus is de-energized.
Possible Causes
- Severe phase-to-phase short circuit in the switchgear bus or cable
- Faulty connection on the busbar creating high-impedance arc fault
- Incorrect breaker rating for the prospective short-circuit current
- Failure of a downstream device causing a bolted fault
Troubleshooting Steps
- Step 1: Do not reclose immediately. Visually inspect the switchgear through the inspection window for smoke or damage
No visible smoke, arcing marks, or deformed busbars are present - Step 2: Megger all outgoing feeders and bus sections at 1000V DC
All sections read >10 MOhm to ground and between phases - Step 3: Verify the breaker short-circuit rating (Icu) exceeds the calculated prospective fault current at the installation point
Icu > Isc_max per IEC 60947-2; documentation confirms coordination - Step 4: Open each outgoing breaker sequentially and re-energize to isolate the faulted section
System remains stable when the faulted feeder is isolated
How to Reset
Reset the Micrologic trip unit by pressing the Test/Reset button after confirming the fault is cleared. Reclose the Masterpact only after verifying all downstream insulation is intact.
Common Replacement Parts
- LV848260SP: Schneider Masterpact MTZ2 20H1 with Micrologic 7.0X (Verify frame size, breaking capacity, and communication options)
Frequently Asked Questions
- Q: What does the I trip mean on Micrologic?
It indicates an instantaneous short-circuit trip. The trip unit detected a current exceeding the instantaneous pick-up (typically 2-15x In).
- Q: Can I test the Micrologic trip unit without tripping the breaker?
Yes. Use the built-in Test function which simulates the trip logic without opening the main contacts. Refer to the user manual.
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Source: Schneider Electric Okken Switchboard User Manual, DOCADOC-001
Category: Electrical
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