Understanding the Self-Power Option for STUs (ACB)
LS Electric ACBs offer a Self-Power option for Smart Trip Units (STUs), which allows the trip unit to operate without external control power. This function has specific characteristics and limitations that vary depending on the STU type.
What is Self-Power?
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Self-Power is the automatic power supply drawn from the main circuit through the built-in CTs (Current Transformers).
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No external control voltage (AC/DC) is needed.
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The STU is energized by the current flowing through the breaker.
Operating Threshold by Trip Unit Type
| Trip Unit Type | Self-Power Activation Condition |
|---|---|
| N-Type (Basic) | Operates when ≥ 30% of rated current (per phase) |
| A-Type (Ammeter) | Operates when ≥ 30% of rated current (per phase) |
| P-Type (Power Meter) | Operates when ≥ 50% of rated current (per phase) |
| S-Type (Supreme Meter) | Operates when ≥ 50% of rated current (per phase) |
Important Limitations of Self-Power
When operating under Self-Power only (no external voltage supplied), several functions are not available:
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❌ Communication (Modbus, Bluetooth, NFC)
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❌ ZSI (Zone Selective Interlock)
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❌ Remote Trip/Reset
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❌ Digital Output Contacts
❌ Incident Waveform Recording
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(A, P and S type) STUs can save 127 event logs, and 6 incident waveforms
In case of operation by self power, incident waveforms are not save
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Summary for Customers
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Self-Power is a convenient way to power the STU using load current alone.
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For full STU functionality, including communication, waveform recording, and remote operations, external control power is required.
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If Self-Power is used, please ensure your application meets the minimum current threshold per STU type.