Cummins/Onan — Testing Circuit Breaker

Testing & diagnosis

From a Cummins/Onan service manual — check it applies to your exact model before use.

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Procedure

1. General: Common maintenance practices such as periodic exercise, checking conductor connections, cleanliness, and security.
2. Exercise breaker: Actuate breaker handle to ON and OFF several times. If equipped with Trip Test button, trip, reset, and actuate to ON several times.
3. Checking insulation resistance: Disconnect load and line conductors. Place breaker to ON. Using ohmmeter R x 10K scale, measure insulation resistance between each pole and ground.
4. Test between phases of opposite polarity and current carrying parts to ground. Also test between line and load terminals with breaker in OFF position. Reading less than one megohm indicates ground.
5. Checking contact resistance: Isolate breaker by placing Remote-Stop-Run switch to Stop position, disconnect generator set starting battery, and disconnect line and load wires.
6. Place circuit breaker handle to ON position. Check resistance values across each pole (line to load). Readings should be relatively equal for all three phases.
7. Voltage drop check: Ensure line and load wire connections are proper and secure. Start generator set. Place breaker handle to Reset and then to ON position.
8. Perform voltage check at line connection then load connection. Compare voltage drop readings across each pole. Unequal or excessive millivolt drops indicate contaminated contacts or loose connections.
9. Checking shunt-trip operation: Refer to order, installation wiring diagrams, and wire routing to identify proper AC or DC signal source connections. Check wire connections are secure.
10. Place circuit breaker to ON position. Apply appropriate signal voltage (DC: 12-, 24-volts, etc.; or AC: 120/240-, 480-volts). Shunt-trip solenoid should energize and trip open the breaker.
11. If breaker did not trip open, remove signal source and perform continuity check of interconnect wiring and shunt-trip solenoid lead wires. Replace defective wiring or solenoid.
12. Checking auxiliary contacts: If equipped, circuit breaker has three leads for form C contacts (1 normally-open, 1 normally-closed) for remote annunciation.
13. Perform continuity checks of wiring with breaker at ON and OFF position to confirm proper lead selection for interconnect. Replace internal contact circuits if shorted or open.
14. Adjusting magnetic trip operation: If equipped with front-adjustable magnetic trip controls, short circuit protection for each pole can be adjusted equally or individually.
15. Adjustors are set equally to high position at factory. Consult on-site requirements and adjust to proper position.

Source: Cummins/Onan service manual. Steps and values are as documented; always cross-check against the official manual and any later revisions for your exact serial number before servicing.

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