Instantaneous Overcurrent Trip Test (MCCB)
Purpose
• Verify
instantaneous short-circuit protection.
• This
ensures the breaker trips immediately during a fault.
Equipment:
• a
calibrated image-retaining oscilloscope, or a highspeed sampling rate digital
ammeter (capable of accurately measuring a half-cycle pulse) is required
How it works
1. Connect
one pole of the breaker to the test equipment according to the test connection
diagram.
2. Verify
the Smart Trip Unit (STU) settings.
• Set
the Instantaneous pickup (Ii) based on the available test current.
• Disable
ground fault protection (Ig = Off).
3. Turn
the breaker ON.
4. Inject
current into the breaker until it trips.
5. Perform
pulse testing:
• Apply
a short current pulse slightly below the trip threshold → the breaker should
NOT trip.
• Apply
a pulse at or above the trip threshold → the breaker SHOULD trip.
6. Compare
the results with the allowable tolerance values specified in the standard.
Important Notes
• If
the breaker has an undervoltage trip, it
must be energized during the test.
• The current
waveform should be sinusoidal for accurate results.
• If
the breaker does not trip within tolerance,
verify:
• Test
connections
• Mounting
conditions
• Test
equipment accuracy