Air Circuit Breaker Tripping? 90% Overlook This Critical Cause!

In daily life and industrial operations, we often encounter circuit breakers tripping. Common causes include faulty breakers themselves or leakage/short circuits in the load. However, some tripping incidents have unexpected origins.

At a mine, an emergency backup power system consisted of a diesel generator (400V), which fed a mining transformer (10,000V–400V) to step up voltage and supply power to the underground shaft. One rainy day, the main grid power failed. To ensure safety underground, the mine immediately started the diesel generator. However, when attempting to close the breaker to energize the step-up transformer, the air circuit breaker tripped instantly. Repeated attempts resulted in the same immediate tripping. At this point, the high-voltage side switch of the transformer had not yet been closed; the only load in the circuit was the transformer itself—leading to suspicion that the transformer might be faulty.

Mine electricians inspected the transformer visually, finding no signs of arcing or burning. Using a megohmmeter, they tested insulation resistance on both high- and low-voltage sides (including cables), all results appearing normal. Due to limited equipment, no further tests could be performed.

The mine contacted me. I arrived on-site with appropriate instruments and measured the transformer’s winding DC resistance and turns ratio. All data fell within normal ranges. Combined with the electricians’ findings, I concluded the transformer itself was likely intact.

Next, I disconnected the output cables from the switching cabinet, started the diesel generator, and tested the power-up. This time, the air circuit breaker closed successfully—indicating the fault lay between the switching cabinet’s output and the transformer’s high-voltage switch.

Upon carefully inspecting the path between the cabinet and the transformer, I noticed that the transformer’s low-voltage junction box lacked a sealing gasket. The cover plate was extremely close to the low-voltage terminals—only about 3mm apart, far below the required electrical clearance and creepage distance for 380V systems (8mm and 12mm, respectively). I concluded this was the root cause of the breaker tripping.

After reinstalling the sealing gasket into the transformer’s junction box, I restarted the diesel generator. The breaker successfully closed, and power was restored.

The fault occurred because the insufficient clearance between the junction box cover and the low-voltage terminals allowed point discharge during the high inrush current at breaker closing. This caused a three-phase-to-ground short circuit, triggering the instantaneous trip of the air circuit breaker.