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Nuisance Tripping of Fuses or Breakers at Energization

When a transformer is energized, it draws a large current for a few cycles to establish the eletromagnetic field. During this time, the transformer load has little to no effect on the initial transformer inrush. Inrush can vary depending on where the sine wave is during initial energization and how long the transformer was denergized before being energized again. Inrush current can be up to 12 times rated FLA current. In some cases custom transformers can be built to have very low inrush values.

Check: Solution:
Are overcurrent device sized correctly per applicable codes. Size overcurrent devices correctly. NEC code should be reviewed. In general buck-boost overcurrent protection should be sized NEC 450-4 which is also found in the back of our buck boost section in the catalog. 600 volt class distribution overcurrent protection should be sized per NEC Table 450.3(A).
Are overcurrent devices time delay? If overcurrent devices are quick trip, replace with time delay version.
Overcurrent devices are sized for a load which is significantly lower than the rated transformer amperage. Overcurrent devices should be sized for the transformer amperage since inrush is dependent on transformer size and not load.
Is the transformer being back fed? Backfeeding is generally done when taking a standard step down transformer and using it to step up the voltage which reverses the order of energization for the windings. Transformers being backfed will experience higher inrush currents since the coil being energized (secondary) is further from the core. In this case the overcurrent devices should be sized to the maximum allowed per the applicable code requirements. In some cases a custom transformer may need to be used instead of backfeeding a standard transformer.
Is the input voltage higher than the input is set up for? Lower the input voltage, adjust the taps or use a transformer capable of adjusting for a higher input voltage.
Check in the input frequency, lower input frequencies can cause higher inrush currents. Adjust the input frequency or replace transformer with a unit capable of using a lower frequency (us a 50/60 Hz core).
Check the core to make sure it is properly aligned, tight and has not shifted. If possible tighten any core clamps. If the core has shifted or is damaged, it may have to be repaired or replaced. Bad core steel or a core built too small for the application would have similar problems.
Check the coil insulation. Shorted turnscan cause nuisance tripping initially but this will quickly turn into a turn to turn fault which will destroy the coil.
Check the primary power factor. Low power factor can cause higher inrush currents. This is a systems problem that will require power factor correction.

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