Slowblow (Time Delay) Fuses for Control Transformers

Question: Can you clarify why you would use a slowblow (Time Delay) instead of a fast acting (one-time) fuse for protecting control transformers?

Answer: [Provided by Vince Saporita, Bussman Fuses]

First of all, let’s note that motor control transformer overload protection is covered in Article 430, Section VI, and not Article 450, where most other transformer requirements are covered. However, the rules for protection with time delay fuses are consistent. In fact, Article 430.72(C)(2) allows control transformers to be protected in accordance with Article 450.3.

Time delay fuses are used for all low voltage transformer applications, including control transformers. Energizing a transformer causes an inrush current. On power transformers, the industry standard is to assume the RMS inrush current is 25 times RMS full load current for .01 seconds and 12 times RMS full load current for .1 seconds. Control transformers can have peak inrush currents that are 100 times the full load RMS current for 1/2 cycle. With these large inrush currents, fast acting fuses would need to be sized many times larger than the transformer full load current, just to hold the inrush. And if they are sized that large, they won't provide adequate short circuit protection. Therefore, time delay fuses are used, so that they can be sized closer to the FLA of the transformer.

Note that even a time delay fuse cannot be sized very close to a control transformer that has a 100 times FLA inrush. That is why motor control transformers of 2 amps and less are allowed to have fuses sized at 500%, per 430.72. Finally, transformers often have a larger inrush when unloaded than they do when fully loaded.

The above newsletter was edited and is managed by: Victor M. Ammons, PE, Director of Electrical Engineering for The Prisco Group, Hopewell, NJ.

Mike Holt's Comment: If you have any comments or feedback, please let me know,


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