Are You Picking the Right Fuse to Protect Your Distribution Transformers?

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Almost every day I get asked by a customer or a member of our sales team on how to select the right speed and ampere rating of the fuse to protect a distribution transformer. Often, it’s new engineers who wonder why a certain fuse has been used for protecting a certain transformer. What’s the reasoning behind the selection?

We have the information to help in making the right fuse selection. But here are some general things to consider when choosing a transformer protection fuse.

For one, every transformer should have individual fuse protection. When making a fuse selection, one of the first parameters to be considered is the voltage rating of the fuse, which needs to match the voltage rating of the transformer. Another important parameter is the fuse fault-interrupting capacity, which should exceed or be equal to the maximum available fault at the transformer location. The ampere rating of the fuse should be selected such that it can handle all the anticipated loading levels of the transformer and also the inrush currents, such as transformer-magnetizing currents.

The Time-Current Characteristic (TCC) curve of the fuse, determined by the speed rating of the fuse, should be selected to ensure the fuse protects the transformer from damaging currents. The best way to ensure this is to make sure the TCC of the fuse is, at a minimum, to the left of the ANSI point on the transformer damage curve and intersects the transformer damage curve at the lowest current possible.

There are other methods of selecting a fuse for transformer protection. For example, some practices include predetermined rules of thumb for ampere rating of the fuses; some other practices include using a certain fuse link speed rating for transformer protection across the distribution system.

I’d be interested in hearing about fuse-selection practices and fuse-selection issues you’ve come across in the Comments section below.