Circuit Protection for Power Distribution Transformers

Medium Voltage Circuit Protection for Power
Distribution Transformers 600V and Up

transformer substation fusing circuit protection


by Greg Carter

Fuses for Transformer and Switchgear Circuit Protection can suffer from an identity crisis. They’re often the same physical size and look similar.

This can lead to mistakes in selecting a replacement fuse. Selecting the right fuse for medium voltage power distribution transformer applications 600V and above is especially problematic. The “in-rush” of current when a power transformer is energized requires fuses especially suited for inrush current, sometimes called input surge or switch-on surge. Since transformers must handle significant fluctuations in voltages, medium voltage fuses for power distribution transformers come in a wide range of system voltages and ratings.

Before talking further about choosing power circuit fuses for power distribution transformers, let’s first look at the basics of power distribution transformers and then consider factors in correctly sizing power transformer fuses.

Transformers and Fusing for Long Distance Transmission

Power Distribution Transformers are second only to motors for needing suitable power distribution fuses for circuit protection of electrical systems and personnel safety. Transformers are electrical devices that transfer – and transform – power between circuits. Sometimes it’s a long journey between power stations. To lighten things up, consider this

Fusing for substation transformers

Fusing in substation transformers provides protection between circuits as voltages are “stepped up” or “stepped down” .

analogy: Just as train passengers moving through transportation substations step up or step down into trains that are slowing, stopping, then accelerating, transformers in electrical power substations “step up” or “step down” voltages to regulate current carried between circuits.

Circuits feeding transformers are Primary Circuits, while circuits drawing the energy are Secondary Circuits that handle the “load”. Selecting fuses for Primary and Secondary transformer circuits is essential to a reliable power distribution system.

7 Factors in Selecting MV Fuses for Power Transformers

Selecting the right fuse for the job depends on the application. Monster Fuses Suggested Fuse Ratings for Power Transformer Applications is a good place to start. Fuse selection depends on many factors, so consult with a professional engineer. Four factors must be considered when applying Medium Voltage Fuses for Transformers: Voltage Rating, Current Rating, Interrupting rating, and fuse coordination with other protective devices.

S&C Power Fuses Power Fuse “Selection Guide” for Distribution Transformers lists seven factors to consider when selecting Medium Voltage (MV) fuses to protect transformers — in descending order of importance:

1. Fuse voltage and interrupting ratings must equal or exceed system requirements.
2. The fuse’s continuous current rating must be large enough to withstand transformer magnetizing (in-rush) current. (Minimum Fuse Rating).
3. The fuse’s continuous current rating must be able to withstand transformer overloading and emergency operation, and meet all NEC® requirements. (Maximum Fuse Rating).
4. Fuses must protect the system on the line side of the fuse from the effects of short circuits on the load side of the fuses. (Utility System Coordination).
5. Fuses must coordinate with the transformer secondary protection where and whenever possible. (Facility System Coordination).
6. Fuses must protect transformer against secondary bolted faults.
7. Where and whenever possible, fuses should protect the transformer against higher impedance secondary faults

Power distribution transformers handle a wide range of currents. You can avoid fuse sizing mistakes when selecting and correctly sizing replacement fuses for power distribution transformers. Proper fuse selection depends on many factors, so consult a professional engineer for your application.

Monster Fuses is a factory authorized supplier of all leading fuses for power transformers. Go to for our selection of power transformer fuses. Call us for help calculating fuse current ratings for transformers or selecting fuse hardwarepower equipment and electrical replacement parts.
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