How To Select Fuse Across The Primary Or Secondary Side Of A Transformer?

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There are different types of fuses having different characteristics. Most common types are categorised as fast-acting and slow-acting fuses. Fuse characteristics can be adapted to different loads. Common fuse types include glass-tube fuses, miniature circuit-breakers, thermal fuses.

While selecting a fuse, current rating is more important than voltage rating; a fuse can operate at any voltage up to its rated voltage. Choose a standard fuse size in relation to the current specified on the transformer. The rule is to find the nearest standard fuse that is rated larger than the rated current.

The secondary fuse is placed after the transformer’s output points of connection on the secondary side. It acts as overload protection for the transformer. Its rating must not exceed the secondary current specified on the transformer. The choice of characteristic, slow or fast, is determined by the load.

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The primary fuse is placed in front of the transformer’s points of connection on the primary side. It acts as short-circuit protection for the transformer. All distribution transformers have fuses on primary side but low-power transformers for electronic circuitry have optional fuses depending on the applications.

When device is switched on, the starting current can be many times higher than nominal current for a short period. It is important that the primary fuse is as slow-acting as possible.

how to calculate the ampere rating of the fuse?

Suppose a transformer is rated at 230V AC primary and 12V, 1A secondary. The voltage turn ratio is given by primary voltage/secondary voltage=230V/12V=19.2A. The primary fuse current can be calculated from current turn ratio, which is secondary rated current/primary current, that is 0.052A.

You can use a primary fuse that is 135% of this value, which is about 100mA. You can select a nearest standard size fuse with current rating not less than 100mA. Note that you can use this fuse across the secondary side but not after the rectifier circuit. A fuse installed in an AC circuit performs differently when installed in a DC circuit.

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