How to Choose a Fuse for Safety?

PublishTime： 2023-12-11 Article Source：XC News

1. How much current can a fuse flow through?

The rated current is the current value that a fuse can operate for a long time, not the operating current.

Due to safety issues related to fuses, they are considered safety devices and must pass relevant certifications before they can be produced and sold.

Usually, safety regulations require that the temperature of a fuse should not eXCeed the allowable value when operating at the rated current. For example, UL specifies that the temperature should not exceed 75℃ when operating at 100% of the rated current.

The safety regulations also impose strict requirements on the fusing time when the actual current exceeds the rated current. For example, a fuse of a certain specification can operate continuously for at least 60 minutes at 1.5 times the rated current; At 2.1 times the rated current, it can work continuously for up to 2 minutes; At 2.75 times the rated current, it can work for at least 400ms ($1.7100) and at most 10s; There is also a requirement for fusing time when the rated current is 4 times or 10 times.

In addition, UL specification fuses have low overload capacity, so a current reduction (Frf=0.75) is required. For 3A applications, when other characteristics are not considered, at least 4A (3/0.75=4) fuses should be selected.

2. How can I work safely?

When a fault occurs in the subsequent circuit (such as a short circuit), almost all of the input voltage is applied to both ends of the fuse! This generates a very high current. Will such a large current cause the fuse to continue to conduct and cannot be pulled open? It's completely possible! So, the concept of breaking current was proposed.

IEC and UL have different requirements for breaking capacity. Taking IEC 127 as an example, there are the following provisions:

The low breaking capacity (LBC) fuse must be able to cut off the larger 35A current or 10 times the rated current.

The EBC capacity fuse must be able to safely cut off 150A current;

The high breaking capacity (HBC) fuse must be able to safely cut off 1500A current.

Breaking capacity is an important safety indicator for fuses. Therefore, when selecting a fuse, careful consideration should be given to this characteristic, and actual testing should be conducted. The maximum short-circuit current that a fuse may experience should not exceed its rated breaking capacity, ensuring that the fuse can safely cut off the circuit in the event of a subsequent fault or large current flow, without causing safety issues.

3. How long does it take for a fuse to blow?

How long is its lifespan? Fusing characteristic, also known as I-T characteristic. This characteristic is the main performance indicator, and fuses can be divided into fast melting, slow melting, and medium speed melting based on the melting time. For resistive circuits or circuits that protect sensitive devices, quick fuse should be selected; For capacitive and inductive circuits or circuits with large pulses, slow fuse should be selected. It indicates the time range of fuse blowing under different current loads.

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