Many non-professionals find fuse selection confusing. With a wide variety of fuses available, how should we make the right choice and what principles should we follow? A brief introduction is provided below.

(I) Selection of Fuse Type

The fuse type shall be selected according to the application scenario.

- Knife-contact fuses are generally adopted for power grid distribution;

- Screw-type fuses are commonly used for motor protection;

- Cartridge fuses are applied to lighting circuits.

For the protection of thyristor components, semiconductor high-speed fuses shall be selected.

(II) Selection of Fuse Specifications

1. Selection of the rated current of the fuse element

(1) The rated current of the fuse element shall be slightly greater than or equal to the load current of transformers, electric furnaces and lighting loads.

(2) For power transmission and distribution lines, the rated current of the fuse element shall be slightly greater than or equal to the safe current of the line.

(3) When used for short-circuit protection in motor circuits, motor starting conditions shall be taken into consideration, and the rated current of the fuse element shall be determined based on the motor starting duration.

For motors with a short starting time, the rated current of the fuse element:

Iₙ(fuse element) = Iₛₜ / (2.5~3)

Where Iₛₜ stands for the motor starting current.

For longer starting durations:

Iₙ(fuse element) = Iₛₜ / (1.6~2)

For fuses installed at the main busbar supplying multiple motors, the rated current is calculated as follows:

Iₙ = (2.0~2.5) × Iₘₐₓ + ΣIₘ

Notes:

Iₙ — Rated current of the fuse;

Iₘ — Rated current of a single motor;

Iₘₐₓ — Rated current of the largest-capacity motor among multiple motors;

ΣIₘ — Sum of rated currents of other motors.

For protection at motor terminal circuits, aM fuses are selected, and the rated current of the fuse-link shall be slightly higher than the motor rated current.

(4) Protection for the main circuit of capacitor compensation cabinets:

- If gG fuses are adopted, the fuse rated current Iₙ shall be approximately 1.8~2.5 times the calculated line current;

- If aM fuses are adopted, the fuse rated current Iₙ shall be approximately 1~2.5 times the line current.

(5) Selective protection between upstream and downstream circuits:

A ratio of rated current Iₙ between the upstream fuse and downstream fuse equal to or greater than 1.6 can avoid unwanted cascade tripping and limit the scope of power outage caused by faults.

(6) Fuses for semiconductor device protection:

The fuse is connected in series with the semiconductor device. The rated current of the fuse element is expressed as the RMS value, while the rated current of semiconductor devices is given as the average forward current. Therefore, the rated current of the fuse element shall comply with the formula:

Iᵣₙ ≥ 1.57 Iₐᵥ ≈ 1.6 Iₐᵥ

Where Iₐᵥ denotes the average forward current of the semiconductor device.

(7) At an ambient temperature of 20℃, the actual operating current of the fuse-link is recommended not to exceed its rated current. Ambient and operating conditions shall be considered during selection, including airflow, cable specifications (length and cross-section), and transient peak currents.

Current-carrying capacity tests are conducted at 20℃; actual performance is affected by ambient temperature variation. Higher ambient temperature leads to higher operating temperature of the fuse-link and shorter service life. Conversely, operation at lower temperatures extends the service life.

(8) In power distribution circuits, the rated current of the upstream fuse element is generally required to be 2~3 times that of the downstream one, preventing expansion of fault outage scope.

2. General fuse parameter selection

(1) Rated voltage of fuse Uₙ(fuse) ≥ Rated line voltage Uₙ(line)

(2) Rated current of fuse Iₙ(fuse) ≥ Operating line current Iₙ(line)

(3) The maximum breaking capacity of the fuse shall be higher than the maximum prospective short-circuit current of the protected circuit.

Fuses are mainly used to protect low-voltage electrical equipment in industrial production and daily life. Selection criteria differ greatly for different electrical equipment. Proper selection and configuration must be implemented strictly in practice; otherwise, the fuse will fail to deliver effective protection.

3. Select high-quality fuses

A wide range of fuses and brands are available on the market. It is recommended to choose brand fuses with reliable quality. Purchase from authorized formal distributors to avoid counterfeit and inferior products.