How Do I Select The Rated Current Of A Spare Fuse Based On The Equipment's Current? What Are The Risks Of Selecting A Higher Or Lower Rated Current? - News

The selection of the rated current of the backup fuse must match the safety margin scientifically according to the operating current characteristics of the device. Improper selection can lead to equipment damage or safety hazards. The following are specific options and risk analyses:
I. Selection Principles for Fuse Rated Current
1.Core formula
The rated current of the fuse (I< suber> rated current ) shall be satisfied:
≥ K_{I_max

Location:

i_max: Maximum continuous current of the device (obtained by actual measurement or from the equipment nameplate).

K: Safety factor, generally 1.25-1.5 (1.25 for fast blow fuses and 1.5 for slow blow fuses).

For example, if the maximum continuous current of the device is 10A:

Fast Fuses:

10A ÷ 1.25 = 8A

(In fact, the most recent values ≥ 8A, such as 10A, in the standard specification should be selected.) Slow blow fuses:

10A ÷ 1.5 ≤ 6.7A

(Select 10A or 15A from the standard specification; further verification is required based on the start-up current of the device).

2. Think of the device's starting current

Electrical equipment (e.g., air conditioners, refrigerator): Start current can be 3-7 times the rated current. Slow fuses should be selected, and their rated current should be able to withstand short-term overload.

Calculation method:

I...

Rating

≥

3

I...

startup

Assuming a starting current of 30A, the fuse must be rated ≥ 10 A.

Pure resistance load (e.g. electric heater, bulb): Start current close to working current; fuse can be selected directly based on maximum continuous current.

3. Ambient temperature correction

The rated current of the fuse will decrease as the temperature increases (for example, at 85 ° C, the rated current may decrease by 20%). If the equipment operates in a hot environment (e.g. engine compartment or outdoors), the following formula should be used to adjust:

I Adjusted = Irated × (1−αΔT)

Location:

α: Temperature coefficient (0.004/° C C).

T: Difference between ambient temperature and standard test temperature (25°C).

For example, if the fuse has a rated current of 10A and an ambient temperature of 60°C:

I Adjust = 10A × [1-0.004 × (60−25)] = 8.6A In this case, a fuze with a high rated current (e.g. 15A) shall be selected to ensure reliability.

ii. Risk analysis for the selection of a Larger or Smaller Fuses

1. Risk of choosing a larger fuse (rated current > Equipment Requirements)

Protective Fault: When overloaded or short-circuited, the fuse does not melt in time, causing the device or circuit to overheat continuously.

Consequences: motor windings, capacitors and other components burn out. This can lead to ageing insulation and even fires (for example, using excessive fuses in car circuit, which can cause wiring bundles to burnout).

Case study: Users replaced air conditioner fuses from 15A to 25A. Because the compressor stalled, the current reached 40A and the fuse did not blow, ultimately causing the compressor to burn.

2. Risk of use of an Undersized Fuse (Rated Current < Equipment Requirements)

Frequent Blowing: Fuses explode when normal equipment is activated or temporarily overloaded, affecting availability.

Consequences: Increased maintenance costs (e.g., an incorrectly selected car fuse in vehicles, resulting in circuit interruption during driving).

It may be the fault of the masking equipment (for example, the motor bearing is jammed, causing an increase in the current, and the root cause is not yet identified after the fuse explodes).

Case study: One user changed her refrigerator fuse from 10A to 5A. Because the compressor starts with a current of up to 15A, the fuse frequently explodes, eventually causing food to spoil.

III. Selection Procedure

Determine Equipment Parameters:

Measure or view the equipment nameplate for maximum continuous operating current (I< subs> max < / subs >) and start current (I< subs> start ).

Select Fuse Type:

Motor equipment: Slow-blow type (flat-flow characteristic curve).

Electrical equipment: Fast blowing type (steep time-current characteristic curve).

Calculate rated current:

Fast Blow:

Irated = Imax ×.

Slow Blow:

Irated = max(Imax x1.5, I start / 3).

Environmental correction:

If the ambient temperature is greater than40°C, the rated current is corrected according to the formula or a higher specification is selected directly.

Verify Compatibility:

Verify fuse size, voltage rating (must ≥ equipment voltage), and certification standard (e.g. automotive industry requirements AEC-Q200). IV. INTRODUCTION Common Misconceptions and solutions

Myth 1: Temporary replacement of a blown fuse with a wire.

Risk: Wire melting point too high to interrupt current and could start a fire.

Solutions: Immediately replace fuses with fuses of the same specification and investigate the cause of fuse detonation (e.g. short circuit, overload).

Myth 2: Use multiple low-rated current fuses at once.

Risks: Parallel connection may prolong the total fusing time, and different fuse resistors may cause the current to be unevenly distributed.

Solution: Choose a fuse with a high rated current or redesign circuit protection scheme.

V. Summary Table

Choice factor corrects incorrect practice

Rated Current Fast Blow:

I...

max

×1.25

*;Slow Blow:

Paramount (I)

max

x 1.5, I

startup

/3)

Select a specification close to the current of the device at random (e.g. 10A fuze for a 10A equipment)

Ambient Temperature High-temperature environments require lower or higher specification. Ignore the temperature effect and choose based on room temperature.

Fuse type motor chooses toblow slowly and the electronic equipment chooses to blow fast. Use only quick-blow fuses (causing frequent engine blowouts) or slow-blow fuses (causing electronic equipment protection failures)

Replace with a fuse of the same specification immediately after blowing and troubleshoot. For the time being, replace it with wire, copper wire or a larger fuse.

Scientific selection of fuses to avoid common hazards, to ensure reliable detonation of fuses when equipment is overloaded or short-circuited, and to avoid malfunctions and to ensure the safety of equipment and personnel.}