What Is The Relationship Between The Fusing Time Of A Spare Fuse And The Overload Current?

The fusing time of the backup fuse is inversely proportional to overload current, that is, the larger the overload current, the shorter the fusing time, and vice versa. This characteristic is the core principle of the overload and short-circuit protection provided by fuzes, the specific relationship of which can be quantified by time current curves.
There are three stages in the process of melting the physical Basis of the Inverse time flow characteristic:
Heating Stage: When an overloaded current passes through a fuse, its resistance produces Joule heat (Q = I2Rt), causing fuse temperature to rise.
The larger the current, the more heat is generated per unit of time (Q∝I2) and the faster the temperature rises.
Melting stage: When the temperature reaches the melting point of the melt material (e.g. lead-tin alloy is about 230 ° C), the melt begins to melt. The melting time depends on the heat accumulation rate (proportional to the square of the current) and heat dissipation conditions (e.g. ambient temperature and fuse heat dissipation area).
Arc break phase: after the fuse is broken, the circuit is fused and the arc in the gap is extinguished to complete the protective function.
ii. Quantitative Relationship Time-Current Curves
The relationship between melting time (t) and overload current (I) of a fuse can be approximated by the following formula:
t= K. (I-irradiated)
Treated)
n
Location:
Anger: Rated current of a fuse.
K, n: Constants associated with fuse material and structures (usually n≈2~4).
Simplified rule: The melting time is inversely proportional to the square of the overload current current (t∝1/I2). Typical curve characteristics:
Overload current close to rated current (I≈I rated current ): extremely long fuse time (possibly hours), fuse does not trip, allowing the device to be overloaded for a short period of time (e.g. motor start).
Overload current 1.5-2 times rated current: Fusing time is between seconds and minutes and is suitable for preventing long-term overload (e.g., equipment stall).
Overload current 5-10 times rated current: Fusing time is milliseconds, suitable for preventing short circuit failure (e.g., direct short circuit).
For example, the time current curve of a 10A fast-blow fuse may show:
At 15A (1.5 times rated current), fusing time was about 10 seconds;
At 30A (3 times rated current), the fuse time was approximately 0.1 seconds;
At 100A (10 times rated current), the fusion time was approximately 0.001 seconds (1 millisecond). III. Factors affecting fuse Blowing Time
Fuse Type:
Fast action: Low melting point material (e.g., pure tin), small heat dissipation area, sensitivity to overload current, short blowing time (suitable for electronic equipment).
Delay: Material with a high melting point material (e.g., lead-tin alloy), filled with arc quenching materials (e.g. silica sand), large heat dissipation area, allows short-term overload (suitable for motor start-up).
Ambient Temperature: Increased temperature reduces the the resistivity of fuse material, accelerates the accumulation of heat and shortens the blowing time.
Calibration Formula:
t
Adjusted
= t
1+αΔT
1
The $\alpha$is the temperature coefficient (approximately 0.004/°C), while the delta T dollar is the difference between ambient temperature and the standard test temperature (25°C).
3. Installation Conditions:
Bad contact between fuse and socket can lead to local overheating, premature blowing and even fire. The air flow around the fuse is insufficient, which reduces heat dissipation efficiency and lengthens the fuse's melting time.
IV. INTRODUCTION Application Scenarios of Time-Current Curves
Equipment protection design: select rated current and fuse type according to the potential overload current range of the equipment.
For example, if the starting current of the air conditioner compressor is 5 times the rated current, a slow blow fuse should be selected, and at 5 times thecurrent curve of the time shows the melting time > 0.1 seconds (to avoid false tripping).
Due to a load short circuit, the output of the power adapter may be 10 times the rated current; a fast fuse should be selected and the time-current curve shows a melting time < 0.01 seconds of 10 times the current (for rapid tripping faults).
Fault Diagnosis: By measuring the current of the fuse (using a current caliper or recorder) and time when the fuse fuses, combined with a time current curve, the fault type can be determined:
* * _ Chronic overload: _ ** melting time (e.g., minutes), current 1.5 to 2 times rated.
**Short Circuit:** Very short melting time (e.g., milliseconds) with an electric current more than 5 times the rated current.
Alternatives: If the original fuse explodes and no replacement of the same specification is available, a compatible model can be selected using a time current curve.
For example, if a 10A fast blow fuse detonates and a a 15A fast-blow fuse needs to be replaced, it will be necessary to verify whether the 15A fast blow fuse initiates for a sufficient period of time at the maximum current of the device (e.g. 12A) (to avoid malfunction).
V. Common Misconceptions and preventive measures
Myth 1: It is thought that the fuse time is determined entirely by the current.
Risk: Ignoring the effects of environmental temperature, installation conditions, etc., may lead to protection failure or malfunction.
Solution: Refer to time current curves provided by the fuse manufacturer (usually including correction values at different temperatures).
Myth 2: Compare the properties of different fuses directly according to fusing time.
Risk: Different manufacturers may have different curves due to different materials and structures, and specific models must be considered.
Solutions: Prioritize fuzes that meet international standards (e.g., IEC 60127, UL 248) and refer their official datasheets.
Myth 3: Think you need to replace a larger fuse as soon as it fuses.
Risk: This can mask faulty equipment (such as an engine stall or short circuit), leading to further damage.
Solution: After a fuse has blown, first investigate the cause of the fault and then select the same or more suitable size for your own needs. VI. INTRODUCTION Summary table Summary
Time characteristics of fuze in Overload Current Range apply to typical Application Scenarios of fuze types
I can take it. '
Rating
Extremely long (several hours) any type of normal equipment operation (no operation)
1.5I
Rating
Rating
Slow blow motor starting, short term equipment overload
2I
Rating
< I < 5I
Rating
0.1 to 1 second Long term overload (e.g., equipment stall) of fast or slow blowing type
I>5I
Rating
Milliseconds Fast-blow type Short-circuit fault (e.g., direct short circuit)
By understanding the reversetime characteristic and time flow curve of the fuse, the specification of the backup fuse can be selected accurately, and the fuse can be reliable when the device is overloaded or short-circuit, and the fault can be avoided and the circuit can be safe.