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Basic Knowledge of Current Fuse Applications

Release Time: 12 Nov,2021

I. The Function of a Fuse:

1. Under normal circumstances, a fuse serves to connect the circuit.

2. Under abnormal (overload) conditions, the fuse, as a safety protection device in the circuit, safely interrupts and protects the circuit by melting itself.

II. Operating Principle of Fuses:

When the fuse is energized, the heat generated by the conversion of electrical energy causes the temperature of the fusible element to rise. Under normal operating current or permissible overload conditions, the heat produced is dissipated to the surrounding environment through radiation from the fusible element and the housing, as well as through convection and conduction, gradually reaching thermal equilibrium with the heat generated. If the heat generated exceeds the heat dissipated, the excess heat accumulates in the fusible element, causing its temperature to rise. When the temperature reaches or exceeds the melting point of the fusible element, the element melts and breaks, thereby interrupting the current and providing circuit protection.

III. Classification of Fuses:

1. Classified by physical dimensions: small fuses, ultra-small fuses (tube fuses), square fuses, and others.

2. Classified by fusing characteristics: fast-acting, medium-time-delay, and time-delay fuses. (They can also be further classified as extra-fast and extra-long-time-delay.)

3. Classified by breaking capacity: low-breaking type and high-breaking type (which can further be categorized as enhanced-breaking type).

4. Classified according to safety standards (or regions of use) as follows: UL/CSA (North America) specifications, IEC (China, Europe, etc.) specifications, MIT/KTL (Japan/South Korea) specifications, and others.

5. Other categories.

IV. Technical Terms Related to Fuses:

1. Rated Current: The nominal operating current of the fuse tube (the maximum current under normal conditions that allows the fuse to operate reliably over the long term).

2. Rated Voltage: The nominal operating voltage of the fuse (the maximum voltage it can safely withstand at the instant of interruption). When selecting a fuse, its rated voltage must be higher than the input voltage of the circuit being protected.

3. Breaking Capacity: When a very large overcurrent occurs in the circuit (such as a severe short circuit), the fuse must be able to safely interrupt (break) the circuit at the maximum current level. This is the most critical safety parameter for a fuse. Safe interruption means that, during the breaking process, no phenomena such as spattering, burning, or explosion occur—phenomena that could endanger surrounding components, other parts, or even personal safety.

4. Overload Capacity (Current-Carrying Capacity): The maximum overload current that a fuse can withstand for a specified duration while continuing to operate. When the current flowing through the fuse exceeds its rated current, the temperature of the fusible element will gradually rise over time and eventually lead to melting and interruption of the circuit.

UL standards stipulate that a fuse must remain operational for more than 4 hours, with a maximum non-blown current of 110% of the rated current (100% for miniature fuse tubes).

According to IEC standards, a fuse shall remain operational for more than 1 hour, with a maximum non-blown current of 150% of its rated current.

5. Fuse Time-Current Characteristics (I-T): The relationship between the load current applied to a fuse and the fuse’s fusing time.

A. Fuse Time–Current Characteristic Curve: In a logarithmic coordinate system where the load current is plotted on the X-axis and the fusing time on the Y-axis, the curve is formed by connecting the average fusing-time data points for a given fuse at various load-current levels. Each fuse model and rating has a corresponding curve that represents its fusing characteristics, which effectively illustrates the fuse’s overload performance and serves as a reference for selecting the appropriate fuse.

B. Fuse Rating Table: A table consisting of several specified, representative load current values and their corresponding fusing time ranges. All relevant safety standards explicitly stipulate that this table serves as the primary basis for acceptance testing of fuses.

For example, quick-blow fuses of specifications such as UL, CSA, and MIT/KTLA are defined as:

In 100% Minimum of 4 hours

At 135% Maximum for 1 hour

At 200% Maximum for 2 minutes

6. Melting Thermal Energy (I²T): The nominal energy value required to melt the fusible element of a fuse and cause partial vaporization, thereby interrupting the current. Simply put, it is the minimum thermal energy needed to blow the fuse.

Total I²t = Melting I²t + Arcing I²t

Among these, the melting I²t (equivalent to the pre-arcing I²t in the IEC standard) refers to the energy required from the moment the fusible element melts until the instant arcing begins; the arcing I²t refers to the energy required from the instant arcing begins until the arc is finally extinguished. For low-voltage fuses, the arcing time is extremely short and can often be neglected, meaning the arcing I²t may be taken as zero.

Neither UL nor IEC specifies requirements for I²t; however, I²t can be helpful in selecting fuses. The I²t of a fuse is typically calculated based on the I²t value corresponding to a fusing time of less than 10 ms (commonly taken as 8 ms). Fuse datasheets provide I–T curves for each rating, along with reference I²t values for the corresponding ratings, which can serve as guidelines when selecting fuses.

7. Voltage Drop: The voltage difference across the fuse terminals when thermal equilibrium is reached under rated current conditions.

8. Temperature Rise: Under specified current conditions, after thermal equilibrium is reached, the temperature rise is defined as the difference between the fuse’s surface temperature and its initial ambient temperature (which can be regarded as the surrounding environment temperature), i.e., Temperature Rise = Fuse Surface Temperature – Ambient Temperature.

V. Safety Standards and Markings for Fuse Tubes:

1. UL and CSA Standards: Safety standards for North America, including the United States and Canada; the standard for small-current fuse links is UL 248-1/14 and CSA 248-1/14.

Safety Signs:

--- UL/CSA LIST (listing mark), a product safety mark indicating that the product has been fully tested and certified in accordance with UL/CSA 248-1/14 standards.

— UL/CSA RECOGNIZED (Recognition Mark), a product safety mark indicating that the product has been tested and certified in accordance with UL/CSA Standard 248-1/14.

— UL-certified and CSA-recognized listed/approved safety marks are equivalent to

2. JIS Standard: Japanese Electrical Safety Standard. The standard for small-current fuse cartridges is JIS C6575.

Safety Signs:

--- PSE

3. KTL Standard: South Korean Electrical Appliance Safety Standard.

Safety Signs:

--- K

4. IEC Standards: Standards developed by the International Electrotechnical Commission, serving as safety standards in Europe and China. The standard for small-current fuse links is IEC 60127 and GB 9364 (China).

Safety Signs:

CCC --- China

SEMKO --- Sweden

VDE --- Germany

BSI --- United Kingdom

IMQ --- Italy

Explanation of Temperature Fuse Terminology

Performance Differences Between Fast-Acting and Time-Lag Fuses

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