[ 05.05.2022 ]

An electric fuse is a switching device designed to protect components in an electrical installation against overloads and short-circuits by opening an electrical circuit.

Principle of operation and design

Let's take a look at the design and operation of the fuse, as it is the most popular fuse. The reasons for its popularity are safety, reliability and ease of use.

The product consists of two basic elements: the fusible link and the base for attaching it with a terminal for connection to the electric circuit. The purpose of the insert is to open the electric circuit in the event of overload. It usually takes the form of a casing with a fusible element inside - a current conductor with a specified cross-section. Parts for high power ratings are supplemented with a filler to extinguish the electric arc. The insert is also equipped with a holder which connects to the fuse base.

Under normal conditions (when there are no overloads or short-circuits), a fuse included in an electrical circuit carries current flowing through it during operation. It heats up, but the heat released is dissipated without deforming the element.

When the current starts to exceed the set value, the fusible element heats up and melts or evaporates completely (if the temperature rises too much). The circuit is broken and the current with substandard values stops flowing to the electrical equipment, protecting it from breakdown and preventing the wiring from catching fire.

The material of the fuse element is selected on the basis of the amperage it has to absorb normally and the amperage at which the circuit is to break.

In simple fuses, the parts of the fused wire are separated from each other by gravity. But with devices for high-power installations, a spring or weight is used. The weights help to quickly separate the wire parts from each other, thus avoiding the formation of an electric arc (which, with its high temperature, can cause burns).

Once the fuse has been destroyed, the circuit can only be restored to operation (with electric current flowing through it) by installing a new fuse.

Key features

Time-current (ampere-second)

This parameter is the most important when selecting a fuse. It reflects the tripping time of the insert in the event of an overload or short-circuit. In mathematical terms, it is the ratio of the melting time of the insert to the current flowing through it. The index demonstrates that the tripping speed depends on the amperage applied to the element. That is, the higher the load, the faster the insert will melt.

Current rating

This is the amperage at which the fuse can carry a certain current without breaking for a specified time. For example, some models of inserts at a rating current can operate for 4 hours. That is, a current will flow through them that has values above the rating, but not enough to melt quickly. This will only happen after 4 hours. During this time, the element will gradually melt while allowing current to flow through.

Temperature dependence of triggering current

The fuse opens the circuit when the insert heats up to its melting point. Therefore, the higher the air temperature, the less energy is needed to melt the insert. In other words, the warmer the environment, the less amperage is needed to trigger the fuse.

It is important here not to confuse the room air temperature with the temperature of the environment that surrounds the element (usually higher, but this is the characteristic that is implied).

I2t (Joule integral)

The time-current characteristic has an important disadvantage: it is designed for alternating currents with a predictable shape (e.g. sinusoidal). But fuses are also used in circuits with pulsed currents where the shape can be different. To determine the energy released by a fuse in such a circuit, the Joule integral I2t, i.e. the integral of the square of the current over a certain period of time, which is expressed in amperes squared per second (A2×s), is used.

Breaking capacity

This is the current value that the fuse can disconnect at a given voltage and certain application conditions. At a higher current, for which the insert is not designed, the element may simply explode (i.e. breakdown will be too fast). The breaking capacity is specified for all fuses.

Voltage rating

When the fuse element trips, the circuit is physically opened. But if the voltage rises strongly, a breakdown may occur (when the current supply is restored, but by air, housing or other method). The voltage rating reflects the load at which this can occur. When selecting a fuse, consideration is given to what the voltage limit may be on a particular mains supply.

Types of fuses

Cylindrical fuse

 

Flag fuse

 

Knife fuse

 

Fuse type D

 

Polymer fuse

 

Thermal fuse

 

According to the principle of operation

The following types of fuses are distinguished according to the way the electric circuit is broken:

• Electromechanical. These have a bimetallic plate that opens the circuit when heated strongly.
• Fusible. The circuit is broken by the melting of a special insert.
• Electronic. The soft key interrupts the circuit when the overload occurs. The key is controlled by an electronic circuit.
• Self-energizing (polymeric) circuits. They consist of polymer materials that can change their properties (get bigger or smaller) and at high current do not allow electricity to pass through. The resistance increases, but then (when the current reaches operating values) decreases. This prevents damage to the external electrical circuit.

 

By time-current triggering characteristics

The fuse does not blow immediately when the current exceeds the value indicated on the enclosure. Normally this value in the circuit must be approx. 2 times the rated current for the fuse to blow quickly.

When calculating the tripping time of a particular fuse, the melting time of the insert (which occurs before the arc is formed) and the extinguishing time of the arc (after which the circuit is opened) are taken into account. The second parameter is usually 1÷10 ms. But the fuse tripping time refers exactly to the time the fuse insert burns out. And this can vary. Depending on this, fuses are differentiated:

• Ultra fast (FF);
• Fast-acting (F);
• Semi-redundant (M);
• Retarded (T);
• Ultra Slow (TT).

 

The time-current characteristic also depends on the insert material, its length and shape.

By design

There are 6 sizes of fuses:

• Type D (first size);
• Cylindrical type A;
• Cylindrical type B;
• Cylindrical type C;
• With pins;
• Cylindrical for use in socket type fuses.

 

All products use gG fusible links with ceramic housing.

A distinction is also made between these types of fuse:

• Low-current inserts. Used in low-power circuits (usually with a current of 6 amperes or less). Generally in the form of a cylinder with metal contacts on the sides and a thin wire inside.
• Forked products. Typically used in DC circuits in automotive applications. The contacts are on one side and the fusible part is on the other side.
• Corked models ("plugs"). The traditional type. These fuses were previously in common use. The porcelain fuse contains a wire which will burn out when overloaded and open the circuit.
• Blade fuses. Generally used in the manufacturing industry for higher loads (up to 1250 amperes).
• Quartz inserts. Used for currents up to 400A in networks up to 36kV.
• Gas generators. When combustion emits gas with an audible popping noise. Used in networks with 35-110 kV load and rated current up to 100A.

 

Fuses can be filled or unfilled. Filled ones have a material (usually quartz sand) inside the insert which extinguishes the electric arc when the conductor burns out. The circuit is only opened when the arc has disappeared.

Advantages and disadvantages of fuses

Key benefits:

• Simple design and high reliability. They burn out quickly, providing effective protection for the circuit and equipment. There is practically nothing to break in the fuse, so it is almost impossible for the mechanism to break, as is the case with automatic relays. In the event of a fault, the element will de-energise the circuit.
• Possibility of using fuses with low tripping speed for selectivity.
• Good breaking capacity at relatively low rated current.
• Many fuse links can be replaced when energised.
• Low cost.

 

Disadvantages:

- Possibility of tripping in only one phase. Therefore, connected equipment such as an asynchronous motor may overheat. For connection of three-phase motors it is recommended to use a phase control relay.
- They only trip once and then burn out. Therefore must be replaced after each trip (unless a self-resetting fuse is used).
- Current, load and breaking capacity limitations.
- Long non-triggering time with a small overcurrent.
Design allows for bugs (bypasses) which can cause fires.

Application

Fuses are used for overload and short-circuit protection of low-voltage and high-voltage electrical circuits. The devices are used in the operation of domestic and industrial electrical equipment. They can be integrated into complete devices.

The following types of fusible links are distinguished according to their application:

• General application. Designed to protect power consumers with high electrodynamic and electrothermal resistance (electric motors, transformer equipment, shop electrical networks, etc.).
• Associated fuses. They are used in conjunction with circuit-breakers and thermal relays. They only trip the circuit at high loads. They either reduce the current value to the values specified for circuit-breakers or deactivate the circuit until the breaker contacts open. They are made only with breaking capacity in a part of the range (in case of small overloads, the protection is provided by auto switches or thermal relays, so there is no point in using full-range bus-bars).
• Fuses for power semiconductors. They can only be made as fast as possible and have current limiting capabilities because semiconductors are sensitive to heat.
• Fuses for transformer installations. They are distinguished by shock and vibration resistance. Particularly suitable for transformers.

 

Today, products are manufactured for use in all climates, on sites with different operating conditions (e.g. level of mechanical stress).