fuses

Like circuit breakers, they are responsible for protecting electrical circuits. The difference between the two is reuse. Circuit breakers can be turned on and off, while fuses, once blown, can no longer be reused.

Fuse summary

• Fuses are safety equipment.
• They are used to stop the flow of electric current.
• When there is a load greater than the fuse can handle, it blows.
• The fuses are formed by: base, fuse holder, protection ring, fuse link and indicator.
• The most common types of fuses are: NH, D and ultra-fast.
• The difference between fuse and circuit breaker is related to their reuse. While the fuse only works once, the circuit breaker works like a switch and can be used multiple times.

What are fuses?

Fuses are safety devices that prevent the flow of current when there is an overload in an electrical circuit . Thus, when there is a passage of current greater than the fuse supports, it burns, preventing this overload from reaching the circuit.

Fuses can be made of different materials and have different load limits . Thus, they have a two-letter classification:

• the first, lowercase, indicates whether the fuse protects against short circuit or also protects against overload;
• the second letter, capitalized, indicates which type of circuit the fuse is used for.

What types of fuses?

The fuse consists of:

• base: fuse holder;
• fuse holder: location that holds the entire fuse assembly;
• protection ring: protects the base thread;
• fuse link: part that burns out when there is a short;
• Indicator: Indicates fuse operation.

The three most familiar types of fuses are:

• NH fuse: used in industrial electrical installations . It has a high breaking capacity and meets rated currents (current of an equipment operating normally) from 6 A to 1250 A.
• Fuse D: used in electrical installations, it can be handled without risk of taking an electrical discharge. Meets a current range between 2 A to 100 A.
• Ultra- fast fuse: used in frequency converter circuits, as this type of circuit is sensitive to irregularities.

What are fuses for?

The applications of fuses are in various electrical components, from circuits in household appliances and automobiles to electrical installations in homes and industries.

In electrical components, when there is a large and unexpected reduction in their capacity, a short circuit occurs. And when the current intensity of a circuit exceeds the value for which it was sized, there is an overload.

Thus, the main function of a fuse is to protect the electrical component from these phenomena. For this, when a short circuit or overload occurs, the fuse blows and burns, interrupting the flow of current that would damage the circuit.

How do fuses work?

Fuses are made with metallic components that have an electrical current limit. When the current intensity exceeds this limit, the metallic component receives this current and, through the Joule Effect (transformation of electrical energy into thermal ), it melts and fuses, hindering the flow of current , leaving the circuit open, like a switch.

Thus, the fuse must have a lower amperage than that supported by the wiring, but greater than the current that can be reached by the installation.

Differences between fuses and circuit breakers

Both are electrical circuit protection components . The big difference between the two is in the reuse of the equipment.

While the fuse, a simpler piece of equipment, breaks its internal filament and prevents the flow of current, the circuit breaker actually works like a switch. When it detects a short or overload, it shuts down, preventing the flow of current without being damaged.

Solved exercises on fuses

Question 1 – (Unespar 2016) We know that the function of fuses is to protect the electrical installation, acting as a safety switch. The fuse, in any installation, must have an amperage less than that supported by the wiring, but greater than the current that can be reached by the installation.

Consider a living room, with installation for a 250 W TV, three 60 W lamps, a 150 W fan and a 600 W home theater, connected to a voltage of 110 V. Calculate the electrical current in the circuit if all they are turned on at the same time. Based on this, mark the alternative that contains the most appropriate wiring and fuse to ensure the safety of the circuit of this installation.

A) Wiring that supports up to 10 A of current and a fuse of 8 A are sufficient because the total current is approximately i = 10.73 A;

B) Wiring that supports up to 12 A of current and a 10 A fuse are sufficient, as the total current is approximately i = 10.73 A.

C) Wiring that supports up to 15 A of current and a 10 A fuse are sufficient, as the total current is approximately i = 9.23 A.

D) Wiring that supports up to 15 A of current and a 12 A fuse are sufficient, as the total current is approximately i = 10.73 A.

E) Wiring that supports up to 12 A of current and a 10 A fuse are sufficient, as the total current is approximately i = 9.73 A.

Resolution

alternative D.

To find the value of the fuse, it is necessary to find the value of the electrical current in the system. As the exercise gives us the value of the power (P) of the circuit and the voltage (U) to which it is connected, it is possible to find the current through the power equation:

P = Wow

Total power: P = 250 + 3(60) + 150 + 600 = 1180 W.

Voltage: U = 110V.

As the fuse must hold a load greater than the circuit, it must be at least the value of the circuit or greater than it, never less.

2 – (Enem 2014) Fuses are devices to protect an electrical circuit, sensitive to excess electrical current. The simplest models consist of a metallic filament with a low melting point, which melts when the current exceeds a certain value, preventing other parts of the circuit from being damaged. The simplest models consist of a metallic filament with a low melting point, which melts when the current exceeds a certain value, preventing other parts of the circuit from being damaged. The figure shows a diagram of a circuit in which the fuse F protects a resistor R of 12, a light bulb L of 6 W, and a speaker carrying 1 A.

Knowing that this fuse is designed to work with a current up to 20% greater than the rated current that flows through this circuit, what is the value, in amperes, of the maximum current that fuse F allows to pass?

A) 1.0

B) 1.5

C) 2.0

D) 2.5

E) 3.0

Resolution

alternative D.

Finding the total electrical current of the system, it is possible to find the maximum current that the fuse allows to pass.

On the resistor :

U = Ri

12 = 12.i

i = 1 A

On the lamp:

P = Wow

6 = 12.i

i = 0.5 A

On speaker:

i = 1 A

So the total circuit current: i = 2.5 A.

As the fuse supports up to 20% more than the system current:

i = 2.5 + 20% 2.5

i = 2.5 + 0.2.2.5

i = 2.5 + 0.5

i = 3 A