Fascinating Features and Laws of Fuses

The speed at which the fuse trips depends on the amount of current flowing through the circuit and the type of material the fuse is made of. The operating time of the fuse is not a fixed range, but decreases as the current increases. The operating time of fuses has different characteristics in relation to current fuses, being called fast or delayed depending on the response time to an overcurrent condition. Typically, a fuse takes twice its rated current to trip in 0.1 second, and a slotted fuse takes twice its rated current to trip in 10 seconds.

Backup selection may depend on load characteristics. Fast or ultrafast fuses are used in semiconductor devices because they heat up quickly when there is an overcurrent. Most fast-response electrical devices require faster-blow fuses because electrical machines can be seriously damaged by an overload current. These types of backups are used for general purposes. The slow blow fuse (delay fuse) is designed to allow current to pass through the fuse above the rated value for a short period of time without blowing the fuse. These fuses are used in motors that can draw higher rated currents for several seconds while reaching their rated speed.

The amount of energy used by the fuse element to correct electrical faults. This expression is normally used in short circuits and the values ​​are used to carry out coordination studies in an electrical network. I2T parameters are provided in the manufacturer's technical data sheet table for each fuse. Backup coordination operations with upstream or downstream devices, I2T fusion and I2T extinction are indicated. I2T fusion corresponds to the energy required to fuse the fuse element. Clear I2T is proportional to the total power allowed by the backup when a fault is cleared. Power depends mainly on the current and time of the fuses and the available fault level and system voltage. Because the fuse's I2T rating is proportional to the energy it allows, it measures the thermal damage and magnetic forces that a fault produces.

The breaking capacity is the maximum current that can be safely interrupted by the fuse. In general, this should be higher than the expected short-circuit current. Miniature fuses can have an interrupting rating of only ten times the rated current. Some fuses are marked HRC (High Rupture Capacity) and are regularly filled with sand or similar material. Fuses for small, low-voltage home wiring systems are typically rated for an interrupt of 10,000 A. Similarly, fuses for larger power systems have higher interrupt ratings, with some low-voltage current-limiting fuses rated at 30,000 amps. Circuit fault level total apparent power rates fuses for high voltage equipment up to 115,000 volts.

The rated voltage of the fuse must be greater than or equal to the open circuit voltage. For example, a glass tube fuse rated at 32V would not interrupt power to a 120V or 230V voltage source. If a 32 volt fuse attempts to interrupt 120V or 230V power, an arc may occur. The plasma in the glass tube fuse can pass the current until the current decreases and the plasma returns to an insulating gas. The nominal voltage must be greater than the maximum voltage source that would be required to disconnect. The voltage rating remains the same for each fuse when related fuses are connected in series. Connecting fuses in series does not increase the combination voltage rating.

The voltage drop across the fuse is usually specified by the manufacturer. The resistance of the fusible element may change as it heats up due to energy dissipation when carrying higher currents. This resulting voltage drop must be taken into account, especially when using a fuse in low voltage applications. With traditional wire fuses, the voltage drop is often not important, but with other technologies, such as B. with reconfigurable fuses (PPTC), it can be significant.

Ambient temperature changes the operating parameters of fuses. A 1 amp fuse at 25 ^° C can conduct up to 10 to 20% more electricity at 40 to 60 °C. ^Ó C and can open at 80% of its rated power at 100 ^Ó C. Operating values ​​vary depending on the fuse family and are specified in the manufacturer's technical sheets.

Fuses are manufactured in various sizes and styles to suit many applications. They are manufactured in standardized housing layouts to make them easily interchangeable. Depending on the application and voltage rating, fuse bodies can be made of ceramic, glass, plastic, fiberglass, molded mica laminates or molded compressed fibers.

It determines the current carrying capacity of a fuse wire. The fuse conducts normal current under stable conditions without increasing its normal temperature to the melting point. In this state, the heat generated by the wind through the fuse wire is equal to the heat released.

Heat generated = I ² R

= l ² ρ (l/a)

= 4I ² Ρl/πd ²

As a = πd ² /4

Where

R – is the resistance of the fuse wire.

ρ – is the specific resistance,

l – is the length and

a – is the cross section of the fuse wire.

= l ² K ₁ (l/d ² ) ——–> 1

Where K ₁ is a constant. Heat loss ∝ Fuse wire surface ∝ πdl.

∴ Heat loss = k ₂ dl ———–> 2

If we equate 1 and 2 we get

EU ² K ₁ (eu/d ² ) =k ₂ dl

EU ² =Kd ³

Where K = K ₂ /K ₁

I = Kd ^3/2

I = Kd ^1.5

This is known as the Security Act .

Magnetic circuit laws

It states that the laws governing the constant flow of electricity in a circuit can be easily changed. To be immediately applicable to the Magnetic Circuit . Therefore

Magnetomotive driving force = flux x reluctance

F = ΦxS

Exactly corresponds to electromotive force = current x resistance

Reluctance = (Length/Area) x (1/Permeability)

= l/Aµ

For a magnet with a uniform cross-sectional area that exactly matches

Resistance = (Length/Area) x (1/Conductivity)

It is often convenient to calculate in unit dimensions. We then have

mmf per unit length = flux x reluctance per unit length

= (flow/area) x (l/permeability)

= flux density x (l/permeability)

Magnetic field intensity H = B/µ

This matches exactly,

For a material with uniformly distributed flow and length l, the total mmf is equal to the mmf per unit length xl

i.e. F=Hl or AT=Hl

By dealing with a Magnetic Circuit where the flux must pass through several parts in a matter of seconds, the methods used to deal with electrical circuits in series can be applied immediately, the total reluctance being the sum of the values ​​of the various parts. Normally, the importance of reluctance is only significant to the extent that it allows determining the mmf necessary to produce a given flow in the circuit. It is often the easiest method to determine this total mmf value by adding together the mmf values ​​needed to produce the change in the various parts of the circuit. This is equivalent to calculating the total voltage drop in an electrical circuit by adding the voltage drop values ​​across multiple components.

Thus, the total value of the mmf around a complete magnetic field is given by

AT (or) F = ∫Hdl

Or if the circuit consists of several homogeneous parts, each of which has a uniform cross section and length ₁ L _2, etc.

total mmf F (or) AT = Σ Hl = H ₁ 1 ₁ +H ₂ I ₂ +….

= Φ (p ₁ +S ₂ +….)

If the values ​​of the area and permeability of the different parts of the circuits are A ₁ μ ₁ etc. becomes all MMF

AT (or) F = (B ₁ /μ ₁ )i ₁ + (B ₂ /μ ₂ )i ₂ +…..

Where B ₁ =Φ/A ₁ etc.

Occasionally it is convenient to express the basic law of the magnetic circuit in form.

Flux = magnetomotive force x permeance

Permeability is nothing more than the reciprocal of Reluctance to interact for parallel paths, and total permeance is the sum of the values ​​of the individual courses.

The main difference between electrical and magnetic calculations arises from the fact that the resistance of an electrical circuit does not directly depend on the value of the reluctance of a magnetic substance, but to a large extent on the value of the flux passing through it.