What is the difference between the protection function of a thermal relay and a fuse?

The difference between the two:

1.Fuse.

Mainly used for short circuit protection, thermal relay.

For overload protection;

2.The fuse uses the principle of thermal fuse, which requires the melt to have a relatively high fusing coefficient;

3.Thermal relays use the principle of thermal expansion, which requires a high coefficient of expansion for bimetallic sheets.

4.Thermal relay protection has a large delay, while short-circuit protection requires that the action of the fuse must be instantaneous.

Similarities between thermal overload relays and fuses:

They are all current protection appliances and have anti-time characteristics.

The fuse is used for short-circuit protection, and the thermal relay is used for overload protection.

Generally, the fuse is small in hot melting, and once the current reaches the action value, the action is very fast, providing sensitive overload protection, and once the fuse cannot be recovered, it can only be replaced. The hot melt of the thermal relay is relatively large, and it will not act immediately when overcurrent, which takes a certain amount of time, and is suitable for overload protection.

The working principle of the thermal relay is to generate heat by the current flowing into the heating element, so that the bimetal sheet with different expansion coefficients is deformed, when the deformation reaches a certain distance, the connecting rod action is promoted, the control circuit is disconnected, so that the contactor loses power, the main circuit is disconnected, and the overload protection of the motor is realized. As an overload protection component of the motor, the relay has been widely used in production due to its small size, simple structure and low cost.

Fuse.

thermal relays).

If the motor is overloaded, the current in the winding increases, the temperature of the bimetal sheet rises higher through the increase of the current in the thermal relay element, and the degree of bending increases, and the herringbone lever pushes the normally closed contact, so that the contact is disconnected and the AC contactor coil circuit is disconnected, so that the contactor releases and cuts off the power supply of the motor.

The fuse is a kind of current protector made according to the principle that the current exceeds the specified value for a certain period of time, and the melt is melted with its own heat, so that the circuit is disconnected, and the thermal relay is used to protect the motor from overloading and preventing the motor from overheating and burning. And the fuse is only used to protect the line.

After the fuse of the fuse is broken, it can only be replaced with a new one.

Whereas, thermal relays can be used over and over again.

Thermal Relay Role:

The function of the thermal relay is to automatically cut off the power supply when the motor is overloaded, and the structure of the thermal relay is composed of two metal sheets with different expansion coefficients, and when the current is too large, the expansion coefficient is large and expands first, which plays the role of cutting off the power supply. There are manual reset and automatic reset after the thermal relay action.

Fuse function:

When the circuit is faulty or abnormal, it is accompanied by the continuous increase of current, and the increased current may damage some important or valuable devices in the circuit, and may also burn the circuit or even cause a fire. If the fuse is correctly placed in the circuit, the fuse will fuse itself to cut off the current when the current rises abnormally to a certain height and a certain time, so as to protect the safe operation of the circuit.

The function of the thermal relay is to control the temperature, when the temperature of the machine rises to the upper limit of the temperature set by the thermal relay, the circuit will be automatically broken to protect the machine.

The fuse is to control the current, the machine load is greater than the rated value, the current will be too large, the fuse is broken in time and protected.

One is the temperature control of the body, and the other is the control of the circuit.

Both fuses and thermal relays can act as "overload protection".

Once the fuse in the fuse is selected, the performance of the overload is fixed. The temperature set by the thermal relay can be adjusted, which is more convenient.

1. The main protection purposes are different:

It is mainly used for overload protection of asynchronous motors, and his working principle is that after the overload current passes through the thermal element, the motor control circuit is disconnected to realize the motor power-off and stop, which plays the role of overload protection. Fuses are widely used in high and low voltage power distribution systems and control systems, as well as electrical equipment, and are one of the most widely used protection devices.

2. The scope of application is different

When the current exceeds the specified value, the fuse blows the melt with the heat generated by itself, which acts as a protector for short circuit and overcurrent. In view of the fact that it takes a long time for the heat to be transferred during the bending process of the bimetal sheet, the thermal relay cannot be used as short-circuit protection, but only as the overload protection of the overload protection thermal relay.

Structural characteristics. The rated current of the melt is not equal to the rated current of the fuse, the rated current of the melt is selected according to the load current of the protected equipment, and the rated current of the fuse should be greater than the rated current of the melt, which is determined with the main electrical appliance.

The fuse is mainly composed of three parts: melt, shell and bearing, of which the melt is the key element to control the fuse characteristics. The material, size, and shape of the melt determine the fusing characteristics. Melt materials are divided into two categories: low melting point and high melting point.

Low melting point materials such as lead and lead alloys, which have low melting points and are easy to fuse, due to their large resistivity, the cross-sectional size of the melt is larger, and more metal vapor is produced during fuse.

Only suitable for fuses with low breaking capacity. High melting point materials such as copper and silver, which have a high melting point and are not easy to fuse, but because of their low resistivity, they can be made into a smaller cross-sectional size than low melting point melts, and less metal vapor is produced during fuse, which is suitable for fuses with high breaking capacity.

Thermal Relay: Overload Protection.

Fuse: overload, short circuit protection.

Because it takes a few seconds for the metal sheet inside the thermal relay to heat, and the short circuit is instantaneous, the thermal relay cannot be protected from short circuits.

The fuse is generally anti-instantaneous short circuit, and the thermal relay is the current overload such as the motor when the electrical appliance is working.

The fuse can be operated instantaneously, but it is not very effective for overload.

Thermal relays work well for overloads, but they don't respond instantaneously.

The fuse can quickly cut off the circuit and protect the line and electrical equipment when the transmission line has a "short circuit and overload" fault. The thermal relay itself can not cut off the circuit, and the thermal relay can only output the control signal or alarm signal through the internal contact when the "overload" fault occurs between the circuit and the electrical appliance, and control other electrical appliances (such as contactors) to disconnect the circuit, so as to achieve the purpose of protecting the circuit and electrical equipment.

The fuse is based on the fact that when the overload current flows through the fuse (fused sheet), the fuse is heated and blown due to the resistance of the fuse, so as to cut off the load current. Thermal relays are normally closed contacts that cut off the control loop based on the deformation of the bimetal sheet when overload current flows through the bimetal. The two are protected in different ways:

The fuse is to directly cut off the power supply of the main circuit, and the thermal relay is to cut off the power supply of the control loop and disconnect the contactor, so that the load loses power.

What protection do fuses, thermal relays, undercurrent relays, low-voltage circuit breakers and contactors have?

Hello dear, I am happy to answer for you, I would like to answer early: the usual role of circuit breaker: (1) control role.

That is, according to the operational needs, part of the power equipment or lines is put into or cut off. (2) Protective. That is, when the power equipment or line has a short circuit, overcurrent, grounding and other faults, the circuit breaker is acted on through relay protection and automatic devices to quickly remove the fault part from the power grid to ensure the normal operation of the non-fault part of the power grid.

AC contactors are used to connect and break lines, or frequently control the operation of motors and other equipment; Thermal relays are mainly used for motor overcurrent protection. The main function of the fuse is to protect the circuit and its electric sparrow device (short rotten vertical circuit).