MCB: Under normal circuit conditions, it can connect and disconnect the load current, and under abnormal circuit conditions (overload, short circuit, especially under short circuit), it can connect, carry for a certain period of time, and disconnect the current of the switching electrical device.

RCCB: In normal operating conditions, it is a mechanical switching device that connects and disconnects the load current, and under specific conditions, it disconnects the contacts when the residual current reaches a certain specified value.

RCD: A protector that causes the contacts to operate and disconnect the main circuit when the residual current in the circuit reaches its specified value under specified conditions. A residual current device can be a combination of various elements that detect residual current and connect and disconnect the current in the main circuit.

The instantaneous value vector sum (root mean square value) of the current flowing through the main circuit of a residual current device.

The total current between the exposed conductive surfaces of the equipment and the ground, including capacitively coupled currents.

A contactor is an automated control electrical device. Contactors are primarily used for frequently making or breaking AC or DC circuits. They have a high control capacity, can be operated from a distance, and when combined with relays, they can achieve timed operations, interlocking control, and various types of quantitative control. They are widely used in automatic control circuits. Their main control objects are electric motors, but they can also be used to control other power loads, such as heaters, lighting, welding machines, and capacitor banks, etc.

A thermal overload relay (hereinafter referred to as a thermal relay) works by generating heat from the current flowing through the thermal element, causing a bimetallic strip with different coefficients of expansion to deform. When the deformation reaches a certain distance, it pushes the lever to actuate, causing the control circuit to open, thereby de-energizing the contactor coil and breaking the main circuit, achieving overload protection for the motor. As an overload protection component for motors, thermal relays are widely used in production due to their small size, simple structure, and low cost.

The rated current of a thermal relay is not the rated current of the thermal element, nor is it the rated current of the thermal relay contacts; it is simply the rated operating current of a specific class of thermal relays.

For any class of thermal relays, there are correspondingly several thermal elements. For example, the "20" in the JR16-20 type thermal relay indicates that the rated operating current of the thermal relay is 20A for that class. For this class, there are thermal elements numbered from 1 to 12, totaling 12 different ratings. Each thermal element has its own rated current, and the rated current of the thermal element can be adjusted within a certain range using a cam-adjusted knob. The adjustment range for the rated current of the number 1 thermal element is 0.25 to 0.3 to 0.35A.

When selecting a thermal relay, we can choose the number of the thermal element based on the rated current of the protected equipment and adjust the set current to the required value by turning the adjustment knob. Therefore, when selecting a thermal relay, the main factor to consider based on the rated current of the protected object is the number of the thermal element. Additionally, the bimetallic strip or heating element of the thermal relay is usually connected in series in the main circuit of the motor, while its contacts (normally closed contacts) are connected in series in the control circuit of the motor.

Thus, when the thermal relay operates due to overload, its normally closed contacts open, cutting off the control circuit of the motor (i.e., the contactor coil circuit), causing the contactor to release due to the loss of excitation in the electromagnet. In this way, the main circuit of the motor is interrupted, providing overload protection for the motor. It is clear that the rated current of the thermal relay is by no means the rated current of the contacts.

Generally, the long-term working current of the thermal relay contacts is only 3 to 5A, the making current is generally the same as the long-term working current, and the breaking current is only 40% to 60% of this value. If used to control a DC circuit, the allowable values of all the above currents would be much lower.

MCR self-protection refers to a situation where, when the breaker Icw

Long-time delay protection can be adjusted with a step length of 1A, and the tripping delay time step length is 0.5 seconds. Short-time delay protection can be adjusted with a step length of 1A. Instantaneous protection can be adjusted with a step length of 10A.