The equipment must have the ability to protect personal safety and prevent electric shock under normal operation and failure. Electric shock protection under normal conditions is called basic protection, which is used to prevent the danger caused by direct contact with live parts.

Firstly, the live parts must be installed in the required electric cabinet, and the electric cabinet must be opened with keys or tools. After opening the cabinet door, all live parts higher than 50V in the door must be protected to prevent accidental electric shock. In addition, the power must be cut off before opening the electric cabinet, that is, the so-called "open the door and cut off the power". This requirement shall be realized by the interlocking mechanism of door and switch, so that the door can be opened only when the power supply is cut off, and the power supply can be connected only after the door is closed. The interlocking switch has the measures that the licensed personnel can release the interlocking and turn on the power switch when the door is opened. When the door is closed, the interlocking mechanism can recover automatically. China now has some manufacturers specializing in the production of such switches.

1. Protection against electric shock under electric leakage

In addition to understanding the protection against electric shock under normal conditions, the protection against electric shock under electric leakage, i.e. smelling protection, should also be considered. It is used to prevent personal danger caused by insulation damage between live part and unclosed conductor part. The protection methods are generally as follows:

(1) In case of electric leakage, it can automatically cut off the power supply to prevent the life of personnel from being endangered by too long contact time;

(2) Connect the unclosed conductor parts to the protection circuit;

(3) Class II equipment or equivalent insulation protection shall be adopted. This measure is used to prevent dangerous voltages on easily accessible parts due to basic failure. Therefore, double insulated appliances (so-called class II equipment) must be used.

2. Short circuit protection

Due to the short-circuit fault of electrical control circuit, such as the damage of conductor insulation or motor winding insulation, it can cause short-circuit fault and generate short-circuit current, which will produce strong electric power and damage electrical equipment. Therefore, when a short circuit occurs, the power supply must be cut off quickly. The commonly used short-circuit protection elements are fuse, overcurrent breaker and automatic. They all have the function of cutting off the power supply instantly.

(1) Protect with fuse

In the branch circuit, the instantaneous current is allowed to be used as protection. The relay acts on the device with sufficient breaking capacity. The device can cut off one or several circuits or branches, as well as the general protection device or emergency stop device.

The rated current of fuse and the setting current of other short-circuit protection devices shall be as small as possible, but must meet the requirements of starting current and transformer closing current. The setting value can be determined according to the minimum load area of the conductor protected by the device.

(2) Protection with automatic switch

Automatic switch, also known as automatic air, has protection functions such as short circuit and overload. In case of fault in the line, this switch can automatically cut off the power supply. It is one of the important protection elements of low-voltage distribution. It is often used as the main power switch of low-voltage distribution board and the closing switch of motor transformer.

3. Overload protection

The long-term overload operation of the motor will make the temperature rise of the motor winding exceed the allowable value, the insulation material in the machine will become brittle, the service life will be reduced, and the motor can be damaged in serious cases. Generally, the motor working continuously above 1kW must have overload protection capacity, and the commonly used overload protection element is. When the motor is rated current, the thermal relay does not act; when the overload current is small When the overload current is large, the thermal relay will act in a short time.

It can be seen that the thermal relay will not act instantaneously under the influence of short-time overload impulse current or short-circuit current of the motor. Therefore, while using the thermal relay for overload protection, it must also be equipped with short-circuit protection.

4. Voltage loss and undervoltage protection

If the grid voltage suddenly disappears for some reason, the running motor will stop running. Then, when the power supply voltage is restored, the motor will start by itself. This situation is easy to cause damage to production equipment and even personal accidents. Therefore, protective measures must be taken, that is, when the grid voltage disappears (such as power failure) In case of subsequent recovery, it is required that the motor and its driving mechanism cannot be started by themselves to ensure the safety of operators and equipment. This protection is called loss of voltage protection. For example, many machine tools are started by button operation, and the loss of voltage protection can be achieved by using the automatic recovery and self-locking of the button.

Some machine tools are operated by control switches and have self-locking links. The circuit of the self-locking link itself has the function of voltage loss protection.

When the motor is in normal operation, some electrical appliances will be released due to excessive voltage reduction, resulting in the operation imbalance of the control circuit, which may lead to accidents. Therefore, the power supply must be cut off when the power supply voltage drops below a certain allowable value, which is undervoltage protection. Generally, electromagnetic voltage relay is commonly used to realize undervoltage protection. When the power supply voltage is too low or disappears, the voltage relay will release In short, the voltage relay can ensure the protection at the predetermined level.

5. Protection against misoperation caused by grounding fault

The grounding fault of the control circuit is easy to cause the accidental start of the machine tool, or cause dangerous movement of the machine tool, or hinder the stop of the machine tool. Therefore, one side of the control circuit should be connected to the protection circuit. The coil of each electromagnetic action device, the primary coil of the signal lamp or the transformer powered by the signal lamp must be connected to the grounding side of the control circuit The contact shall be connected to the other side of the coil and the control circuit. If the wire between the contact of the protection relay and the coil of the device controlled by it is in the same electrical cabinet, the contact of the protection relay can be connected between the grounding side of the control circuit and the coil.

When the control circuit is directly connected between the two phase lines of the power supply, or between the phase line and the neutral line, and the neutral line is not grounded, or is grounded through high impedance, a bipolar switch must be used to "start" and stop the machine action, so as to prevent personal danger and mechanical damage caused by the accidental start of the machine tool and the failure to stop the machine movement.