Crane safety and protection measures

Electrical safety of 1. cranes

Bridge and gantry cranes all use electric energy to obtain electric energy from the power grid. If you need to start, you only need to connect the power supply and close the switch to move. After deceleration, you can drive various mechanisms. However, electricity is a kind of invisible, inaudible, inaudible, and intangible energy. Because non-electrical workers do not know much about electricity, they often cause electric shock accidents.

There are two main types of electrical injuries in cranes. Electric shock is a deadly accident, and spark burns often make it difficult to restore the body's skin to its original state. Therefore, in addition to daily use and maintenance, in case of electrical equipment failure, the failure should be informed to the electrician, who is responsible for repair.

When the current passes through the human body, the degree of harm to the human body is mainly related to the size of the current passing through the human body, the duration of the current passing through the human body, the path of the current passing through the human body and other factors.

The greater the current passing through the body, the more obvious the shock response and the stronger the sensation. The shorter the time required to cause ventricular fibrillation, the greater the fatal risk.

The minimum current that causes a person to produce a perception is on average about 1.0 mA. When people can get rid of the current independently after electric shock, the maximum current is about 15 mA on average. From the safety point of view, we put the 10 mA to get rid of the current positioning. The smallest current that can cause death in a short period of time is called a lethal current. The lethal current is generally 50 mA. We believe that ventricular fibrillation or ventricular rest caused by electric current is the main cause of fatal electric shock.

Burns from electric sparks are another form of electric shock injury. Because the burn of electric spark is usually deep, the healing of the wound is slow. Electric sparks usually cause extensive burns to the body.

In general, a short circuit between wires with a strong current will produce an explosive strong flash. The disconnection or damage of the knife switch through which a large current flows occurs even when the fuse is pulled out.

Therefore, in the crane, electrical damage to the human body, mainly electric shock and short circuit caused by spark burns. Therefore, we require that the electrical equipment and lines of the crane must be kept in good condition at all times, especially for open-air cranes, whose electrical lines are easily damaged in advance due to various reasons such as sun and rain. In order to ensure safety, the crane driver should always clean the electrical equipment to avoid the pollution flashover caused by the deposition of filth or dust on the line, especially on the line joint. Crane drivers regularly check the electrical equipment of the crane. If problems are found, they should immediately report to the equipment department and send electricians to repair them. Do not do it yourself. When the electrician repairs the equipment, the crane driver should be present to observe and check whether the repair quality is qualified. Pay attention to whether the parts removed from the electrical equipment, such as arc extinguishing cover, are installed back to their original places. The replaced damaged parts should be taken away and cannot be left on the crane. It is not allowed to throw them down from high altitude to hurt people.

After an electric shock accident, the wire should be separated from the body of the electric shock with an insulating rod. Never touch the body of the electric shock with your hands to avoid chain electric shock reaction. After picking up the wire, it is necessary to lose no time to immediately rescue the electric shock. Generally, the method of chest compression or mouth-to-mouth artificial respiration is used to restore the respiratory system, especially the nervous system that controls respiration. The person who is injured by electric shock should do everything possible to rescue him without stopping until the attending doctor diagnoses that he is indeed dead. Generally speaking, the electric shock injured by the rescue, in addition to the parts burned by electric sparks, few are permanently disabled.

Second, the crane electrical protection measures

In order to protect the safety of electrical equipment and staff, the crane electrical control system should be set up the necessary electrical protection measures. Generally, there are main isolating switch, short circuit protection and voltage loss protection of main power supply, zero position protection, overload protection of motor, magnetic loss protection and overspeed protection, emergency power switch, limit protection and stroke protection, interlocking protection of channel port and grounding protection.

(I) and main isolating switch

Maintenance and inspection of electrical equipment should generally be carried out in the event of a power failure. Therefore, there should be isolation switch or other isolation measures between electrical equipment and power supply grid. When the isolating switch is in the disconnected state, it must maintain an effective disconnection distance and a clearly visible disconnection point, so that the maintenance personnel can intuitively confirm that the main power supply circuit is indeed disconnected.

When the air switch and the iron shell switch are in the off position, they sometimes do not match the state, and there is no obvious disconnection point, so they cannot be used as isolating switches. Can only be used as a load switch.

Short circuit protection of (II) and main power supply

When the insulation of the electrical equipment on the crane is damaged, the shell or the phase short circuit occurs, the short circuit protection device of the total power supply should act quickly to cut off the faulty power supply.

The short-circuit protection of the total power supply requires that each phase must be set to ensure that any two-phase or any relative short circuit occurs when the fuse fuse is blown or the automatic circuit breaker acts.

When the crane uses cable power supply or a set of sliding lines to supply power to a crane, if the main power switch has been set on the ground and has a short-circuit protection function, the crane can not set the short-circuit protection of the main power supply. If multiple cranes share a set of sliding lines and set up a ground total power switch, a fuse or circuit breaker shall be set up separately on each crane as a short-circuit protection device for the total power supply.

(III), total power loss protection

The voltage loss protection of the main power supply means that the main power supply circuit can be automatically disconnected after the power supply is interrupted; when the power supply is restored, the main power supply circuit cannot be connected without manual operation. Generally, the button that can automatically reset is connected into the main power contactor coil or the loss-of-voltage trip coil circuit of the automatic circuit breaker to realize the loss-of-voltage protection of the main power supply.

If the main power supply has no voltage loss protection, the main power supply can be switched on without manual operation when the power supply is interrupted and the power supply is restored. In this regard, sometimes the operator did not find, mistakenly believe that the total power supply or no electricity, intentionally or unintentionally touch the controller, will cause misoperation, accidents.

(IV) and zero protection

Zero protection is mainly used to ensure that the controller can only be started when the controller is at zero, so as to prevent the power supply of the crane from losing voltage or cutting off the power supply when the controller handle is not at zero, thus causing the motor to start by mistake and possibly causing accidents.

Overload protection of (V) and motor

If the rated power of the motor on the crane is greater than 1KW, overload protection shall be set separately. There are many ways of overload protection, generally using over-current relay or thermal release circuit breaker; large capacity, the use of built-in thermal element overload protection.

Overcurrent protection is the current crane commonly used motor overload protection. Generally, the overcurrent relay coil is connected to the main circuit of the motor, and the interlocking contact is connected to the safety interlocking circuit of the main power supply or the zero relay coil control circuit. In case of overcurrent, disconnect the main power supply or the mechanism motor control power supply to stop the motor.

Overspeed protection of (VI) and motor

When the speed of the crane hoisting mechanism is adjusted, the supporting brake of the mechanism is fully opened and does not play a braking role. At this time, if there is only one load torque acting on the motor shaft due to the small loss of voltage or control failure, the motor is out of control, causing the motor to overspeed and a heavy object falling accident will occur. If the installation position allows, should be set up overspeed protection switch. When the switch is operated, the motor stops running.

(VII), emergency power-off switch

Each control point of the crane must be equipped with an emergency power-off switch that can quickly disconnect the main power supply in case of emergency, and be located in a place convenient for the driver to operate. The structure of the emergency power-off switch should not be reset automatically.

Safety interlock protection for (VIII) and access

The crane shall be equipped with an access interlock switch to disconnect the main power supply when the access is open.

The trolley of bridge crane is powered by bare sliding line. If someone boards the platform of the trolley from the hatch door or the end beam door, the safety interlock switch on the door will automatically disconnect the main power supply and cut off the power supply of the trolley sliding line after opening the door, so as to prevent the staff boarding the walking platform from getting electric shock by mistake, and also prevent the sudden start of the mechanism from causing crushing injury and falling accidents.

Loss-of-excitation protection for (IX) and DC motors

The DC separately excited motor shall be provided with loss-of-excitation protection. If under-current relay is used for field loss protection, the setting value of the operating current of the relay is about 0.85-0.90 times of the motor excitation current (when field weakening speed regulation is not required) or the minimum excitation working current (when field weakening speed regulation is required).

(X) and zero connection protection

When the crane power supply is a low-voltage system with neutral point directly grounded, the metal structure (or cart track) should be connected to zero protection, that is, the metal structure (or cart track) is connected to the zero line of the power supply grid. When the electrical equipment on the crane due to insulation damage leakage of the shell, can use the zero line to form a single-phase short circuit, resulting in a large enough single-phase short-circuit current, forcing the protection device on the line to act quickly, cut off the fault power supply.

Short-circuit protection devices are usually fuses and circuit breakers, and their action speed or whether they can act depends on the size of the single-phase short-circuit current.

(XI) Grounding protection

In the low-voltage power supply system where the neutral point is not grounded, the metal structure of the crane (or the trolley track) should be protected by grounding. When the grounding resistance is not more than 4Ω, once the electrical equipment on the crane hits the shell, the ground voltage of the metal structure (or cart track) will not exceed 36V, which is safe.

When the power supply is a high-voltage power supply (generally 3,6,10KV) with ungrounded neutral point, the metal structure and electrical equipment of the crane shall be protected by grounding. In this system, even if single-phase grounding occurs, the grounding current does not exceed 10A, which is a small grounding short-circuit current system. If the current is considered as 30A, high voltage and low voltage share a set of grounding device, and the ground voltage on the metal shell of high voltage electrical equipment with grounding protection is 120V, the grounding resistance Rd≤ 120/30 = 4Ω, but the voltage on the leakage high voltage electrical equipment is 120V, which is still dangerous to human body, and the electrical equipment shall also be provided with relay protection device capable of automatically removing grounding fault.