The MV grounding switch is an important part of the MV switchgear as the grounding protection of the high voltage circuit during maintenance and overhaul. The grounding switch needs to withstand short-term withstand current, peak withstand current, that is, maintenance and repair, grounding switch closed, accidental power supply, the grounding switch needs to be stable in the ground connection state, will not boom open cause ground protection disconnection. Depending on the application, it is also necessary to consider the ability to close, when the switchgear cabinet is energized and the grounding switch is closed accidentally, the grounding switch also needs to be reliably closed in place to realize the grounding protection.

Grounding switch does not have a separate actuator drive, but with the switch as a whole, simply put, the usual medium-voltage grounding switch is a spring mechanism over the dead center, that is, the spring is compressed to store energy, through the dead center after the rapid release, to achieve a certain closing speed, that is, conducive to closing to the static contacts, but also to achieve the shutdown, to reduce the pre-breakdown arc ablation.

Considering that the closing requires a large closing speed, the design of the ground switch requires a large closing angle and a small breaking angle. That is to say, increase the spring compression stroke, that is, as much as possible when the slammer can store energy, thus providing a larger closing speed for the closing, the slammer as long as the dead center to achieve a stable position can be maintained, but also can quickly realize the closing.

The spring energy passes through 1/2*k*l*l, where k is the stiffness factor of the spring and l is the spring travel. With the energy available, and the moment of inertia of the entire moving contact system calculated, the closing angular velocity, and thus the line speed, can be calculated.

The general air-insulated earthing switch with 80kA closing capacity needs a closing speed of more than 5m/s. The operating mechanism needs to have an empty stroke after the dead center.

The operating mechanism over the dead center needs to have air travel, there are operated by the operating handle air travel operation, more is the mechanism with air travel, the purpose of the air travel is the spring over the dead center after the automatic fast closing, will not be affected by the speed of human operation, and the spring release of energy, force, no air travel will be on to the operator caused by personal injury.

The following contact pressure, in addition to closing, contact pressure is the contact to withstand the key parameters of the short-term withstand current, contact pressure, resistance to dynamic thermal stability, and thermal stability of the effective value is also the contact pressure determines the contact resistance, the decision of the voltage drop at the contact, determines whether or not the contact welding, whether or not it can withstand thermal stability.

The contact contact pressure is large, the operating torque required is large, then the simplest calculation is to withstand the short-circuit peak current torque = contact pressure friction coefficient movable arm length, the need to operate the torque is large, it means that the mechanical strength of the grounding switch to be high to meet the requirements of the mechanical life of 2,000 / 3,000 times the overall strength of the switch to be large;.

The operating force of the earthing switch should meet the requirements of the national standard, i.e. the operating force is less than 250N, and the instantaneous small angle is allowed to be less than 450N. If the switch is operated by a crutch arm, due to the space limitation, the operating torque is directly equal to the contact torque and the operating force is large, whereas if the switch is operated by an umbrella gear, the torque is reduced by the ratio of 1:2 or 1:2.5 of the umbrella gear, which makes the operating angle larger and the operating force smaller.

By calculation, the operating force required for a solidly designed earth switch is very large under the joint action of the contact pressure and the mechanism spring, e.g. the JN22 standard earth switch requires a special operating mechanism with an enlarged transmission ratio in order to reduce the operating force.

After the dynamic thermal stability test, according to the standard, the operating handle needs to be used at the first time after the test to open the earth switch with a force that does not exceed the standard force, and the use of other external forces to knock or exceed the standard operating force will be recorded in the test report.

The solutions are.

1. Optimize the design of the grounding switch to reduce contact pressure and thus operating force by, for example, using electromagnetic force-assisted blocking.

2. the use of a longer operating handle, which at the same time requires a high strength of the operating shaft and the earthing switch, especially the hexagonal head of the operating shaft, which can be rounded off if the hardness is not sufficient, resulting in inoperable.

3. the use of turbine worm gear operating mechanism, the opening of the gate needs to be rotated 10 turns to open the gate, MV NEX switchgear is the use of such a mechanism. Or other labor-saving mechanism that reduces the operating force by increasing the stroke.