The critical structural dimensions of switchgear cannot be solely determined by specific quantitative measurements. For instance, clearance distance depends on factors like electric field distribution and insulation materials. It is not universally appropriate to apply a single standard value for all cases. For example, the clearance distance of 12 kV, which is greater than 125 mm, is based on a lightning impulse withstand voltage of 75 kV. However, slight variations in the electric field may alter the pure air gap required, since the electric field conditions within the conductive circuit of switchgear vary and have different degrees of field non-uniformity. Circular busbars and rectangular busbars cannot be evaluated using the same standard.

This can only be used as a reference. For other parameters, we can provide additional reference values. Today, let's discuss the structural dimension requirements related to the internal arc resistance rating of switchgear.

Typically, air-insulated switchgear uses a design where one side has hinges, and the other side has bolted door locks. The number of hinges and bolts can serve as an indicator of whether the switchgear is capable of resisting internal arc pressure. Some switchgear designs, upon visual inspection, reveal that their trolley compartment or cable compartment doors have only two hinge locks, which clearly do not meet the Internal Arc Classification (IAC) standards.

When an internal arc occurs, the total pressure exerted on the door panel, divided by the total number of bolts and hinges, results in the pressure stress each bolt needs to withstand. If this stress exceeds the allowable tensile stress, the door panel may open. Based on the required stress for bolts, basic judgment criteria can be derived:

For the circuit breaker compartment door:

· At 31.5 kA for 0.5/1 second, the door must withstand a force of 8 tons. While an M10 bolt can withstand 3 tons of force, the rivet nut can only handle 2 tons. Considering different materials and safety factors, each bolt is safely capable of bearing 800 kg of force.

· Therefore, the use of hinges and door lock bolts with an M10 diameter, with at least 5 hinges and 5 bolts, is required.

For higher levels:

· At 40 kA for 0.5/1 second, the hinges and door lock bolts must also have an M10 diameter, with at least 6 hinges and 6 bolts.

· At 50 kA for 0.5/1 second, the number of hinges and door lock bolts (M10) increases to at least 8, and the door panel width must be reinforced to reduce deformation, using a labyrinth plate design to prevent arc ejection.

For higher current levels, a design incorporating hinges on one side and hooks on three sides with a quick-release handle to activate locking hooks simultaneously is preferable. The stress on the hook plates and locking bolts must also be calculated to determine the necessary quantity.

Cable Compartment in Ring Main Units (RMU)

The cover of the cable compartment in RMUs typically uses a hook design. The cover has cylindrical lock bolts on both sides, or sometimes bolts are used. The switchgear sides have locking hook plates with slots that engage the bolts when the cover is lifted, and the cover can be easily closed by aligning the cover with the slots and lowering it into place. This design is very convenient.

However, to withstand the pressure generated by an internal arc in the cable compartment, additional attention is required. For RMUs, a 20 kA for 1 second internal arc rating is commonly required, and the pressure involved is approximately 5 tons. The thickness of the hook plates becomes critical, as a 3 mm thick plate can handle around 200 kg of force per hook. Thus, a large number of hooks is necessary.

Typically, 8 hooks are required on each side, and the total force the two sides can withstand is about 3.2 tons. The lower hooks directly engage the base of the cabinet, while the upper part is usually secured with bolts, meeting internal arc requirements.

Designs with fewer hooks or thinner plates are unlikely to pass internal arc testing.

In conclusion, switchgear that genuinely meets internal arc resistance standards can often be judged by the number of hinges and bolts used. It is important to note that clients are often well-informed, and weak design elements that do not meet procurement standards will not go unnoticed.