The customer’s requirement is to pass an internal arc fault test with a 40kA 1s rating for a 17.5kV switchgear cabinet that is 650mm wide.
Switchgear Cabinet Dimensions:
- Width: 650mm
- Depth: 1400mm
- Height: 2200mm
- Phase Spacing: 150mm
- Rated Voltage: 17.5kV
- Rated Power Frequency Withstand Voltage: 38kV
- Rated Lightning Impulse Withstand Voltage: 95kV
- Rated Current: 1250A
- Rated Short-Time Withstand Current: 31.5kA
- Rated Peak Withstand Current: 82kA
- Rated Internal Arc Class: IAC A FLR 40kA/1s
The first mapping test revealed some of the following problems:
(1) Firstly, the rivet pulling nut will be pulled apart under the huge shock wave impulse;
(2) As a result, electric arcs are easily ejected from the gaps between the fastening bolts of the cover plate and the door plate;
(3) The cable room pressure relief channel is too narrow, resulting in poor pressure relief, and the rear cover plate of the cable room is subjected to excessive pressure to punch open;
(4) The SMC insulation board continued to burn inside the cabinet, requiring a fire extinguisher to extinguish the fire and scrapping the entire cabinet;
(5) In accordance with the requirements of IEC62271-200 standard, the internal arc needs to be tested under the simulated roof with a distance of 600mm from the top of the cabinet, and the arc is directly sprayed out of the cabinet roof to the simulated roof reflection back to ignite the level indicator, the test failed.
To address any issues that may arise during testing, the following main measures were taken:
- Deepening the rear cover to enlarge the cable chamber’s pressure relief path, lowering the current transformers for improved airflow release.
- Replacing riveted nuts with welded nut strips.
- Reducing the bolt spacing to 150mm.
- Designing a labyrinth structure for the cover and door panels, sealing them with EMKA gaskets, ensuring that the arc does not escape. EMKA gaskets provide an attractive and reliable sealing solution, meeting both IP42 protection requirements and arc resistance testing.
- Using flame-retardant insulation materials for the main circuit scheme and the terminal box.
- Designing a buffer compartment on top of the cabinet to reduce pressure rapidly. Pressure is released through the common compartment shared by multiple cabinets, significantly reducing flame temperature and gas pressure, preventing horizontal indicator ignition.
This design approach has allowed for the successful completion of internal arc tests for a 17.5kV switchgear cabinet with a width of 800mm, achieving an IAC AFLR 50kA/1s internal arc rating. These results were obtained without employing methods such as permanently welding door panels or locking internal bolts. Instead, the design ensures that all switchgear cabinets can withstand internal arc faults without concern, safeguarding personnel and minimizing damage.
By applying the same design approach in actual projects, the cost increase is minimal, ensuring consistent performance across all switchgear cabinets. This approach provides peace of mind in passing inspection tests and ensures the safety of operators. In the event of an internal arc fault, there is minimal risk of personnel injury, and losses are kept to a minimum.
