Preventing RMU Insulation Failures: Key Causes

1. Insufficient Creepage Distance or Air Clearance

Inadequate creepage distance and air gaps are the main causes of insulation failure and accidents in solid-insulated ring main units (RMUs). Especially in drawer-type cabinets, manufacturers reduce cabinet size by minimizing space for circuit breakers, significantly decreasing isolation distances between plug contacts and ground. Without adequate reinforcement of insulation structure, such designs increase the risk of flashover under overvoltage conditions.

2. Poor Contact Connection

Insufficient contact pressure or poor connection leads to localized temperature rise. In severe cases, moving parts may be burned out, causing grounding faults or arc discharge, ultimately resulting in insulation flashover. There have been reports of fire and explosion incidents at substation disconnectors due to overheating of metal burrs leading to short circuits.

3. Environmental Influences

Operating environment is a key factor in insulation failures. Increasing air pollution gradually contaminates insulators, bushings, and busbars, reducing surface insulation performance and increasing the risk of tracking and flashover, especially in humid or coastal areas.

4. Manufacturing and Assembly Issues

Manufacturing and assembly quality significantly affect the overall dielectric strength of solid-insulated RMUs. Some components may pass individual withstand tests, but poor integration can prevent the entire unit from passing system-level tests. Irregularly tightened fastening screws may protrude excessively after tightening, reducing insulation clearance and causing electric field concentration. Additionally, low-quality support porcelain columns with poor dynamic stability may crack under short-circuit current impact, leading to cascading failures.

5.Design Recommendations

Designers of solid-insulated RMUs should select highly reliable switching components and ensure sufficient insulation levels to achieve maintenance-free operation. The main circuit is fully encapsulated within an expanded enclosure, isolated from external environmental factors. The sealed chamber can be filled with SF₆ or nitrogen—oxygen-free gases—with moisture content controlled at low levels, preventing insulation degradation due to contamination or condensation, and avoiding corrosion of metal parts caused by oxidation.