Why RMUs Fail? Condensation & Gas Leak Explaine

1. Introduction

Ring main units (RMUs) are primary power distribution equipment that house load switches and circuit breakers within a metal or non-metal enclosure. Due to their compact size, simple structure, excellent insulation performance, low cost, easy installation, and fully sealed design [1], RMUs are widely used in medium- and low-voltage power systems across China’s grid network [2], especially in 10 kV distribution systems. With economic growth and increasing electricity demand, requirements for safety and reliability in power supply systems continue to rise [3]. As a result, RMU manufacturing technology has advanced accordingly. However, issues such as condensation and gas leakage remain common operational failures.

2. Structure of Ring Main Units

An RMU encloses key components—load switches, circuit breakers, fuses, disconnectors, earthing switches, main busbars, and branch busbars—within a stainless steel gas tank filled with SF₆ gas at a specific pressure to ensure internal insulation strength. The SF₆ gas tank primarily consists of a stainless steel shell, cable feed-through bushings, side cones, viewing windows, pressure relief devices (bursting discs), gas charging valves, pressure gauge ports, and operating mechanism shafts. These components are assembled into a fully sealed enclosure through welding and sealing gaskets.

RMUs can be classified in several ways:

• By insulating medium: Vacuum RMUs (using vacuum interrupters) and SF₆ RMUs (using sulfur hexafluoride).

• By load switch type: Gas-generating RMUs (using solid arc-extinguishing materials) and puffer-type RMUs (using compressed air for arc quenching).

• By structural design: Common-tank RMUs (all components in one chamber) and unit-type RMUs (each function in a separate compartment) [4].

3. Common Fault Types in RMUs

During long-term operation, RMUs inevitably experience various faults due to multiple factors. The most common include condensation (moisture ingress) and gas leakage.

3.1 Condensation in RMUs

When condensation occurs inside an RMU, water droplets form and fall onto cables under gravity. This reduces cable insulation performance, increases conductivity, and may lead to partial discharge. If left unchecked, prolonged operation under such conditions can result in cable explosions—or even catastrophic RMU failure [5]. Additionally, since most RMU enclosures and structures are made of metal, moisture causes rusting of operating mechanisms and cabinet components, shortening the equipment’s service life.

3.2 Gas Leakage in RMUs

Field and manufacturer investigations reveal that gas leakage from RMU gas tanks is a widespread and serious issue. Once leakage occurs, the internal insulation strength drops. Even normal switching operations can generate transient overvoltages that exceed the weakened dielectric strength, leading to insulation breakdown, phase-to-phase short circuits, and posing a major threat to the safe operation of the power system.

4. Causes of Gas Leakage in RMUs

Gas leakage primarily occurs at welded joints, dynamic seals, and static seals. Welding leaks commonly appear at panel overlap joints, corners, and where external metal components (e.g., bushings, shafts) are welded to the main tank. Incomplete penetration, micro-cracks, or poor weld quality during manufacturing can create tiny leakage paths. Dynamic seals—such as those around operating shafts—are prone to wear over time, while static seals (e.g., gaskets between flanges) may degrade due to aging, improper compression, or temperature cycling, leading to gradual gas loss.