In power distribution systems, metal-clad switchgear and gas-insulated switchgear (GIS / RMU) are the two most commonly used types of medium-voltage equipment.
Many people are confused when first encountering them:
Why do some projects specify metal-clad switchgear, while others prefer compact gas-insulated units?
Although both serve the same purpose—power distribution, control, and protection—they differ fundamentally in insulation medium, structure, safety, and application scenarios.
1. Definition and Basic Characteristics
1) Metal-Clad Swithgear (Withdrawable Type)
Metal-clad switchgear is widely used across generation, transmission, and distribution systems. It performs switching, control, and protection functions.
It typically includes: Circuit breakers, Disconnectors, Load switches, Operating mechanisms, Current transformers, Protection devices
The insulation medium is air, and the circuit breaker is mounted on a withdrawable truck, it can be moved between service / test / disconnected positions, making operation and maintenance intuitive. (eg. KYN28)
2) Gas-Insulated Switchgear (GIS / RMU)
Gas-insulated switchgear encloses all high-voltage components (busbars, breakers, switches) inside a sealed stainless-steel tank filled with insulating gas such as: SF₆.
The excellent insulation strength of gas allows much smaller electrical clearances.
2. Key Differences Explained
1） Insulation & Principle
Metal-clad switchgear uses air insulation, so it needs enough safety distance between live parts (e.g., ≥125mm for 10kV). That’s why the cabinet is relatively large.
GIS uses gas insulation (SF₆ or similar). Its insulation strength is about 3 times higher than air, with excellent arc-quenching performance.
All high-voltage parts are sealed in a slightly pressurized tank (0.02–0.05 MPa), allowing a much more compact design.
2）Structure
Metal-clad switchgear has a compartment design (breaker, busbar, cable, control).
The breaker is on a withdrawable truck, with clear positions (service / test / disconnected), making operation very intuitive.
GIS is fully sealed in a stainless steel tank (robot-welded).
Only operating handles and cable connections are outside.
It’s usually modular, easy to combine into different schemes.
3） Size & Space
Metal-clad:
Large size (800–1000mm width)
Needs dedicated switchgear room
GIS:
Compact (500–600mm width for 1250A)
Saves 40–60% space
Ideal for limited spaces (underground stations, wind towers)
4）Protection & Environment
Metal-clad：IP4X
GIS： core unit (gas tank) IP67, Cabinet shellIP4X, Outdoor enclosure IP54
5） Maintenance
Metal-clad:
Easy to repair and replace parts
Needs regular maintenance
GIS:
Very low maintenance (or maintenance-free)
High reliability
But harder to repair if something goes wrong
6）Cost
Metal-clad:
Lower price
Good for most standard projects
But takes more space (higher building cost in some cases)
GIS:
Higher initial cost
Saves space + lower maintenance
More cost-effective in the long run for compact or critical projects
3. Comparision of Advantages and Disadvantages
| Item | Metal-Clad Switchgear | Gas-Insulated Switchgear (GIS / RMU) |
| Advantages | 1. Cost-effective, high value for money 2. Easy maintenance, good interchangeability of withdrawable parts 3. Visible isolation gap, intuitive operation | 1. Compact size, saves space 2. Strong environmental adaptability (resistant to humidity and dust) 3. High safety, fully enclosed live parts 4. Flexible modular design 5. Maintenance-free or low maintenance |
| Disadvantages | 1. Large footprint 2. Poor environmental resistance 3. Exposed live parts, risk of electric shock | 1. Higher cost 2. More difficult to repair 3. Risk of gas leakage 4. Internal components are not visible |
Conclusion
Metal-clad switchgear and gas-insulated switchgear are not substitutes, but complementary solutions.
Metal-clad switchgear remains dominant in conventional projects due to its cost-effectiveness and ease of maintenance.
GIS / RMU is becoming the preferred choice for modern applications thanks to its compact size, high reliability, and strong environmental adaptability.
