A piece of switchgear may be a simple open-air isolator switch or it may be insulated by some other substance. An effective although more costly form of switchgear is gas insulated switchgear (GIS), where the conductors and contacts are insulated by pressurized sulfur hexafluoride gas (SF6). Other common types are oil or vacuum insulated switchgear.
The combination of equipment within the switchgear enclosure allows them to interrupt fault currents of thousands of amps. A circuit breaker (within a switchgear enclosure) is the primary component that interrupts fault currents. The quenching of the arc when the ciruit breaker pulls apart the contacts open (disconnects the circuit) requires careful design. Circuit breakers fall into these four types:
- Oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil through the arc.
- Gas (SF6) circuit breakers sometimes stretch the arc using a magnetic field, and then rely upon the dielectric strength of the SF6 to quench the stretched arc.
- Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other than the contact material), so the arc quenches when it is stretched a very small amount (<2–3 mm). Vacuum circuit breakers are frequently used in modern medium-voltage switchgear to 35,000 volts.
- Air circuit breakers may use compressed air (puff) to blow out the arc, or alternatively, the contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air thus blowing out the arc.
Circuit breakers are usually able to terminate all current flow very quickly: typically between 30 ms and 150 ms depending upon the age and construction of the device.
Several different classifications of switchgear can be made:
- By the current rating.
- By interrupting rating (maximum short circuit current that the device can safely interrupt)
- Circuit breakers can open and close on fault currents
- Load-break/Load-make switches can switch normal system load currents
- Isolators may only be operated while the circuit is dead, or the load current is very small.
- By voltage class:
- Low voltage (less than 1,000 volts AC)
- Medium voltage (1,000–35,000 volts AC)
- High voltage (more than 35,000 volts AC)
- By insulating medium:
- Gas (SF6 or mixtures)
- By construction type:
- Indoor (further classified by IP (Ingress Protection) class or NEMA enclosure type)
- Draw-out elements (removable without many tools)
- Fixed elements (bolted fasteners)
- Metal enclose & Metal clad
- By IEC degree of internal separation 
- No Separation (Form 1)
- Busbars separated from functional units (Form 2a, 2b, 3a, 3b, 4a, 4b)
- Terminals for external conductors separated from busbars (Form 2b, 3b, 4a, 4b)
- Terminals for external conductors separated from functional units but not from each other (Form 3a, 3b)
- Functional units separated from each other (Form 3a, 3b, 4a, 4b)
- Terminals for external conductors separated from each other (Form 4a, 4b)
- Terminals for external conductors separate from their associated functional unit (Form 4b)
- By interrupting device:
- Air Blast Circuit Breaker
- Minimum Oil Circuit Breaker
- Oil Circuit Breaker
- Vacuum Circuit Breaker
- Gas (SF6) Circuit breaker
- By operating method:
- Solenoid/stored energy operated
- By type of current:
- Alternating current
- Direct current
- By application:
- Transmission system
- By purpose
A single line-up may incorporate several different types of devices, for example, air-insulated bus, vacuum circuit breakers, and manually-operated switches may all exist in the same row of cubicles.
Ratings, design, specifications and details of switchgear are set by a multitude of standards. In North America mostly IEEE and ANSI standards are used, much of the rest of the world uses IEC standards, sometimes with local national derivatives or variations.
Maaf belum sempat translate neeh ;-)