This type of electrical switchgear has essentially an earthed metallic enclosure. This type of switchgear has essentially three compartments, the front compartment is for circuit breaker, the top compartment is for bus bar assembly and back or rear compartment is for current transformer and cable connection. Generally metal enclosed switchgear has compartments with metal partitions that are intended to be earthed hence this is called metal clad switchgear. Although some of these electrical switchgears may have one or more nonmetallic partitions and then this switchgear is known as compartmented switchgear.
Design and Construction of Metal Enclosed Switchgear
This type of switchgear is designed and constructed in such a way, that, normal operation, maintenance, inspection can be carried out very easily and safely. All the components can easily be replaced. Another essential feature of metal enclosed switchgear is that it should have a proper interlocking arrangement between different components to ensure most safe operation.
To ensure safe and proper sequence of operation, the following provisions are to be provided,
- The circuit breaker cannot be removed or drawn out from its compartment unless it is in OFF condition.
- Not only that, it should be impossible to isolate the circuit breaker from bus bar unless it is in OFF condition.
- The operation of the CB is impossible unless its service position, isolated position, earthed position, and removed from the compartment.
- It is impossible to switch ON the circuit breaker in its service position unless auxiliary circuit is connected with it.
Now let’s discuss the constructional details of metal enclosed switchgear.
Metal Enclosed Indoor Switchgear
There are mainly two types of metal enclosed indoor switchgear
- Vertical isolation and horizontal draw out,
- Horizontal isolation and horizontal draw out.
Generally, indoor switchgear housing has three chambers, namely the circuit breaker chamber (Front), bus bar chamber (Top), and the current transformer cum cable connection chamber (Rear). The metering and really panel consist of a separate low voltage chamber may or may not be fixed with the main gear housing.
The withdrawable partition comprises a wheel mounted truck with an operating mechanism, interrupters and isolating units. The spring charging mechanism may be either hand operated or motor operated. For motor operated spring charging mechanism, a manual charging provision should be provided to charge the spring during failure of spring charging motor or absent of auxiliary supply.
The withdrawal portion with CB generally has three position i.e. SERVICE, TEST, and ISOLATED position. The positions should be clearly visible from front of the switchgear.
Three bus bars for three-phase run parallel inside the bus bar chamber. The bus bars are made of either aluminum or copper and cross-section of the bars depends upon the current rating of the system. The bus is supported on epoxy support insulators.
The current transformers and cable termination chamber of earthed metal constriction are designed to be able to mount the CT and have a provision for cable termination.
Gas Insulated Medium Voltage Switchgear
This is fixed type metal enclosed design which does not have any withdrawal portion. This design of metal enclosed switchgear, mainly has two main compartments, namely CB compartment and bus bar compartment. Here bus bar compartment is provided with three position switch marked with SERVICE, ISOLATED and EARTH. The CB compartment consists of three interrupters. The interrupters are generally of vacuum type but some manufacturers also design gas isolated medium voltage switchgear with SF6 interrupters.
Metal Enclosed Outdoor Type Medium Voltage Switchgear
This is basically similar to metal enclosed indoor type medium voltage switchgear, except its external housing. The external housing or metal enclosure is made of welded sheet steel with slanting roof and rain shields. The housing is designed in such a way, that it becomes perfectly suitable for outdoor application. This type of design of medium voltage switchgear is not commonly used except for some special utilities like city distribution networks with underground cable systems.