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Metal-Clad Switchgear: The Industry’s Workhorse
January 3, 2018 - Power & Communication
By Sherry Rollins
Everyone knows the phrase, “If it’s not broke, don’t fix it.” This is the case when we refer to the robust design of metal-clad switchgear. Metal-clad switchgear has remained the industry’s go-to choice for medium-voltage applications for more than 20 years because of its proven design.
With the design containing a circuit breaker rather than a fuse, there are three dominant reasons metal-clad remains the most popular industry choice: customization, maintenance and safety. Today, new technologies in the market are enhancing these benefits more than ever. Let’s break down the reasons for this as a reminder of the basic benefits that make this equipment so valuable in its use.
The customizability of metal-clad switchgear is considered one of its most attractive benefits. This is because metal-clad size and internal capabilities allow for each switchgear section to be fully customized to any customer’s specifications.
What do we mean by fully customizable? Metal-clad switchgear provides the most basic of load protection and operates based on the instructions provided by the connected relays. As such, it can easily be used in the most basic or the most intricate of system applications, depending on relay design.
Essentially, regardless of how advanced or basic the relay might be, metal-clad switchgear can operate and perform its function as a protection circuit breaker switchgear. This “one size fits all” feature of customization proves useful in the electrical system industry.
For example, metal-clad switchgear can be used for simple applications, where the relay usually contains minimal ANSI/IEEE functions, including overcurrent, undervoltage, overvoltage and tripping the circuit breaker. For more demanding relay applications, more complex relays can be chosen for tasks such as zone-selective interlocking. This would consist of current, voltage, and frequency protection features, such as direction ground fault and phase overcurrent.
For more custom applications requiring such functions as phase sequence and phase balance, an even more advanced relay with additional features — including non-directional phase, ground faults, and rate of change frequency — can be used with metal-clad switchgear.
Further, customization does not just affect the relay and wiring schemes but also the structure of the switchgear. Internally, it is possible to have various layouts of auxiliary and breaker sections, allowing for metal-clad breakers to be stacked on top of one another based on the size and shape of the switchgear. This design allows for the low-voltage compartment and doors to also be customized because space usually is more abundant for laying out additional switches, lights, alarm signals, racking ports, plugs, and communication ports. The customizable structures allow for designs with multiple main sources that can be supplied from generators, utility services or other upstream energy sources. The use of tie-breaker sections allows for the switchgear to have a main bus A and a main bus B that will provide the main source of power to the entire switchgear lineup in the event either source is lost abruptly.
If both sources are needed simultaneously, paralleling of the two sources can be done using a synch-check relay to ensure both sources are synchronized, inhibiting closure of the circuit breaker if synchronizing parameters such as voltage, frequency and phase angle are outside limits. This will prevent damage to the generator and disturbance of the busbar.
Ease of maintenance
While maintenance needs vary based on customer application requirements, metal-clad switchgear generally is easy to maintain across all types of scenarios. The inherent nature of metal-clad switchgear to function as a disconnect that operates in the on position at all times means that for many customers, maintenance can be deferred until the annual facility shutdown, when all equipment will be inspected. However, regular planned maintenance of any type of electrical equipment is highly encouraged for ensuring system health and reliability and for lowering total cost of ownership.
Alternatively, metal-clad switchgear can operate under consistent-use conditions, where it is required that it switch from an open-to-close or on-to-off state more frequently. In these cases, metal-clad switchgear can handle the wear and tear required to constantly operate the loads without interruption to the daily process flow of manufacturing facilities.
Another benefit of metal-clad switchgear is that the breaker can be withdrawn from its compartment, which allows for the electrical and mechanical parts to be inspected and maintained on a more frequent basis.
The ease of maintenance for metal-clad switchgear also means it saves time for facility and electrical technicians. Metal-clad switchgear feature breakers with withdrawable finger clusters that allow the breaker to be racked out of its compartment with the push of a button or with a racking handle. With this feature, providing a visual means of disconnect is not required, which simplifies the process for service professionals. Instrument transformers also can be withdrawn easily, and the low-voltage compartment isolated from the rest of the switchgear, easing maintenance.
Last but certainly not least is the enhanced safety metal-clad switchgear provides over traditional metal-enclosed switchgear. Because the fully grounded unit features grounded barriers, mechanical interlocks, breaker shutters and a fully insulated bus, operators are segregated from the any live medium-voltage sections. Metal-clad switchgear is also fully compartmentalized, so technicians can access the low-voltage compartment without having to also access any other live parts in the switchgear. The main bus and incoming bus are also separate from each other.
One additional safety feature in metal-clad switchgear is the remote breaker racking option, which puts distance between the system and the worker, removing personnel from the line of fire in the event an electrical incident occurs. With a remote breaker option, the breaker can be racked out with a push-button station by personnel standing 50 feet away from the unit. An active arc-mitigation system is also available in metal-clad switchgear as an optional feature.
While there are other options for switchgear, metal-clad has remained the most popular for good reason. As technology and electrical systems become more advanced, it is important to have reliable switchgear that can adapt to modern systems. Customization, ease of maintenance and enhanced safety make metal-clad a clear choice for evolving electrical designs.
Sherry Rollins is product manager with Schneider Electric, www.schneider-electric.us/en/