The Application of Switchgear and Switchboards

Switchboard Panel | Source: GE Evolution Switchboards

One important element of good power system design is the proper selection of the distribution equipment. The choice of using either switchgear or switchboards must be based on many different criteria and the design of the power system requires thought to be given to each one. Following are insights into just a few of the differences to help in making those decisions. 

Related Article: Design Considerations to Enhance Safety and Reliability for Service Entrance Switchboards

Standards and Testing

Switchgear and switchboard structures are built and tested to different standards: Switchgear to ANSI standard C37.20.1, UL standard 1558, and NEMA standard SG-5, switchboards to NEMA PB-2, and UL-891. Switchgear incorporates only low-voltage power circuit breakers (LVPCB) which conform with ANSI C37.13, NEMA SG-3 and are listed per UL-1066, whereas switchboards may include any combination of protective devices including an insulated case (ICCB), molded-case circuit breakers (MCCB) listed per UL-489, fusible switches listed per UL-508 and 977 and power circuit breakers listed to UL-1066.

Switchboard testing is based on system nominal voltage rating levels (i.e. 240V, 480V, 600V). Switchgear standards, however, recognize actual conditions may vary beyond that and stipulate maximum voltage levels at nominal plus 5.8% (i.e. 480V nom, 508V max). Dielectric tests are also different: switchboard tests are limited to twice rated voltage plus 1000V phase-to-phase and phase-to-ground (i.e. 240V system = 1480V test); switchgear is tested to 2200V in all cases. 

Structures

Switchgear and individually compartmented switchboards require rear access for terminating incoming line and outgoing feeder cables. Group-mounted switchboards, however, are typically front-access-only for incoming line and feeder cables. As a result, switchboards 45” deep or less can be placed with their rear against a wall. The largest feeder that may be group mounted is a 1200 amp molded case breaker. In group-mounted switchboards, feeders rated above 1200 amps will be individually mounted. Generally, switchgear and individually compartmented switchboards take more space than group-mounted switchboards. GE switchgear and individually compartmented switchboards offer an insulated and isolated bus system. This option can significantly reduce the opportunity for arcing type faults to occur on the bus or to the ground and should be considered when safety from such faults is of high concern. 

Protective Devices

In switchgear, all protective devices are draw-out and individually mounted as well as individually compartmented. Switchboard protective devices may be group-mounted or individually compartmented. Individually compartmented switchboards have each breaker positioned behind its own door and barrier-isolated from surrounding devices; these are also identified as AV3 Access and Power Break 2 Switchboards. Some switchboards may incorporate a combination of individually mounted as well as group mounted protective devices. In these cases, the individually mounted devices are not individually compartmented.

Application Consideration

In the presence of a high current fault exceeding the instantaneous set points, all devices could operate immediately, compromising the continuous flow of power. Fortunately, there are many options available to the design engineer.

System Grounding

When selecting the type of equipment, the grounding method of the electrical system must be considered. Molded case and insulated case circuit breakers are only suitable for solidly grounded distribution systems. This is due to their relatively low single-pole interrupting ratings.

System Coordination

Here we must investigate the level of selectivity between protective devices required. LVPCB’s can be built without an instantaneous trip function. This is true for the main as well as the feeder breakers. Without an instantaneous, the main, feeders and possible other downstream devices can more fully coordinate with each other. 

Size and Accessibility

Size is always an important consideration. Some applications are so space restrictive that only group-mounted switchboards can be used. If the equipment must be backed against a wall eliminating rear access, the only choice may be group-mounted switchboards with front-accessible terminations. Applications requiring higher levels of reliability, maintainability, and remote operation must be designed to contend with rear access of that gear and in compliance with NEC 110.26 and 110.34.

Related Article: How to Select Circuit Protection Devices?

Economics 

In most applications, economics is an important consideration. The primary consideration is often the initial cost. Generally speaking, group-mounted switchboards are the least costly. Individually mounted switchboards can cost between 15 and 50% more than group mounted switchboards, depending on the number and type of devices required. Switchgear is typically more expensive than the individually mounted switchboards, but it may have certain features that preclude the use of any other equipment.

Match The Project’s Need

There are projects that will require the use of switchgear, others that will preclude its use. For instance, the continuity of power needs in a middle school is not as critical as in a health care facility or steel mill. A switchboard with its size and cost advantage may be well suited for the first while the unsurpassed reliability and maintainability offered by the switchgear may offer no viable alternative for the latter projects. Thorough knowledge of the customer’s needs and the demands of the customer’s process is key in matching the proper equipment with the project. 

Reference: 

Title: The Application of Switchgear and Switchboards

Author: Jerry Milton, GE Systems Engineer

Publisher: ABB/ GE Industrial Solutions

Document Type: PDF | Download