Switchboards are used to safely distribute electricity throughout commercial and industrial facilities. A switchboard is a component of an electrical distribution system which divides an electrical power feed into branch circuits while providing a protective circuit breaker or fuse for each circuit and a common enclosure. On the outside, they look just like many other large gray metal enclosures. So sometimes switchboards can be confused for switchgear, motor control centers, or even panel boards.

What is the difference between switchgear and switchboards?

Differences between switchgear and switchboards

Switchboards are most often confused with switchgear because from the outside both are freestanding metal enclosures that house bus bar, power cables, circuit breakers, meters, relays, sensors and even transfer schemes for redundant power. However, when it comes to the internals, switchboards differ dramatically from switchgear. Switchboards are governed under the UL 891 standard. One of the biggest differences is that switchboards are made with an open chassis. This means that behind the dead front cover there are no barriers between cabling, bus bar or terminations. This differs from switchgear, which under UL 1558 requires separate compartments to shield and isolate each section from another. Because of this structural difference, switchgear can be designed to fulfill UL 891 switchboard requirements. But switchboards can never satisfy UL 1558 switchgear standards.

Another visual difference is the type of breakers used. Switchboards will typically use fix mounted molded case circuit breakers, or MCCBs, that fall under UL 489. Since MCCBs can’t be serviced, the entire enclosure must be the de-energized before replacement of existing breakers or the addition of new breakers. On the other hand, switchgear will have drawout power circuit breakers that can be removed and maintained without fully interrupting a power system.

Another major difference is short time current withstand rating. A switchboard generally has three cycles for selective co-ordination with downstream components before tripping and switchgear is rated to withstand 30 cycles. This extra time allows for co-ordination with downstream devices.

What is the difference between switchboards and panelboards?

The first thing you would notice if you’re comparing them is that panelboards mount on the wall and switchboards are freestanding. Panelboards are designed to UL 67 and are typically limited to a maximum of 1200 amps. While switchboards can contain bussing and overcurrent devices up to 6000 amps similar to panelboards, switchboards can be front connected, requiring only front access. However, switchboards can allow rear access if desired.

So where switchboards installed and who uses them?

Where are switchboards installed and who uses them

Switchboards are key pieces of low voltage distribution equipment. They can either be fed from a power circuit breaker and upstream switchgear assembly or fed directly from the electric utility service transformer in smaller facilities, which is referred to as service entrance. If that’s the case, then the utility may have extra requirements for landing power directly to the board and there will be requirements for grounding and bonding that will need to be considered.

What exactly is a switchboard and what does it consist of?

What is a switchboard and what does it consist of

Switchboards integrate circuit breakers, fuses, metering, surge protection and other electrical products to safely distribute and monitor power. Switchboards can range up to 600 volts, 6000 amps and 200 kAIC and offer flexible configurations to maximize standardization, safety and convenience to accommodate for commercial construction, industrial, healthcare, data center and other power distribution needs. Switchboards are typically 90 inches tall with a minimum depth of 18 inches. There are four main structure types common to all switchboards. But not all switchboards require all these structure types. Main structure, pull structure, distribution structure, and an Integrated Facility System or IFS structure. A switchboard can consist of a single vertical structure or multiple structures, depending on the number and sizes of the loads being powered.

A main structure will contain the main disconnects or main lugs and will often contain utility and or customer metering equipment and surge protection. Cabling can enter directly into the main structure or through a dedicated pull structure. A pull structure is a blank enclosure containing empty space through which cabling can be pulled and is commonly used in service entrance switchboards. Cables can enter through the structure from the floor, referred to as bottom entry, or from above also known as top entry.

Distributions structures divide and send power to branch circuit protection devices and then to branch circuits to power downstream loads. Power flows from the main structure to the distribution structure via cross bus or horizontal bus. Last but not least, an Integrated Facility System switchboard structure includes panelboards, dry-type transformers, transfer switches and blank back pans for field mounting other equipment. The IFS is helpful to integrate other equipment into a switchboard. In most installations, switchboards are mounted close to the wall and are front accessible only. If required the switchboard can be constructed to allow both front and rear access. Rear access switchboards provide easier access for installation and maintenance, but they are typically deeper than front access only switchboards.

Components of a switchboard

The primary components of a switchboard structure are the frame, bus, overcurrent protective devices, instrumentation, enclosures, dead fronts, gutters and exterior covers. The switchboard frame is the metal skeleton that houses all of the other components. The bus, which is either copper or aluminum, is mounted within this frame. The bus distributes power from the incoming cable conductors to the branch circuit devices. A horizontal bus distributes power to each switchboard section. The standard orientation for a horizontal bus is A, B, C from top to bottom. In contrast, a vertical bus distributes power to the circuit protection devices within an individual section and is normally oriented either left to right or front to back. In most cases, insulation or dielectric strength between the three bus phases is provided via an adequate air gap. In locations where bus clearances are not sufficient to provide the necessary dielectric strength insulation is applied to the bus.

In switchboards molded circuit breakers are group mounted, while insulated case breakers are independently mounted with separation between breakers. There is no separation between insulated case or molded case breakers in low voltage controls, horizontal or vertical bus or the lugs and breakers. The switchboard enclosure houses a number of critical components. Overcurrent protective devices are mounted to the vertical bus bars from the front of the unit. Four common types are insulated case circuit breakers, molded case circuit breakers, fusible switches and bolted pressure switches.

Other protective devices used on the switchboard interior may include meters, surge protective devices or SPDs, utility compartments, transfer switches, transformers and other equipment. The switchboard interior may also house specialized instrumentation. Keep in mind that switchboards are not completely compartmentalized like switchgear, but there can be dedicated compartments for the control and metering components for electrical isolation. Meters can be used in incoming section to measure current, voltage power usage, peak demands, and other important power characteristics. Standard enclosures for switchboards include NEMA type 1 for indoor and NEMA type 3R for outdoor uses. After equipment installation the frame is enclosed with the exterior cover panels. Like the dead front of a panel board, these covers allow for access to protective devices while sealing off the bus and wiring from accidental contact.

Main parts of a switchboard

Now let’s discuss the main parts of a typical switchboard metering, control and instrumentation, insulated case circuit breakers, molded case circuit breakers, or MCCBs, panel boards, transformers, and other custom equipment. The metering section would include CT shorting blocks, meter bases, control power transformers, DC power supplies, potential transformers, switches, fuses and much more. This is where low voltage control monitoring and networking equipment will be installed. It may even include special equipment like a UPS.

Insulated case breakers can be used and larger switchboards typically applied as the main breaker or where remote operation or control schemes are required. These devices are tested to UL 489 and are rated at either 80% or 100%. Insulated case breakers are designed to be somewhat maintainable and some parts can be replaced.

Molded case circuit breakers are the most common breaker used in switchboards and are typically applied at 80% of continuous current rating when installed in equipment per NEC 210.20 and rated at 80% in accordance with the UL 489. These breakers are sealed devices, so they are not maintainable. They are used in distribution systems to provide main and branch circuit protection, up to 2500 amps. Some switchboards include advanced molded case circuit breakers that can connect to your network or the cloud with built-in communication capability, generate data to help optimize your facilities performance, and mitigate arc flash to help keep people safe. All frame sizes of these breakers have global certifications, including IEC, CCC, UL, and CSA.

Switchboard standards and ratings

There are several important standards and ratings that apply to switchboards, but the main one is NEMA PB2 which is the general instructions for proper handling, installation, operations and maintenance of dead front distribution switchboards rated 600 volts or less. Another important rating for switchboards is short circuit current rating.

So what is short circuit current rating? The short circuit rating for standard bus and connectors on all switchboards are designed for use on systems capable of producing up to 65 kA short circuit current at the incoming terminals. Increased bus short circuit ratings equal to that of connected switchboard devices, up to 200 kA, are available in most switchboards when approved main devices are installed. UL labeled switchboard sections are marked with their applicable short circuit rating.

Switchboards are also available with a wide array of advanced arc flash safety technologies which can reduce incident energy limit arc flash damage, increase uptime and improve safety. These can include arc flash reduction maintenance systems or ARMs capability with MCCBs and insulated case breakers, as well as remote operation IR windows and other monitoring control technologies to improve arc flash safety. This equipment, combined with engineering services such as arc flash studies and labeling, as well as preventative maintenance, upgrades and support, can help keep your switchboards operating safely with the latest technology and advancements.

Types of switchboards

General purpose switchboards can include integrated insulated case circuit breakers, molded case circuit breakers, fuse switches, metering and surge protection. The vast majority of switchboard applications can be served with a general purpose switchboard. These switchboards provide higher density and a reduced footprint, while accommodating higher current rated power breakers, and they require a rear accessibility to barrier cable compartments.

This switchboard can be configured to a variety of applications, including single ended and double ended substation lineups, and can easily align with existing switchboard configurations and design. Switchboards can be expanded beyond their basic circuit distribution and protection functions to include various other equipment as part of an Integrated Facility System or IFS. For example, IFS systems will often include low voltage distribution transformers, automatic transfer switches, uninterruptible power supplies and datacom equipment.

NEMA Standard PB2 and UL 891 Integrated Facility System switchboards combine circuit breakers, chassis and transformers as well as other electrical products. IFS switchboards integrate electrical distribution components that are traditionally separate and control equipment into a single space saving factory assembled and connected product. By integrating the various equipment on the factory floor, IFS systems reduce footprint and installation time at the customer site. IFS type solutions are popular in multi-site applications like chain stores and restaurants, where common repetitive design is used and in general construction, where prefab solutions are expanding.

Fusable switchboards are designed for commercial, industrial and service entrance applications to protect and switch feeder and branch circuits. Each panel, board unit includes a switching contact structure with an instantaneous trip element. Draw-out molded case switchboards provide the same basic capabilities as general purpose switchboards with the added functionality of offering draw-out capability for one or more molded case feeder breakers to be serviced, interchanged or tested. Draw-out molded case switchboards are designed to meet UL 891 standard for critical applications like health care and data centers to reduce system downtime when replacing a breaker.

And finally, generator quick connect switchboards provide a solution to getting your facility back online safely and quickly during unexpected power outages. This configuration with a portable or temporary generator is a great option for regions that experience long term outages from hurricanes and other natural disasters, but don’t want to have permanent generators at every site. Designed to meet UL 891, the switchboards’ color coded, cam type plug receptacles are readily accessible and eliminate cable connection guesswork, avoiding hazards associated with opening electrical panels and exposing workers to potentially dangerous conditions.