In power system design, there is usually an ambiguity about the application of switchboard, switchgear, or panelboard. The basic purpose of all these switching gears is to supply a common node of power distribution at various levels while offering varying types of protection to their associated circuits. The proper choice of either of these requires technical knowledge as well as the consideration of electrical system applications where these gears are going to be used. Having a good understanding will help us to evaluate the significance and application of each in practical power systems.
Panelboard is a unit of electrical distribution system, which splits the electrical power to various branches or circuits. It offers electrical circuit control and protection, mainly used in low voltage distribution systems. The NEC defines a panelboard as:
“A single panel or group of panel units designed for assembly in the form of a single panel, including buses and automatic overcurrent devices, and equipped with or without switches for the control of light, heat, or power circuits; designed to be placed in a cabinet or cutout box placed in or against a wall, partition, or other support; and accessible only from the front” [NEC 100].
An electrical panelboard has three important components which ensure supply of electricity to consumer end and powering various appliances.
The main breaker has the key function of controlling the electricity coming from the power line to distribution end. Without the main protection, panelboard can become exhausted by huge amount of power, wires may get damaged, and appliance may blow up which will start an electrical fire. That is why, main breakers play a significant role in protection of distribution panels.
Circuit breakers supply protection to their respective branches/circuits against overflow of electricity. If a circuit breaker opens, supply of the only particular circuit will get interrupted, and the remaining branches will have no effect.
Bus bars are metallic bars (usually made of copper or aluminum), used to transfer power from the incoming supply to various feeder branches.
Different feed options are available in panelboards. Feed options are top feed, side feed and bottom feed. Panelboards are mainly categorized as Power panelboards and lighting panelboards.
We have different panel Lighting panelboards, which are also known as appliance branch circuit panelboards, are defined in the NEC (Article 384) as:
“One having more than 10% of its overcurrent devices rated 30 amperes or less for which neutral connections are provided.”
Electrical panelboards that do not fulfill the above definition will be classified under power panelboards which are also known as distribution panelboards.
Further types of Distribution Panelboards based on the application and requirements are:
Main circuit breaker panels are used to protect the circuits from overheating and keep an eye on the panel’s ampere ability.
In the case of the main breaker panel, it can cut the power of the house including the circuit breakers.
The main lug panel can be used as a sub-panel when the main panel relates to it through the breaker. In case of emergency, power is cut before entering the house by disconnecting the meter.
Subpanels are easy to install and efficient for homes. They are helpful in distributing the power to a specific part of your home by getting power from the main circuit breaker panel.
They are uses where all the electricity is controlled and distributed having three components which are the main switch, circuit breaker and residual current device.
Transfer switches are best for backup sources. They convert generator power to electrical power through the breaker panel. The generator has a transfer switch with the same rating as the main circuit breaker panel. There are two diverse types of transfer switches.
The motor control center (MCC) is the center from where we have controlled the operation of the electric motor. It is a collection of various components to control motors which includes diverse types of starters like DOL (Direct online Starter), ASD (Automatic star-delta) starter, busbars and control equipment having all function to control the operation of the electric motor and place these components in an integrated panel. We have several types of MCC’s Panels based on voltage type, based on types of motor operations, based on module division etc.
Switchboard is an electrical unit that regulates the electricity from power line to various smaller consumers. It is a combination of one or more panels in an assembly. The NEC defines a switchboard as:
“A large single panel, frame, or assembly of panels on which are mounted on the face, back, or both, switches, overcurrent, and other protective devices, buses, and usually instruments. These assemblies are generally accessible from the rear as well as from the front and are not intended to be installed in cabinets” [NEC 100].
An electrical switchboard has following main components:
It is to hold units like switches, indicators and other power delivering and controlling devices.
It has the function to link up and check single or multiple electrical sources to and from switchboard. It includes synchroscopes, gauges, and other tools to assess synchronization of electrical generators.
It has the function to receive power and transfer it to multiple circuits mounted through the switchboard.
Switchboards consisting of integrated insulated case circuit breakers, molded case circuit breakers, fused switches, metering, and surge protection are general purpose switchboards. The vast number of switchboard applications can be served with a general-purpose switchboard.
Switchboards made for commercial, industrial, and service entrance purposes to safeguard and switch feeder and branch circuits are known as fusible switchboards. Each panelboard forms a switching contact arrangement with an immediate trip element.
Draw-out molded case switchboards offer the same basic functions as a general-purpose switchboard, but the added feature is to supply draw-out function for feeder breakers. Draw-out molded case switchboards are often used in crucial applications like healthcare and data centers to lower system interruption when substituting a breaker.
Commercial metering switchboards blend circuit breakers, chassis, surge protection and meter sockets in an enclosure. Metering can be of two types: bulb-type watt-hour metering and electronic tenant metering, as demanded by the utility.
In the case of switchgear, all devices are drawn-out individually mounted and individually compartmented. The NEC finds switchgear as:
“An assembly completely enclosed on all sides and top with sheet metal (except for ventilating openings and inspection windows) and containing primary power circuit switching, interrupting devices, or both, with buses and connections. The assembly may include control and auxiliary devices. Access to the interior of the enclosure is provided by doors, removable covers, or both.”
An electrical switchgear has following main components:
The power conducting components of switchgear consist of switches, fuses, circuit breakers, insulators, lightning arrestors, etc. These components are primarily employed to break the flow of electric power through the circuit.
The control system components of the switchgear consist of control panels, current or potential transformers, relays, etc. These components are mostly employed to check, control, and safeguard the system along with the power conducting components.
Metal-clad switchgear is the structure of an electrical switchgear where all electrical components involving the incoming bus, outgoing bus, metering equipment and main circuit breaker or switch, are inserted in separate metal partitions to supply an added security, strength, and comfort of maintenance.
Metal-enclosed switchgear includes circuit protection devices including circuit breakers, power fuses and fusible switches along with control and metering equipment. These devices can be installed in common compartments and do not require the separate compartments.
Pad-mounted switchgear is constructed for underground distribution systems rated from 5 to 38 kV that are needed to be above grade workable. Pad-mounted switchgear’s outdoor rated, low profile and strong construction makes it perfect for utility distribution, feeder sectionalizing and circuit protection purposes. Switches, fuses etc. are used to safeguard loads, restrict faults, and lessen interruptions.
Vault or subsurface switchgear is devised for electrical distribution systems rated from 15 to 38 kV demanding that the switch and its accessories to run from inside a vault or below-grade site. These positions can be dry or exposed to water. Vault or subsurface switchgear can also allow the user to control the switch from above ground by hand or with the use of relays and uses vacuum interrupters to safeguard loads and restrict faults.
Conclusively, the most important thing which needs to be considered while selecting a panelboard, switchboard or switchgear is the project requirement. Matching the scheme requirement is the most crucial factor because there will be some applications which need switchgear and the use of a switchboard in these places will prevent the system from certain possible functions. A critical industrial infrastructure, for example, a hospital or a glass factory, will require crucial continuity of power. Here the switchgear is more sophisticated, offering maintainability and higher reliability. Whereas in a middle school, switchboards will be more suitable due to their size and cost advantage. So, the right choice of an equipment and its particular type is essential for a power system to make it safe, reliable and cost effective.
About The Author
Abdur Rehman is a professional electrical engineer with more than eight years of experience working with equipment from 208V to 115kV in both the Utility and Industrial & Commercial space. He has a particular focus on Power Systems Protection & Engineering Studies.