Power Distribution Equipment

Power Distribution Equipment is a term generally used to describe any apparatus used for the generation, transmission, distribution, or control of electrical energy. This section concentrates upon commonly used power distribution equipment: Panelboards, Switchboards, Low-Voltage Motor Control Centers, Low-Voltage Switchgear, Medium Voltage Power and Distribution Transformers, Medium-Voltage Metal Enclosed Switchgear, Medium Voltage Motor Control Centers, and Medium-Voltage Metal-Clad switchgear. Each has its own unique standards and application guidelines, and one facet of good power system design is the knowledge of when to apply each type of equipment and the limitations of each type of equipment. The equipment described herein are typically custom-engineered on a per-order basis.

One common characteristic of the equipment types covered in this section is that they are all enclosed for safety. The enclosures for enclosed equipment generally follow the guidelines set forth in NEMA 250-2003 Enclosures for Electrical Equipment (1000 Volts Maximum) NEMA Standards Publication 250-2023.*, and, although this standard is intended for equipment less than 1000 V, it is also true of medium-voltage power equipment.

The most common NEMA enclosure types are as follows (see Enclosures for Electrical Equipment (1000 Volts Maximum) NEMA Standards Publication 250-2023.*):

Type 1: Enclosures constructed for indoor use to provide a degree of protection to personnel against access to hazardous parts and to provide a degree of protection of the equipment inside the ingress of solid foreign objects.

Type 3R: Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt and windblown dust); to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (rain, sleet, snow); and that is undamaged by the external formation of ice on the enclosure.

Type 4: Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt and windblown dust); to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (rain, sleet, snow, splashing water, and hose directed water); and that is undamaged by the external formation of ice on the enclosure.

Type 4X: Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (windblown dust); to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (rain, sleet, snow, splashing water, and hose directed water); that provides an additional level of protection against corrosion; and that is undamaged by the external formation of ice on the enclosure.

Type 5: Enclosures constructed for indoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against the ingress of solid foreign objects (falling dirt and settling airborne dust, lint, fibers, or other items); and to provide a degree of protection with respect of harmful effects on the equipment due to the ingress of water (dripping and light splashing).

Type 12: Enclosures constructed (without knockouts) for indoor use to provide a degree protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt and circulating dust, lint, fibers, or other items); and to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (dripping and light splashing).

Panelboards are the most common type of power distribution equipment. A panelboard is defined 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” (see The National Electrical Code NFPA 70, The National Fire Protection Association, Inc., 2003 Edition.*). It typically consists of low-voltage molded-case circuit breakers arranged with connections to a common bus, with or without a main circuit breaker. Panelboards shows typical examples of panelboards.

Panelboards are used to group the overcurrent protection devices for several circuits together into a single piece of equipment. In small installations they may serve as the service equipment. The NEC The National Electrical Code NFPA 70, The National Fire Protection Association, Inc., 2003 Edition.* divides panelboards into two categories:

Lighting and Appliance Branch-Circuit Panelboard: A panelboard having more than ten percent of its overcurrent devices protecting lighting and appliance branch circuits.

Power Panelboard: A panelboard having ten percent or fewer of its overcurrent devices protecting lighting and appliance branch circuits.

Separated Distribution Panelboard: A panelboard combining the above Lighting and Appliance Branch-Circuit and Power Panelboards.

Lighting and appliance branch-circuit panelboards are limited to a maximum of 42 overcurrent devices, excluding mains. UL 67 UL Standard for Safety for Panelboards, UL 67, Underwriters Laboratory, Inc. November 2003.* designates Class CTL Panelboard as the marking for appliance and branch circuit panelboards; CTL stands for “circuit limiting”. In some manufacturer’s literature lighting and appliance branch-circuit panelboards for residential or light commercial use are referred to as load centers.

Panelboards are available with built-in main devices or as main lugs only (MLO). The NEC The National Electrical Code NFPA 70, The National Fire Protection Association, Inc., 2003 Edition.* requires appliance and branch circuit panelboards to be individually protected on the supply side by not more than two main circuit breakers or two sets of fuses having a combined rating no greater than the rating of the panelboard. Lighting and appliance branch circuit panelboards are not required to have individual protection if the feeder overcurrent device is no greater than the rating of the panelboard.

Power panelboards must be protected by an overcurrent device with a rating not greater than that of the panelboard (see The National Electrical Code NFPA 70, The National Fire Protection Association, Inc., 2003 Edition.*).

Various methods for attaching the circuit breakers to the panelboard bus are available, such as plug-on, bolt on. The circuit breakers are typically purchased separately. Often, the enclosure, interior, and trim assemblies for the panelboard itself are purchased separately as well. This is typically true of larger panelboards and gives a great deal of flexibility about use of the same interior with different enclosures and trims.

Panelboards are available with several accessories. Subfeed lugs allow taps directly from the panelboard bus without the need for overcurrent devices. Circuit breaker locking devices allow locking of circuit breakers in the open or closed position (the breakers still trip on an overcurrent condition). Various types of trims are available, with various locking means available for trims that are equipped with doors. Various digital solutions are available for communications, metering, maintenance mode switching and surge protection.

The definition of a switchboard is “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” (see The National Electrical Code NFPA 70, The National Fire Protection Association, Inc., 2005 Edition.*). Switchboards are free-standing equipment, unlike panelboards, and are generally accessible from the rear as well as from the front. They may consist of multiple sections, connected by a common through-bus. Unlike panelboards, the number of overcurrent devices in a switchboard is not limited.

Switchboards generally house molded case circuit breakers or fused switches. They are generally the next level upstream from panelboards in the electrical system, and in some small to medium-size electrical systems they serve as the service equipment. Switchboards shows an example of a switchboard.

Switchboards are available with a main circuit breaker or fusible switch, or as main lugs only. The available ampacities and multi-section availability makes them more flexible than panelboards. They are generally available utilizing either copper or aluminum bussing, and with a variety of bus plating options. Custom bussing for retrofit applications is also possible.

Switchboard circuit breakers may be stationary-mounted (also referred to as fixed-mounted), where they can be removed only by unbolting of electrical connections and mounting supports, or drawout-mounted, where they can be without the necessity of removing connections or mounting supports. It is possible to insert and remove drawout devices with the main bus energized. The section that contains the main circuit breaker(s) or service disconnect devices is referred to as a main section. A section containing branch or feeder circuit breakers is referred to as a distribution section.

Devices mounted in the switchboard may be either panel mounted (also referred to as group mounted), where they are mounted on a common base or mounting surface, or individually mounted, where they do not share a common base or mounting surface. Individually mounted devices may or may not be in their own compartments. A device which is segregated from other devices by metal or insulating barriers and which is not readily accessible to personnel unless special means for access are used is referred to an isolated device. Group and Individually-mounted Devices shows examples of sections with group and individually-mounted devices.

The main through-bus is often referred to as the horizontal bus. The bussing in a section which connects to the through-bus is referred to as the section bus (also known as vertical bus). The bussing that connects the section bus to the overcurrent devices is referred to as the branch bus. Section and branch busses may be smaller than the main through-bus; if this is the case UL 891 (see The National Electrical Code NFPA 70, The National Fire Protection Association, Inc., 2005 Edition.*) gives the required section bus size as a function of the number of overcurrent devices connected to it.

Switchboards are available with several accessories, including custom-engineered options such as utility metering compartments, automatic transfer schemes, and modified-differential ground fault for switchboards with multiple mains. However, the internal barriering requirements are minimal.

A motor control center (MCC) is defined as “a floor-mounted assembly of one or more enclosed vertical sections typically having a common power bus and typically containing combination motor control units” (see UL Standard for Safety for Motor Control Centers UL 845, Underwriters Laboratories, Inc., August 2021.*). Motor control centers are used to group several combination motor controllers together at a given location with a common power bus. MCCs are typically found in large commercial or industrial buildings where there are many electric motors that need to be controlled from a central location. Low-Voltage Motor Control Center shows an example of a motor control center.

MCCs are classified into two classes by UL Standard for Safety for Motor Control Centers UL 845, Underwriters Laboratories, Inc., August 2021.*and Motor Control Centers NEMA Standards Publication ICS 18-2007.*:

Class I Motor Control Centers: Mechanical groupings of combination motor control units, feeder tap units, other units, and electrical devices arranged in an assembly.

Class II Motor Control Centers: A Class I motor control center provided with manufacturer-furnished electrical interlocking and wiring between units, as specifically described in overall control system diagrams supplied by the user.

MCC wiring is classified by UL Standard for Safety for Motor Control Centers UL 845, Underwriters Laboratories, Inc., August 2021.*and Motor Control Centers NE MA Standards Publication ICS 18-2001.* into three types:

Type A Wiring: User (field) load and control wiring are connected directly to device terminals internal to the unit; provided on Class I MCCs only.

Type B Wiring: User (field ) control wiring is connected to unit terminal blocks; the field load wiring is connected either to power terminal blocks or directly to the device terminals.

Type C Wiring: User (field control wiring is connected to master terminal blocks mounted at the top or bottom of vertical sections which contain combination motor control units or control assemblies; the field load wiring is connected to master power terminal blocks mounted at the top or bottom of vertical sections or directly to the device terminals.

MCCs generally consist of a common power bus and a vertical bus for each section to which combination motor controllers are plugged on. The combination starter consists of motor starter, fuses or circuit breakers, and power disconnect. MCCs may also have push buttons, indicator lights, variable-frequency drives (VFDs), programmable logical controllers (PLCs), and metering equipment. The individual plug-in units are often referred to as buckets and may be inserted and removed with the main bus energized so long as the disconnecting device for the individual unit is open. A vertical wireway is supplied internal inter-unit connections and field connections within each section.

MCCs offer the opportunity to group several motor starters together in one location with a space-efficient footprint versus individual control cabinets, and like switchboards are available with many options. Removable plug-on units allow quick change-outs if spare units are kept on hand for the most common sizes of starters in the facility. Low-voltage soft-starters and variable-speed drives may also be mounted within MCCs.

Low-voltage switchgear, more properly termed metal –enclosed low-voltage power circuit breaker switchgear, is defined per IEEE Standard for Metal-Enclosed Low-Voltage Power Circuit Breaker Switchgear IEEE Std. C37.20.1-2001, October 2002.* as ”LV switchgear of multiple or individual enclosures, including the following equipment as required:

• Low-voltage power circuit breakers (fused or unfused) in accordance with IEEE Std. C37.13-2015 or IEEE C37.14-2015

• Bare bus and connections

• Instrument and control power transformers

• Instruments, meters, and relays

• Control wiring and accessory devices

Low-voltage power switchgear is the preferred equipment for medium to large industrial systems where the advantages of low-voltage power circuit breakers, discussed in System Protection, can be utilized to enhance coordination and reliability. It is typically used as the highest level of low-voltage equipment in a facility of this type and, if the utility service is a low-voltage service, the service entrance switchgear as well. Low-voltage Switchgear shows an example of low-voltage switchgear.

Low-voltage switchgear, although it performs the same functions and has comparable availability of voltage and ampacity ratings as switchboards, represents a different mode of development from switchboards and is, in general, more robust, both due to the construction of the switchgear itself and due to the characteristics of low-voltage power circuit breakers versus. molded-case circuit breakers. For this reason, it is preferred over switchboards where coordination, reliability, and maintenance are of primary concern.

Low-voltage switchgear is typically built as “rear accessible” meaning the access to load cabling and connections are from the rear of the equipment. In situations where space is limited, low-voltage switchgear may be built as “front accessible” where the access load cabling and connections are from the front of the equipment. This allows the low-voltage switchgear lineup to be placed against a wall. This configuration is also useful in electrical house applications where no exterior doors are necessary to access the back side of the equipment.

Low-voltage switchgear is compartmentalized to reduce the possibility of internal fault propagation. ANSI C37.20.1 (see IEEE Standard for Metal-Enclosed Low-Voltage Power Circuit Breaker Switchgear IEEE Std. C37.20.1-2001, October 2002.*) requires each breaker to be provided with its own metal-enclosed compartment. Optional barriers are usually available to separate the main bus from the cable terminations, forming separate bus and cable compartments within a section, as well as side barriers to separate adjacent cable and bus compartments.

All low-voltage switchgear is required to pass an AC withstand test of 2.2 kV for one minute (see IEEE Standard for Metal-Enclosed Low-Voltage Power Circuit Breaker Switchgear IEEE Std. C37.20.1-2001, October 2002.*).

As with switchboards, low-voltage switchgear is available with many options. The options are generally more numerous than those for switchboards due to the nature of switchgear service conditions.

“A transformer is a static electrical device that transfers energy between two or more circuits through electromagnetic induction” (https://www.se.com/). The main applications of a transformer include converting utility voltage to building distribution voltage, and converting distribution voltage to application voltage requirements. These units can be classified into multiple types: Dry-type, Control Power, and Mini Power-zone.

Dry-Type: Dry-type distribution transformers are designed to transform voltages for supplying electrical power to a building or load center. For classification as a low voltage dry-type distribution transformer, the transformer must have an input voltage of 34.5 kV or less, have an output voltage of 600 V or less, be rated for operation at a frequency of 60 Hz, and have the capacity of 15-2500 kVA (see 0100CT1901 Low-voltage Transformers).

Dry-type transformers windings are air-cooled; and are not permitted to use oil as a coolant. When voltage is applied to the input winding of the transformer, there can be a brief period of inrush current until the transformer core stabilizes, which lasts for about six power cycles (0.1 seconds). The magnitude of the inrush varies depending on when the switch closes on the power wave, meaning the inrush can range from zero to greater than the full load current rating of the transformer (7400CT1901).

Additionally, the impedance of the supply system can influence the amount of inrush current the transformer can draw. To avoid tripping circuit breakers or blowing fuses on the primary side of the transformer during energizing, it is essential to carefully coordinate fuse sizes or breaker handle ratings, and magnetic trip settings. To provide optimal coordination and minimize inrush nuisance tripping, adjust the primary overcurrent protection based on the maximum inrush current. This results in the primary overcurrent protection exceeding the 125% allowance in the NEC for primary-only protection, and secondary protection is required (see 7400CT1901).

Due to concerns regarding the impact on the efficiency of the transformers and market needs for improvements in energy consumption, low voltage distribution transformers are regulated through the Energy Policy and Conservation Act (7400CT1901). The Department of Energy (DOE) evaluates and sets minimum efficiency standards for low voltage dry-type distribution transformers. Transformer efficiency can be defined as the percentage of power out compared to the percentage of power in. A perfect zero loss transformer would have the same power in as out and would be 100% efficient (see 7400CT1901). The efficiency of the transformers shall be no less than that which is required for their kVA rating, as shown in Energy Conservation Standards for Low Voltage Dry-type Distribution Transformers.

The following low-voltage dry-type transformers must comply with the DOE efficiency standards as shown in Energy Conservation Standards for Low Voltage Dry-type Distribution Transformers:

• Three- and single-phase transformers

• Step-up and step-down transformers

• General purpose ventilated transformers

• Harmonic-mitigating transformers

• General purpose open core and coil transformers

The following low-voltage dry-type transformers are not required to comply with the efficiency standards as shown in Energy Conservation Standards for Low Voltage Dry-type Distribution Transformers:

• Auto-transformers

• Drive-isolation transformers

• Non-ventilated transformers

• Resin-encapsulated transformers

• Buck-boost transformers

• Control-power transformers

• Medical isolation panel transformers

Control Power: Industrial control power transformers are designed with low impedance windings for voltage regulation and can accommodate the high inrush current associated with contractors, starters, solenoids, and relays. Their function is to meet the diverse needs of panel builders and machinery OEMs. They are typically 50/60 Hz rated and are designed with various temperature classes as shown in Temperature Rises for Low-voltage Transformers.

Mini Power-Zone: Mini Power-Zone combines a transformer and circuit breaker distribution panel into a single wall mounted substation. The substation includes a primary main circuit breaker, sealed step-down transformer, secondary main circuit breaker, and distribution panelboard. They are typically built in a NEMA Type 3R enclosure, making them suitable for indoor and outdoor use, but can also be built for Type 4X applications. Typical applications for these units can include assembly lines, emergency power, temporary power, areas with limited space, and more.

For information on medium-voltage basics according to IEC and IEEE standards please reference Medium Voltage Technical Guide.

Medium-voltage power and distribution transformers are used for the transformation of voltages for the distribution of electric power. They can be generally classified into two different types:

Dry-Type: The windings of this type of transformer are cooled via the circulation of ventilating air. The windings may be one of several types, including Vacuum Pressure Impregnated (VPI), Vacuum Pressure Encapsulated (VPE), and cast-resin. The cast-resin types generally are more durable and less likely to absorb moisture in the windings than the VPI or VPE types. In some cases, the primary windings are cast-resin and the secondary windings are VPI or VPE.

Liquid-Filled: The windings of this type of transformer are cooled via a liquid medium, usually mineral oil, silicone, or paraffinic petroleum-based fluids.

Liquid-filled units have a generally low in first-cost, but the requirements in NEC The National Electrical Code, NFPA 70 The National Fire Protection Association, Inc., 2005 Edition.* Article 450 must be reviewed so that installation requirements can be adequately met, and maintenance must be taken under consideration since fluid levels should be monitored and the condition of the fluid examined on a regular basis. They have an expected service life of approximately 20 years. VPE and VPI dry-type transformers also generally have low first-costs, have longer lifetimes than liquid-filled units, and are much easier than liquid-filled types to install indoors; however, give consideration to the absorption of moisture by the windings if these are used outdoors. Installed indoors, these have expected service lifetimes of around 30 years. Cast-resin dry-type transformers have generally high first-costs compared to the other types but have the same installation requirements as dry-type transformers and have the longest expected service life (around 40 years).

Enclosure styles may also be divided into two basic types: pad-mounted, which is a totally-enclosed type generally mounted outdoors and with specific tamper-resistance features to minimize inadvertent access by the general public, and unit substation type, which is an industrial-type enclosure suitable for close-coupling into an integrated unit substation lineup with primary and secondary equipment (unit substation-style transformers may also be equipped with cable termination compartments as well).

Medium Voltage Power and Distribution Transformers shows typical examples of medium-voltage power and distribution transformers.Top to bottom: cast-coil dry type with unit substation-style enclosure, VPI dry-type with unit substation-style enclosure, liquid-filled type with unit substation-style enclosure, and dry-type with pad-mounted enclosure.

Medium-voltage power and distribution transformer capacities may be increased with the addition of fans. Cooling types are listed as AA (ambient air) for dry-type transformers without fans, and AA/FA (ambient air/forced air) for dry-type transformers with fans, for an increase of 33% in kVA capacity. The cooling type for a liquid-filled transformer is listed as OA for units without fans, OA/FA for units with fans, with an increase of 15% kVA capacity for units 225 - 2000 kVA, and 25% for units 2,500 – 10,000 kVA. “FFA” (future forced air) options are usually available for both dry and liquid-filled types, although experience has shown that the fans are almost never added in the field.

Typical BIL Levels for Medium-voltage Power and Distribution Transformers gives typical BIL levels for medium-voltage power and distribution transformers. These apply to both the primary and secondary windings. Typical Design Temperature Rises for Medium-voltage Power and Distribution Transformers gives typical design temperature rises.

Impedance levels vary; the manufacturer must be consulted for the design impedance of a specific transformer. In general, units 1000–5000 kVA typically have 5.75% impedance ± 7.5% tolerance.

Medium-voltage power and distribution transformers are typically available with several types of accessories, including connections to primary and secondary equipment, temperature controllers and fan packages, integral fuses for transformers with padmount-style enclosures.

Metal-enclosed power switchgear is defined by IEEE Standard for Metal-Enclosed Interrupter Switchgear IEEE Std. C37.20.3-2001, August 2001.* as “a switchgear assembly enclosed on all sides and top with sheet metal (except for ventilating openings and inspection windows) containing primary power circuit switching or interrupting devices, or both, with buses and connections and possibly including control and auxiliary devices. Access to the interior of the enclosure is provided by doors or removable covers.” Metal-enclosed interrupter switchgear is defined by IEEE Standard for Metal-Enclosed Interrupter Switchgear IEEE Std. C37.20.3-2001, August 2001.* as “metal-enclosed power switchgear including the following equipment as required:

• Interrupter switches (or vacuum circuit breaker)

• Power fuses (current-limiting or noncurrent- limiting)

• Bare bus and connections

• Instrument transformers

• Control wiring and secondary devices

Metal-enclosed interrupter switchgear is typically used for the protection of unit substation transformers and as service-entrance equipment in small- to medium-size facilities. Metal-enclosed Interrupter Switchgear shows an example of metal-enclosed interrupter switchgear.

In the case of fusible equipment, overcurrent protection flexibility is limited, however with current-limiting fuses this equipment has high (up to 65 kA rms symmetrical) short-circuit interrupting capability. The load interrupter switches in this class of switchgear are designed to interrupt load currents only and may use air as the interrupting medium or SF₆. They may be arranged in many configurations of mains, but ties, and feeders as required by the application.

As for Vacuum-circuit breaker Metal-enclosed gear, all use a combination of vacuum circuit breaker and protective relays; design features like compartmentalization, bus insulation and for breakers to be withdrawable Is not required by IEEE standards.

This type of switchgear is frequently used as the primary equipment of a unit substation line-up incorporating primary equipment, a transformer, and secondary equipment.

Voltage Withstand Levels for Metal-enclosed Interrupter Switchgear shows the BIL levels of metal-enclosed interrupter switchgear, per IEEE Standard for Metal-Enclosed Interrupter Switchgear, IEEE Std. C37.20.3-2001, August 2001.*. The power frequency withstand is a one-minute test value. Momentary (ten cycle) and short-time (two seconds) current ratings are also assigned for this type of switchgear.

Medium-voltage motor controllers are used to control the starting and protection for medium-voltage motors. They generally utilize vacuum contactors rated up to 400 A continuous, in series with a non-load-break isolation switch and R-rated fuses, fed from a common power bus. The motor starting methods in Arc Flash Hazard are all generally supported, including soft-start capabilities. Class E2 units per Industrial Control and Systems: Medium Voltage Controllers Rated 2001 to 7200 Volts AC NEMA Standards Publication ICS 3-1993.*, which employ fuses for short-circuit protection, are generally the most common. Medium-voltage MCC shows a typical example of a medium-voltage MCC.

Medium-voltage MCC’s are generally available with several options depending upon the manufacturer, including customized control and multi-function microprocessor-based motor protection relays The contactors are generally of roll-out design to allow quick replacement.

Above 7200, metal-clad switchgear is generally used for motor starting.

Metal-clad switchgear is defined by IEEE Standard for Metal-Clad Switchgear, IEEE Std. C37.20.2-1999, July 2000.* as “metal-enclosed power switchgear characterized by the following necessary features:

• The main switching and interrupting device is of the removable (drawout type) arranged with a mechanism for moving it physically between connected and disconnected positions and equipped with self-aligning and self-coupling primary disconnecting devices and disconnectable control wiring connections.

• Major parts of the primary circuit, that is, the circuit switching or interrupting devices, buses, voltage transformers, and control power transformers, are completely enclosed by grounded metal barriers that have no intentional openings between compartments. Specifically included is a metal barrier in front of, or a part of, the circuit interrupting device so that, when in the connected position, no primary circuit components are exposed by the opening of a door.

• All live parts are enclosed within grounded metal compartments.

• Automatic shutters that cover primary circuit elements when the removable element is in the disconnected, test, or removed position.

• Primary bus conductors and connections are covered with insulating material throughout.

• Mechanical interlocks are provided for proper operating sequence under normal operating conditions.

• Instruments, meters, relays, secondary control devices, and their wiring are isolated by grounded metal barriers from all primary circuit elements except for short lengths of wire such as at instrument transformer terminals.

• The door through which the circuit interrupting device is inserted into the housing may serve as an instrument or relay panel and may also provide access to a secondary or control compartment within the housing.

Medium-voltage metal-clad switchgear is generally used as the high-level distribution switchgear for medium- to large-sized facilities. It is also the preferred choice for service entrance equipment for these types of facilities.Metal-clad Switchgear shows an example of metal-clad switchgear.

Metal-clad switchgear uses high-voltage circuit breakers, as described in System Protection, fed from a common power bus. It is configurable in many different arrangements of main, bus tie, and feeder devices to suit the application. Relays are usually required since the circuit breakers generally do not have integral trip units. This type of switchgear is the preferred means for accomplishing automatic transfer control and complex generator paralleling applications; the control may be placed in the switchgear itself or in a separate panel, depending upon the application and specific end-user preferences.

A new generation of Metal-clad switchgear with narrower footprint, offers an average space saving of 25% when compared to traditional metal-clad. Its native digital operation and monitoring enhances safety and efficiency.

The construction requirements per IEEE Standard for Metal-Clad Switchgear, IEEE Std. C37.20.2-1999, July 2000.* ensure that metal-clad switchgear is the safest type of switchgear in terms of operator safety.

The BIL and withstand voltage requirements for this switchgear are the same as for metal-enclosed switchgear as given in Medium-voltage Metal-clad Switchgear.

This type of switchgear has many options available to suit the application, such as electric racking for circuit breakers, ground and test units that allow the grounding/testing of stationary contacts with a circuit breaker withdrawn.

Retrofit solutions are pre-engineered solutions that are designed with a new breaker element truck and carriage used to be installed in the existing switchgear. The interfaces with the existing structure maintain all safety interlocks inherent with the original design. The interior of the existing cell is not modified. These types of solutions work well with any brand of legacy switchgear.

• For low-voltage switchgear a cradle-in-cradle design is utilized as replacement to make a connection to the existing cell bus connections. The retrofit solutions allow interchangeability of the breaker element with other OEM switchgear of the same rating.

• For medium-voltage switchgear a new truck element is designed to match the existing cell. Design the electrically operated mechanism to match the existing air-magnetic circuits. The mechanism utilized in the design have a passive interlock to block the insertion or removal of a closed breaker.

• The solutions are designed and tested to IEEE Std C37.59 or to ANSI C37.50 standards.

Gas Insulated switchgear is a relatively compact, multi-component assembly, enclosed in a grounded metallic housing in which the primary insulating medium is a compressed gas. It is typically characterized by the following features:

• 1-high or Single-high construction

• Fixed vacuum circuit breaker

• Front-access only design

• Front cable connections and testing

• Internal disconnect and grounding mechanism

• Bottom and top cable entry, with T-body connectors

• Arc resistant and seismic design

• CTs and VTs accessible outside the gas tank

• VTs are isolated with integrated grounding switch

• Modular design of switchgear sections

Medium-voltage gas-insulated switchgear is generally used as the high-level distribution switchgear for medium- to large-sized facilities. It is also the preferred choice for Power or E-houses and power distribution rooms where gas-insulated switchgear’s compact design maximizes equipment space savings.

A retrofill solution is a custom engineered solution that involves modifying the cell and bus to accept the new low or medium voltage circuit breaker. The new circuit breaker provides new racking mechanism, primary and secondary disconnects, as well as new doors. For low-voltage switchgear a cradle-in-cradle design is utilized as replacement to make a connection to the existing cell bus connections. The retrofit solutions allow interchangeability of the breaker element with other OEM switchgear of the same rating:

• For medium-voltage switchgear a new truck element is designed to match the existing cell. Design the electrically operated mechanism to match the existing air-magnetic circuits. The mechanism utilized in the design have a passive interlock to block the insertion or removal of a closed breaker.

• The solutions are designed and tested to IEEE Std C37.59 or to ANSI C37.50 standards.