Learning Module 13: Panelboards and Switchboards. 101 Basic Series

Transcription

1 Learning Module 13: Panelboards and Switchboards 101 Basic Series

2 What You Will Learn We ll step through each of these topics in detail: Introduction 4 Definitions 4 Similarities Between Panelboards and Switchboards 4 Power Supply Systems 5 Service Entrance Equipment 5 Distribution Equipment 6 Grounding and Ground Fault 7 Equipment Ratings 8 Panelboards 10 Panelboard vs. Loadcenter 10 Components of a Panelboard 10 Circuit Identification 11 Enclosures 12 Review 1 13 Circuit Breakers 14 Balancing the Load 14 Types of Panelboards 14 Lighting and Appliance Panelboard 14 Power Distribution Panelboard 14 Installation and Mounting 16 NEC Mounting Specifications 16 Review 2 17 Switchboards 18 Components of a Switchboard 18 Sections of a Switchboard 21 Service Section 21 Pull Section 21 Distribution Section 22 IFS Section 22 Spare, Provision and Blank Space 23 Helping the Customer 24 Matching a Panelboard to an Application 24 Matching a Switchboard to an Application 25 Review 3 26 Glossary 27 Review 1 Answers 31 Review 2 Answers 31 Review 3 Answers 32 Page 2

3 Welcome Welcome to Module 13, which is about Panelboards and Switchboards. In Module 10, we discussed power distribution with a loadcenter. Loadcenters are used mostly in residential applications. Distributing power in commercial and industrial facilities is more complex than residential distribution. A typical industrial distribution system is shown in Figure 1. In additional to distribution devices such as switchgear, switchboards, transformers, and panelboards, such a system usually contains metering equipment, main and branch disconnects, protective devices, power switching devices, and conductors. We will look at switchboards and panelboards in detail in this module. These expandable devices are used to supply electric service in industrial applications. Figure 1. Typical Components of a Commercial and/or Industrial Distribution System A Note on Font Styles Viewing the Glossary Like the other modules in this series, this one presents small, manageable sections of new material followed by a series of questions about that material. Study the material carefully, then answer the questions without referring back to what you ve just read. You are the best judge of how well you grasp the material. Review the material as often as you think necessary. The most important thing is establishing a solid foundation to build on as you move from topic to topic and module to module. Key points are in bold. Glossary items are italicized and underlined the first time they appear. Printed versions have the glossary at the end of the module. You may also browse the Glossary by clicking on the Glossary bookmark in the left-hand margin. Page 3

4 Introduction Definitions Similarities Between Panelboards and Switchboards Both panelboards and switchboards contain the circuit feeder cables and protective devices required for the Branch Circuits. Each, however, do slightly different jobs. A panelboard provides circuit control and overcurrent protection for light, heat or other electrical loads. It is used in applications in commercial and industrial facilities. NEC (National Electrical Code) offers this definition for a panelboard: A single panel, or group of panel units designed for assembly in the form of a single panel; including buses, 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 or partition and accessible only from the front. A switchboard divides large blocks of electrical current into smaller blocks of current used by electrical equipment. Applications can be as small as a small office building, or as large as a major industrial complex. NEC offers this definition for a switchboard: A large single panel, frame, or assembly of panels on which are mounted, on the face or back, or both, switches overcurrent and other protective devices, buses and usually instruments. Switchboards are generally accessible from the rear as well as from the front and are not intended to be installed in cabinets. Figure 1 shows the panelboard downstream from the switchboard. Distribution systems, whether simple or complex, typically include a panelboard. However, a distribution system does not always need a switchboard. There are several similarities between panelboards and switchboards. These are: Power supply systems Service entrance equipment / Distribution equipment Main device and main lug only types Grounding Ratings Page 4

5 Power Supply Systems Panelboards and switchboards get their power from a variety of sources. Both pieces of equipment can serve as Service Entrance Equipment, receiving power directly from the utility transformer. In addition, both can serve solely as distribution points, receiving their power from a panelboard or switchboard upstream. In any case, power originates at the power company or local generator, and may be stepped down through transformers for distribution. There are three main power supply systems in use today for panelboards and switchboards. These are: Single-phase, three-wire system Three-phase, four-wire, wye-connected transformer Three-phase, four-wire, delta-connected transformer This system can deliver both 240-volt and 120-volt power. Making a connection across both hot wires provides 240 volts. Connecting with either hot and the third wire neutral provides 120 volts. This system is predominantly found in residential applications. In a 208Y/120 or 480Y/277 wye-connected system, a connection across any two of the three hot wires yields 208 or 480 volts, respectively. Connecting across any hot wire and the neutral provides 120 or 277 volts. This system is a bit more complex. Connecting across any two of the three hot wires yields 240 volts. A connection made from the neutral to either of the two adjacent hot wires (C or A) provides 120 volts. Finally, a connection across the neutral to the non-adjacent hot wire (B) provides 208 volts. This non-adjacent hot wire has to be specially marked so the electrician does not accidentally connect to it when only 120 volts are desired. As a result, older delta-connected systems are being replaced by wyeconnected systems. Figure 2. Power Supply Systems Service Entrance Equipment Sometimes panelboards and switchboards can be used as service entrance equipment. The Service Entrance is the single point at which electrical service enters a building. Service entrance equipment enables an operator to control and cut off the electrical supply to the entire building from one point. To be classified as service entrance equipment, the panelboard or switchboard must meet these requirements: Must be approved and labeled Suitable for use as Service Equipment Must have a means of disconnection and overcurrent protection Must ground the neutral service conductor Must follow the six subdivisions of the main rule: The service entrance conductors must have a readily accessible means of being disconnected from the power supply. This is why the NEC has a Six Subdivisions of the Main Rule. This ruling states that you must be able to throw no more than six handles into the off position to disconnect electrical service. Page 5

6 In other words, the service entrance panelboard or switchboard can contain up to six overcurrent devices without the need for a single Main Disconnect Device. If more than six branch circuits are required, then a main device must be supplied upstream to disconnect all the branch circuits at once. For this reason, there are two ways to configure a panelboard or a switchboard: The Main Breaker or main switch unit has a single main Disconnect Device that will disconnect power to all equipment being supplied by the service. It also protects the system from short circuits and overloads (as well as Ground Faults, if equipped with Ground fault protection). The Main Lug Only (MLO) unit is equipped with up to six devices to disconnect power to all equipment being supplied by the service. It does not have a single main device. The incoming supply cables are connected directly to the Bus Bar lugs. Figure 3. Main Device (on left) Vs. Main Lug Only (on right) Distribution Equipment Both switchboards and panelboards can be used as Distribution Equipment. This is the term given to a panelboard or switchboard used at a point downstream from the service entrance equipment. Electrically speaking, service entrance panels and distribution panels or switchboards differ in only two respects: Distribution panels or switchboards may or may not be protected by an integral main breaker. This means the MLO is utilized for distribution. A feeder cable from the service entrance equipment supplies the power to the distribution panels. Therefore, it may be protected by the feeder cable s Circuit Breaker in the service panel. The service entrance is the only point at which the neutral is connected to ground. The neutral in any downstream equipment is isolated. The explanation for this is in the next section on grounding. A switchboard that serves both roles simply has a service entrance section and one or more distribution sections, as shown below. Figure 4. Simplified Service Entrance/Distribution Switchboard Page 6

7 Grounding and Ground Fault We have mentioned the concept of grounding a few times throughout this module but we haven t really defined it. The National Electrical Code defines ground as a conducting connection (intentional or accidental) between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth. Proper grounding of any electrical system is vital, not just for personal safety, but also for equipment longevity. There are two objectives to the intentional grounding of electrical equipment. These are to: Reduce the potential for electrical shock by minimizing the potential voltage differentials between various parts of a system. Minimize the ground path s impedance. Lower impedance means higher current when a fault occurs. That translates into faster-opening overcurrent protection devices. Ground fault protection is required per the National Electrical Code for all service entrance equipment mains of solidly grounded wye systems where the voltage to ground exceeds 150 volts and the overcurrent main is rated 1000 ampere or greater and where a neutral is provided. This includes all 480Y/277 volt systems. If the six disconnect rule is used, all overcurrent devices that exceed the requirements above will require ground fault protection. As an example, a 3000 ampere service entrance switchboard at 480Y/277 volts has six mains. Two mains are 1000 ampere, one main is 800 ampere, and three mains are 200 ampere. The two 1000 ampere mains would require ground fault protection; the other four mains would not. As we have already stated, the neutral service conductor on service entrance equipment must be grounded. The neutral is connected to ground only at the voltage service. In addition to the service entrance, additional grounding is required at separately derived services, such as distribution transformers. Distribution panelboards and switchboards benefit from that upstream grounding in case of a short circuit or overcurrent problem. A circuit is grounded only at the service entrance or separately derived services, never at any downstream equipment. Figure 5. Grounding the Downstream Panel For example, in Figure 6, the computer has a short circuit. If you trace the thick line back, you will see how fault current is returned to the source. This is why the Page 7

8 downstream panelboard contains a branch circuit breaker. It trips, disconnecting power from the load. Figure 6. A Short in a Downstream Load Equipment Ratings Although properly grounded equipment is of vital importance, ground fault protection is equally important. This type of protection is designed to save lives and protect equipment. A ground fault can occur when someone is washing down a countertop and water accidentally reaches an electrical appliance or outlet. A special circuit breaker called a ground fault circuit interrupter breaker can provide protection against this type of fault. It contains the normal thermal magnetic circuit protection, along with a ground fault sensor. A ground fault breaker can detect extremely low levels of current leakage to ground from four to six milliamps. Standard circuit breakers can t do that. The level of four to six milliamps was selected because, above this current level, it would be difficult for a person to physically let go of a conductor. When only equipment protection is needed, a level of 30 milliamps is used for detection. You must have two key pieces of information about an application before you can select panelboards, switchboards and overcurrent protection devices. These are: Maximum continuous amps Available fault current NEC article states: Equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment. Equipment intended to break current at other than fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted. Section was changed in the 1999 code by substituting the word interrupt for the work break in two places. The interrupting rating of overcurrent protective devices is determined under standard test conditions. It is important that the test conditions match the actual installation needs. Section states that all fuses and circuit breakers intended to interrupt the circuit at fault levels must have an adequate interrupting rating wherever they are used in the electrical system. Fuses or circuit breakers that do not Page 8

9 have adequate interrupting ratings could rupture while attempting to clear a short circuit. There are two ways to meet this requirement: the Full Rating Method and the Series Rated Method. The full rating method selects circuit protection devices with ratings equal to or greater than the available fault current. Consider a building with 65,000 amps of fault current available at the service entrance. All downstream circuit protection device must be rated at 65,000 Ampere Interrupt Capacity (AIC). Although switchboards are available with short circuit Current Ratings up to 200,000 amps, anything over 100,000 AIC starts to get cost-prohibitive because additional bus bracing is required. In Figure 7, the main circuit breaker and all branch breakers are rated for 65,000 AIC. Figure 7. Full Rating Method (on left) Vs. Series Rated Method (on right) The series rated method states that the main upstream circuit protection device must have an interrupting rating equal to or greater than the available fault current of the system, but downstream devices connected in series can be rated at lower values. Under fault conditions, both the main device and the downstream device would open to clear the fault. Consider a building with 42,000 amps of available fault current. Although the breaker at the service entrance is rated at 42,000 amps, additional downstream breakers could be rated at only 22,000 amps. To receive UL listing, series-rated breaker combinations must first pass testing in series, then pass tests installed in panelboards and/or switchboards. There are additional rating terms that need to be understood when selecting panelboards, switchboards and appropriate circuit protection devices. These are: Current Rating: This is the level of fault current a piece of equipment can withstand without sustaining damage. Interrupting Rating: This is the current rating a protective device (such as a Fuse or circuit breaker) can safely interrupt. Ampere Rating: This is the current a protective device will carry continuously without deteriorating or exceeding temperature rise limits. Voltage Rating: The voltage rating of a switchboard or panelboard can be higher than the system voltage, but never lower. For example, a 480 VAC switchboard could be used on a 240 VAC system. A 240 VAC switchboard could not be used on a 480 VAC system. Page 9

10 Panelboards Panelboard vs. Loadcenter Components of a Panelboard Now that we have looked at the commonalities between the switchboard and the panelboard, We will look at their specific features separately. Let s begin with the panelboard. We looked at the Loadcenter in Module 10 of this series. The loadcenter and the panelboard perform similar functions. They both serve to protect branch circuits from overloads and short circuits. A panelboard is for use in commercial and industrial applications, while a loadcenter is primarily for use in residential applications. This is because the bus bars in a loadcenter are typically rated at a maximum of 200 amps, while those of a panelboard can accommodate up to 1200 amps. Along the same lines, a loadcenter can handle a maximum of 240 volts, but a panelboard can handle up to 600 volts. Now that you have a basic understanding of the functions a panelboard performs, let s take some time to consider the panelboard s construction. There are several components to a panelboard: a can, interior, circuit protection devices, label, dead front and trim, and filler plates. Figure 8. Typical Panelboard Meeting NEC Definition Can The Can, also called the box, is the housing in which the other components reside. Typically, it is made of galvanized steel. The design of the can provides protection for both personnel and the internal components. The can s end panels are removable, allowing the installer to locate and cut holes for conduit installation. Optionally, the end panels can be provided with stamped Knockouts. Interior Inside the can, you will find overcurrent protection devices, bus bars, insulated Neutral Bars, and other components, depending on the application. The centerpiece is the set of bus bars. A bus bar is a common conductor, used as a connection point for multiple circuits. Bus bars are usually aluminum, but can also be copper. They provide a mechanical means of affixing branch circuit breakers. The bus bars are mounted on the interior, which is mounted on studs in the box. The neutral bar provides the termination point for the neutral wires from both the incoming service and the load circuits. The neutral bar can be mounted on studs in the back of the box, or on the panelboard interior. Page 10

11 Figure 9. Bus Bars and Neutral Bar Circuit Protection Devices The circuit protection devices are usually circuit breakers. They are mounted directly to the bus bars. We will talk more about circuit breakers later. Label The panelboard label provides information regarding the unit s voltage rating and ampacity. Dead Front and Trim These components cover the front surface of the panelboard. A hinged access door is provided as part of the trim. The dead front provides access to the circuit breakers themselves, while preventing contact with interior components such as the bus bars and internal wiring. Figure 10. Components of a Panelboard Circuit Identification Filler Plates Filler plates are used to cover any unused Pole spaces not used by a circuit breaker. All circuits in the panelboard must be clearly identified with a number. This can be done in two different ways. The NEMA (National Electrical Manufacturers Association) numbering scheme assigns odd numbers to the poles on the panelboard s right (your left), and even numbers to the poles on the panelboard s left (your right). This is shown in Figure 11 on the left. The other method simply used a vertical numbering sequence, shown on the right. Page 11

12 Figure 11. NEMA Numbering (on left) Vs. Vertical Numbering (on right) Enclosures Attached to the inside of the door, you will find a label containing a circuit directory. NEMA has established guidelines for electrical equipment enclosures. Most panelboards are supplied as standard in a NEMA Type 1 enclosure. Descriptions of the enclosure options are listed below. NEMA Type 1 General Purpose NEMA Type 3R Raintight NEMA Type 4X Corrosion Resistant NEMA Type 12 Dusttight This enclosure type is for general purpose, indoor use. It is suitable for most applications where unusual service conditions do not exist. It provides protection from accidental contact with enclosed equipment This enclosure type is intended for outdoor use. It provides protection against falling rain and sleet, and damage from external ice formation. It has a gasketed cover. This enclosure type is intended for indoor or outdoor use, where corrosion resistance is required. It is constructed of stainless steel, polymeric, or fiberglass. It also provides protection from splashing or hose-directed water, wind-blown dust or rain, and damage from external ice formation. This enclosure type is for indoor use. It provides protection from dripping non-corrosive liquids, falling dirt, and dust. Page 12

13 Review 1 Answer the following questions without referring to the material just presented. Begin the next section when you are confident that you understand what you ve already read. 1. There are several similarities between panelboards and switchboards. Name four areas of similarity. 2. In your own words, explain the difference between the full rating method and the series rated method of selecting overcurrent protection devices. 3. Define the following ratings: Withstand rating: Interrupting rating: Ampere rating: 4. Briefly explain the function of each panelboard component listed here: Can Bus Bars Dead Front and Trim Filler Plate Page 13

14 Circuit Breakers Balancing the Load One circuit breaker is installed in a panelboard for each branch circuit the panelboard serves. In this section, we will consider the proper way to install circuit breakers in a panelboard. In the case of a single phase system, there are two bus bars in the panelboard. When installing breakers, it is important to balance the load so that both bars are doing the same amount of work. This prevents strain on the system. Compare this to loading clothes into a washing machine. It is important to get an even distribution of clothes all the way around the tub. And it s not enough just to have an equal number of items on each side of the tub. Having a pair of jeans and a sweater on one side and two socks on the other side is NOT a balanced load. The same is true for the panelboard. It is not enough to make sure there are an equal number of breakers on each bus bar. The total amperage ratings of the breakers on each bus bar need to be equal. Figure 12. Balancing the Load Types of Panelboards Lighting and Appliance Panelboard Power Distribution Panelboard There are two common types of panelboards: Lighting and Appliance Panelboard Power Panelboard The NEC definition for lighting and appliance panelboards has three primary elements: A maximum of 42 overcurrent devices (poles) installed in one cabinet At least 10% of the overcurrent devices must be rated at 30 amps or less Neutral connections must be provided The NEC defines power distribution panelboards as all panelboards that are not lighting and appliance. The only restriction is physical limitations. It is used to feed other panels, three-phase motors, and transformers. Page 14

15 Figure 13. Types of Panelboards Consider this panelboard as an example: Number of Circuit Breakers Description Number of Overcurrent Devices 4 1-Pole, 30 A Pole, 40 A Pole, 40 A 16 Total 42 There are only four overcurrent devices which are rated at 30 amps or less. This panelboard does not qualify as a lighting and appliance panelboard, so it is a power and distribution panelboard. In the Workplace Consider what happens when a light switch is turned on. If the circuit takes 18 amps to power the lights, it will continue to take 18 amps until the lights are turned off. Now consider a motor circuit. The start button is pressed and the motor experiences a large inrush of current. This inrush could easily be 6 times the current needed to run the motor at full rated load at full speed. For example, a one-horsepower motor that requires 21 amps to start will only need 3-1/2 amps when running at full speed. Because the power panel is designed to supply branch circuit loads such as this, it cannot be regulated as closely as a lighting and appliance panelboard. Page 15

16 Installation and Mounting Flush Mount vs. Surface Mount A panelboard can be Flush-Mounted or Surface-Mounted against a wall. A flushmounting is recessed into a hole in the wall. A surface-mounting is attached to (and projects out from) the wall. Many panelboards are flush mounted in commercial, office, school and public buildings. Flush-mounting offers some important benefits, such as: Space savings Because the box is behind the wall, the room is not robbed of space. Appearance This option is more attractive, as the wiring and box are concealed. Safety Except for the trim, the panel does not jut out to possibly catch clothing or other objects. Figure 14. Flush-Mount Vs. Surface Mount NEC Mounting Specifications Surface-mounted panelboards are generally used in industrial buildings and basements of office and commercial buildings. Because most of these areas have walls made of poured concrete, flush-mounting is impractical. Steel columns are often used for surface mounting. The NEC specifies clearances around panelboards. This is to provide access and working space. There are three basic rules: Headroom in the location must be a minimum of 6-1/2 feet. For systems up to 150 volts, the minimum distance from the panelboard to the ground shall be 3 feet, 4 feet for volt systems. For access, there must be 30 inches minimum space in front of the panelboard, and sufficient space to allow the hinged door to open and rotate 90 degrees. Figure 15. Panelboard Clearance Requirements Page 16

17 Review 2 Answer the following questions without referring to the material just presented. Begin the next section when you are confident that you understand what you ve already read. 1. Underwriter's Laboratory (UL) has a number of stringent guidelines for application of the lighting and appliance panelboard. These include: A maximum of overcurrent devices installed in one cabinet At least % of the overcurrent devices rated at amps or less must be provided 2. Does this drawing show a balanced breaker load? Circle YES or NO. 3. The two mounting styles for panelboards are -mounted and -mounted. 4. There are a number of requirements that the installation location for a 240 volt panelboard must meet. In your own words, explain these requirements. Page 17

18 Switchboards Now let s turn our attention to switchboards. Switchboards are low voltage equipment, meaning 600 volts or less. The current range is 400 to 6000 amps. Again, the purposes of a switchboard are to disconnect loads for safe maintenance, and to protect conductors and equipment against excessive current due to overloads, short circuits, and ground faults provided ground fault protection is supplied. The basis for a switchboard consists of a frame, bus, Overcurrent Protective Devices, instrumentation, and exterior covers. Figure 16. Typical Switchboard Meeting NEC Definition Components of a Switchboard Frame The frame is the metal skeleton in which the other components reside. Bus Bus is mounted within the frame. Like the bus bars of the panelboard, bus is used to distribute power. Where bus bars move power from incoming utility power cables to the branch circuits, bus move power between two or more components or circuits. Horizontal Bus distribute power to each switchboard section. Vertical Bus distribute power through the circuit protection devices to the branch circuits. Figure 17. Switchboard Frame, Before and After Installing Bus NEMA requires that the bus be installed with the phases in sequence when viewed from the front. In the case of the three-phase switchboard shown in Figure 17, the phases must be in the order A-B-C from left to right (for vertical bus) or from top to bottom (for horizontal bus). Page 18

19 If a Neutral Bus is required, it is placed at the end of the sequence. With vertical buses, it is placed at the left, yielding an N-A-B-C sequence. In the case of horizontal buses, it is placed at the bottom, resulting in an A-B-C-N. (No vertical neutral bus is shown above.) Protective Devices Protective devices, such as circuit breakers and disconnect switches, are mounted to the vertical bus bars from the front of the unit. Other devices installed at this time include meters, transient voltage surge protection (TVSS), utility compartments, panelboards, transformers, and other equipment. Figure 18. Installing Protective Devices Page 19

20 Types of Protective Devices There are four types of protective devices commonly used in switchboard applications. These are: Power Circuit Breaker The term power is applied to a large circuit breaker with a wide range of adjustable magnetic overcurrent and solid state. trips. It does not have a thermal trip. It is built on an open framework for easy servicing and adjustment. It can be manually or electrically operated and is available with ratings up to 4000 amperes. Power air circuit breakers can be stationary-mounted or draw-out type. Draw-out breakers can be tested without removal from the switchboard. Low voltage power circuit breakers are covered in Module 7. Molded Case Circuit Breaker This breaker is normally both a thermal and magnetic trip device. Ratings run 15 2,500 amps, with a variety of short circuit interrupting capacities. Some breakers feature interchangeable trip elements. Others have solid state trip units. Molded Case Circuit Breakers are covered in Module 8. Fusible Switch Bolted Pressure Switch This is a hand-operated disconnect switch with a fuse on each pole. When an overload condition arises, the fuse link melts and opens the pole, protecting the circuit. Fuses are available in two styles. The time delay fuse is suitable for load surges, such as motor start-ups. The instantaneous fuse is designed to clear in a fraction of a second in the event of a high current short circuit. They are typically in rated for v and amps. The main difference between a circuit breaker and a fusible switch is that the circuit breaker can be reclosed after an overload has been interrupted and corrected. The fuse needs to be replaced. This switch type is primarily used for service entrance and feeder circuits. The switch is quick-make/quick-break. When the switch moved to the ON position, the line contacts are squeezed together under pressure by the contact locking mechanism. This stored up energy is released for quick make. The same energy quickly breaks the contacts when the manual trip button is pushed. The protective elements are heavy duty, current-limiting high interrupting capacity fuses. Bolted pressure switches are rated volts and 800 4,000 amps. Accessories are available to open the switch automatically or remotely. It can be manual or an electric trip Instrumentation Meters can be used in the service section to measure current, voltage, power usages, peak demands just to name a few. This will be explored in Module 15, Metering and Communication. Enclosures Switchboards are usually only available in a NEMA Type 1 or NEMA Type 3R enclosure. Exterior Covers Once the protective devices are installed, exterior cover panels are put in place on Page 20

21 the frame. Like the dead front and trim of the panelboard, these covers allow access to the protective devices while sealing off the buses and wiring from accidental contact. The cover panels also serve as trim for the sake of product appearance. An informational is attached to the cover panel, providing information on the switchboard type, voltage and amperage ratings, and part number. Figure 19. Installing Dead Front and Trim Sections of a Switchboard Service Section Now that you understand how a switchboard is put together, let s take some time to consider the function of each section. We have made a few brief references various sections of the switchboard without really describing them. There are four main section types that a switchboard may have. Note that all switchboards do not utilize all four of these section types. They are: Service section Pull section Distribution section IFS section This section always contains a main disconnect and usually also has utility and/or customer metering equipment. The service section can be fed directly from the utility company s transformer to serve as a service entrance. Service from the utility can enter the service section in a variety of ways. Cable can be brought into the switchboard from the top or bottom, giving additional application flexibility. You will need to work with the customer to determine what service entrance needs exist. This decision will affect the cost. Figure 20. Bringing Service into the Service Section Pull Section The center example in Figure 20 shows the use of a Pull Section. The pull section is a common switchboard component. It is simply a blank enclosure containing empty space through which cables can be pulled, hence the name. Page 21

22 Distribution Section A pull section is used most often with service entrance switchboards where the utility feed comes up through the floor. This allows the service section to be fed from the top without any exposed conductors. The fourth example shows the use of a Pull Box, which is basically a top-mounted pull section. This is useful if there are extra connections to be made because it permits more space in which to work. Power moves from the service section to the distribution section. Here, it is divided and sent through branch circuit protection devices, then out to the branch circuits, to provide power to loads downstream. In some applications, the size of the service section s main disconnect device and associated bus requirements may necessitate the service section cabinet to be deeper than the distribution section cabinet. Because the rear of all cabinet sections are always aligned (for installation against a wall), the front of the service section cabinet will protrude. This is referred to as a Rear Alignment. Figure 21. Front and Rear Alignment Vs. Rear Alignment IFS Section If the depth of the service section and distribution sections are the same, the switchboard installation has Front and Rear Alignment. Some switchboards may require a deeper distribution cabinet for the circuit protection devices and bus. Or, extra depth may be added as an option. Again, work with the customer to understand the need. This decision may affect the cost. An IFS section, or Integrated Facility Switchboard, may include panelboards, dry type transformers and blank back pans for mounting other equipment. If panelboards and dry type distribution transformers are used within the same room as switchboards, it may make sense to consider IFS sections. IFS sections can reduce the need for lineal wall space required for equipment, thus reducing the area needed for equipment. Additionally, IFS can also significantly reduce the installation time while reducing the number of pieces of equipment for handling. Page 22

23 Spare, Provision and Blank Space Customer specifications for protective devices will also affect cost. We have already covered the types of protection devices (switches and circuit breakers) commonly used in the switchboard. But there are some extra terms used in this area pertaining to the protective devices that you should know. For future expandability, the customer could request a Spare protection device. This means that the protection device is bolted in place and connected to the bus. All the customer has to do is connect a branch conductor to it. This is very different from ordering a provision. With this arrangement, a space is left in the panel for the future insertion of a protection device. The bus connectors are in place and ready to use when the customer is ready to expand. Different still is the specification of a space. It is space on the panelboard/switchboard chassis that is simply covered with a blank panel. There is vertical bus in back of the panel, but a connector kit would be required to install a breaker in the future. The space can be defined for a defined overprotective device, in which can an appropriately sized filler cover will be supplied. Page 23

24 Helping the Customer Matching a Panelboard to an Application Throughout our discussion of panelboards and switchboards, we have identified a number of questions you will need to ask the customer to match a product to the application. It may be helpful to spend a moment reviewing the information needed so that you are more prepared to knowledgeably recommend a product to the customer. As you talk with the panelboard customer, be sure to obtain the following information: System type (including voltage, number of phases, and number of wires) Required amperage rating AIC rating or short circuit current rating Service entrance equipment or distribution? Main breaker or main lug only? Top- or bottom-fed? Flush- or surface-mount? NEMA Type enclosure Any desired special modifications Required shipping timeframe Shipped assembled or as components? Armed with this information, you should be able to consult your product catalog and make a good product recommendation. Page 24

25 Matching a Switchboard to an Application The questions you are required to ask of a switchboard customer are somewhat different. To save time, we have come up with a questionnaire-style form that should speed up the interview process. It is reproduced here. Figure 22. Switchboard Questionnaire Help the customer to complete this form thoroughly. This information should provide a good start on matching a switchboard to the customer s application. With this information in hand, you might then try using the form below to sketch out the proposed switchboard. Figure 23. Switchboard Layout Form This should help you and the customer fine-tune the switchboard s features to best serve the application. Page 25

26 Review 3 Answer the following questions without referring to the material just presented. 1. Like the bus bars of the panelboard, are used to in a switchboard. 2. There are four types of protective devices commonly used in switchboard applications. Name three of them. 3. Switchboards are usually only available in enclosure types and. 4. The four main switchboard section types are: 5. In your own words, explain the difference between a spare and a space. Page 26

27 Glossary Ampere Interrupt Capacity (AIC) Ampere Rating Blank Space Branch Circuit Bus Bar Bus Can Circuit Breaker Current Rating Disconnect Device Distribution Equipment Flush-Mounted Front and Rear Alignment Full Rating Method Fuse Also Interrupting Rating. A rating of the amount of current that a protective device, such as a fuse or circuit breaker, can safely interrupt. A rating of the amount of current a protective device will carry continuously without deteriorating or exceeding temperature rise limits. A space on a panel of a switchboard that will never be used for expansion. It is covered with a blank panel and no bus runs behind it. A circuit that supplies power to the electrical loads in a building and is terminated at the panelboard or switchboard. A component of a panelboard that serves as an extension of the main service conductors. Simplifies the connection of circuit protective devices to the main service conductors. Components of a switchboard that serves as an extension of the main service conductors. Simplifies the connection of circuit protective devices to the main service conductors. Utilized to move power between two or more components or circuits. Also Box. Component of a panelboard used to contain the other components. It is designed to provide component and personnel protection. A reusable overcurrent protection device. After tripping to break the circuit, it can be reset to protect the circuit again. A rating of the level of fault current a piece of equipment can withstand without sustaining damage. A blanket term used to describe a number of circuit protection devices, such as fuses and circuit breakers. Also Distribution/ Power Only Panelboard. A panelboard or switchboard used to feed lighting and appliance panelboards and three-phase motor loads. Also can power small transformers to provide other voltage levels. Recessed into a hole in the wall. A type of switchboard arrangement where the front and the rear faces of the switchboard sections are aligned. A method of selecting circuit protection devices where all device ratings are equal to or greater than the available fault current. A non-reusable overcurrent protection device. After tripping to break the circuit, it must be replaced to restore power to the circuit. Page 27

28 Ground Ground Fault Horizontal Bus Interrupting Rating Knockouts Lighting and Appliance Panelboard Loadcenter Main Breaker Main Disconnect Device Panelboards and Switchboards A conducting connection (intentional or accidental) between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth. An electrical fault where contact is accidentally made between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth. Distributes power to each section of a switchboard. Also Ampere Interrupting Capacity (AIC). A rating of the amount of current that a protective device, such as a fuse or circuit breaker, can safely interrupt. Circular perforations in the top and bottom panels of a panelboard that can be removed to provide entrances and exits for feeder wires enclosed in conduit. A panelboard that supplies electrical power to branch circuits that use a constant amount of power. A device that delivers electricity from a supply source to loads in light commercial or residential applications. Also Main Circuit Breaker. A reusable overcurrent protection device designed to protect an entire panelboard or switchboard. Power from the mains is fed through a main breaker to the bus bars. A blanket term used to describe a number of panel protection devices, such as a main circuit breaker or main switch. Main Lug Only (MLO) A panelboard where power from the mains is fed directly to the bus bars. Neutral Bars Neutral Bus Overcurrent Protective Device Panelboard Provides the termination point for the neutral wires from both the incoming service and the load circuits in a panelboard. Provides the termination point for the neutral wires from both the incoming service and the load circuits in a switchboard. A device such as a circuit breaker or fuse. In the event of an overload or short circuit, this device will quickly terminate power to the circuit. A wall-mounted electrical power distribution device for use in commercial and industrial applications. It provides circuit control and overcurrent protection for light, heat or power circuits. NEC defines it as a single panel or group of panel units designed for assembly in the form of a single panel; including buses, 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 or partition and accessible only from the front. Page 28

29 Pole Power Panelboard Pull Box Pull Section Rear Alignment Series Rated Method Service Entrance Service Entrance Equipment Service Section Six Subdivisions of the Main Rule Space Spare Surface-Mounted A space in a panelboard (or switchboard) where a branch circuit protective device can be attached to connect a branch circuit to the bus bars (buses) and protect the branch circuit from overload. Any panelboard that does not qualify as a lighting and appliance panelboard. It is used to feed lighting and appliance panelboards and three-phase motor loads. Also can power small transformers to provide other voltage levels. A top-mounted pull section. A common switchboard component, a blank enclosure containing empty space through which cables can be pulled. A type of switchboard arrangement where only the rear faces of the switchboard sections are aligned. A method of selecting circuit protection devices where the main upstream circuit protection device must have an interrupting rating equal to or greater than the available fault current of the system, but downstream devices connected in series can be rated at lower values. The single point at which electrical power enters a building. The term used to describe a panelboard or switchboard used as a service entrance. This section of the switchboard is where upstream power enters. It always contains a main disconnect and usually also has utility and/or customer metering equipment. An NEC ruling which states that you must be able to throw no more than six handles into the off position to disconnect electrical service. If throwing more than six handles would be required, one upstream disconnect must be provided to disconnect all the branch circuits at once. A specification when ordering a switchboard. Room is purposely left for a protective device to be installed in a later expansion of service. Bus bars are in place. The customer needs to buy and install a protection device and connect a branch circuit to it. A specification when ordering a switchboard. An extra protection device is bolted in place and connected to the bus. For future service expansion, all the customer has to do is connect a branch circuit to it. Attached to (and projects out from) the wall. Page 29

30 Switchboard Vertical Bus Voltage Rating A floor-standing electrical power distribution device for use in commercial and industrial applications. It divides large blocks of electrical current into smaller blocks of current used by electrical equipment. NEC defines it as a large single panel, frame, or assembly of panels on which are mounted, on the face or back, or both, switches overcurrent and other protective devices, buses and usually instruments. Distributes power through the circuit protection devices of a switchboard to the branch circuits. A rating of the voltage at which a piece of equipment is designed to operate. Page 30

31 Review 1 Answers 1. Any four of the following: Power supply systems Service entrance equipment / Distribution equipment Main breaker and main lug only types Grounding Ratings 2. Answer should basically say: The full rating method selects circuit protection devices with ratings equal to or greater than the available fault current. The series rated method states that the main upstream circuit protection device must have an interrupting rating equal to or greater than the available fault current of the system, but downstream devices connected in series can be rated at lower values. 3. level of fault current a piece of equipment can withstand without sustaining damage; current rating a protective device can safely interrupt; current a protective device will carry continuously without deteriorating or exceeding temperature rise limits 4. Answer should basically say: houses the other components, designed to provide component and personnel protection common connection for two or more circuits, mechanical means of affixing branch circuit breakers provide access to the circuit breakers while sealing off the bus bars and internal wiring from contact cover any unused pole spaces not used by a circuit breaker 5. flush, surface Review 2 Answers %; 30 amps Neutral connections 2. Yes 3. Flush; Surface 4. Answer should basically say: The minimum distance from the panelboard to the ground must be 4 feet. There must be at least 30 inches of space in front of the equipment to allow the hinged door to open 90 degrees. Headroom in the area must be at least 6-1/2 feet. Page 31

32 Review 3 Answers 1. bus; distribute power Panelboards and Switchboards 2. Any three of the following: power circuit breaker molded case circuit breaker fusible switch bolted pressure switch 3. NEMA Type 1, NEMA Type 3R 4. service pull distribution IFS 5. Answer should basically say: A spare protection device is bolted in place and connected to the bus. All the customer has to do is connect a branch circuit to it. A space is left in the panel for the future insertion of a protection device. The bus bars are in place and ready to use when the customer is ready to expand. Page 32