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Both panelboards and switchboards contain the circuit feeder cables and protective devices required for the Branch Circuits. Each, however, do slightly different jobs.
Definitions
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.
Similarities Between Panelboards and Switchboards
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
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 | 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. |
| Three-phase, four-wire, wye-connected transformer | 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. |
| Three-phase, four-wire, delta-connected transformer | 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 wye-connected 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.
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).
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
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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
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 CodeŽ 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 downstream panelboard contains a branch circuit breaker. It trips, disconnecting power from the load.
Figure 6. A Short in a Downstream Load
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.
Equipment Ratings
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 110-9 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 110-9 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 110-9 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 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.
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