A CURRENT Look at Electrical Circuit Breakers

When electricity was first used commercially before the turn of the century, fuses were the only method available to protect the building wiring from potential overcurrent or short circuits.

When electricity was first used commercially before the turn of century, fuses were the only method available to protect the building wiring from potential overcurrent or short circuits (and subsequent overheating and fire hazard). A fuse is a filament of wire in a mechanical package that can be placed in series with the load wiring. The fuse is designed to vaporize and open the electrical circuit when the amount of electrical current (amperes) exceeds what the wire can safely handle.

Blowing a fuse
Most people who have lived in older homes have seen the screw-in type fuses that were used to protect the various electrical circuits in the house. When properly applied, fuses do a great job in protecting these electrical circuits, but they have two primary problems.

Problem #1: When a fuse blows, it self-destructs and must be replaced with a new fuse.

Problem #2: If a properly sized fuse is not readily available, a different size fuse that might be available fits in the same space as the blown fuse. If a smaller fuse is used as a replacement, it may blow again when the equipment is turned on. If a larger fuse is used, the design protection for circuit wiring is compromised; this can lead to the overheating of the wiring and a potential fire hazard.

The electrical circuit breaker was first introduced by the Westinghouse Corporation in the early 1900s, but was not widely used until decades later. It offers the overcurrent protection of the fuse in a package that will not self-destruct. The circuit breaker can be reset, and not replaced, after the electrical circuit is opened up. Since the circuit breaker doesn’t need to be replaced, the risk of the improper circuit protection associated with replacing the fuse with the wrong size circuit breaker has also been greatly diminished.

As stated in Article 100 of the 1999 National Electrical Code (NEC), the definition of a circuit breaker is “a device designed to open and close a circuit by nonautomatic means and to open the circuit automatically on a predetermined overcurrent without damage to itself when properly applied within its rating.”

An electrical circuit (hereinafter called a circuit) is defined as (1) the electrical load (such as a coffee maker, cooler or submersible pump); and (2) the current carrying conductors (wiring) associated with the electrical load. There are, therefore, two basic functions of a circuit breaker:

• to provide a means to manually turn a circuit ON and OFF for installation and maintenance of equipment.
• to automatically turn a circuit OFF when the current exceeds the design rating of that circuit without damaging itself and, therefore, being able to be reset.

The size of the breaker (in amperes) is determined by the electrical power requirements of the load that is to be served on each circuit. According to NEC Article 384-16(D), a continuous load (one that in normal operation continues for three hours or more) shall not exceed 80 percent of the ampere rating of the breaker. In general, the size (or gauge) of the wire on each circuit is determined by the calculated load that is to be served.

The ampere rating of the breaker is then chosen to protect the rated ampacity of the conductor. The ampere rating of the conductor is based on the amount of current it can carry without exceeding its allowable temperature rise. For example, a 20 ampere rated breaker can carry up to 16 amperes (80 percent of rating) of the continuous load with a #12 AWG (American Wire Gauge) wire. A 30 ampere rated breaker can carry up to 24 amperes (80 percent) of continuous load with a #10 AWG wire.

Circuit protection
There are two types of protection that most circuit breakers provide:

• Thermal protection from overcurrent on the circuit. When the current on the circuit exceeds the breaker rating, a thermal element reacts to the heat caused by the overcurrent and unlatches the breaker to open a set of contacts that interrupts current flow on the circuit.
• Magnetic protection from the rapid rise in current on the circuit. When the current on the circuit changes very rapidly, the magnetic field caused by this rapid change in current trips a solenoid. The solenoid unlatches the breaker and opens the set of contacts to interrupt current flow on the circuit. Some breakers do this immediately; others have a time delay in this action to prevent nuisance tripping from short inrush currents during motor startups and other inductive loads.

In addition to standard circuit breakers, there are also specialized circuit breakers to perform enhanced functions:

• The Switched Neutral (SWN) breaker opens the neutral conductor in addition to the line conductor on the circuit. This is used for loads in hazardous locations (such as multi-product dispensers) where a spark from the neutral conductor could ignite the hazardous material.
• The Shunt Trip (ST) breaker can be turned off remotely by either manual or automatic means. This is used typically for emergency shutdown applications (such as a car wash) where the shutdown switch is in a different location from the electrical panel room. These types of breakers are also used in some cases when the local electrical authority requires a single, accessible disconnecting means for the main electrical service. This is done by having a remote switch (normally located outside) that, when activated, closes a contact which sends a control voltage to the shunt trip coil assemblies located inside the breakers. The breakers simultaneously open up, thus removing power from all electrical loads within the building.
• The Ground Fault Interrupting (GFI) breaker turns off automatically when a dangerous ground fault is detected. Like the related ground fault receptacles (the ones with the test and reset buttons that are seen in bathrooms, kitchens, garages, unfinished basements and other outdoor locations), the ground fault breaker turns off when the current flowing on the line conductor is different from the current flowing on the neutral conductor. This indicates that current is flowing to ground (shock hazard) and the circuit breaker trips automatically.
• The Switched Duty (SWD) breaker is designed to be used as a means to switch circuits on and off manually on a regular basis. Standard type breakers are designed for overcurrent protection or occasional switching, but not for use as a switch on a daily basis. The SWD breaker is normally found in 120-volt and 277-volt fluorescent lighting circuits and has a 15- or 20-ampere rating.
• The Heating and Air Conditioning Rated (HACR) breaker is a heavier duty breaker designed for the large inrush currents and the continuous load associated with large heating and air conditioning equipment.

Panelboards and bus bars
Breakers are sometimes used in individual, stand-alone enclosures on the wall. However, they are more commonly grouped together with other breakers in a piece of electrical equipment called a panelboard.

The breakers are connected by either a “snap-on” type connection or a “bolt-on” type connection to one or more bus bars. The bus bars provide the electrical power to each of the circuit breakers in the panelboard. They must be sized to carry the electrical design load of the sum total of breakers installed, along with some expansion for future loads.

According to Article 100 of the 1999 National Electrical Code (NEC), the definition of a panelboard is “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.”

Diagram 1: Panelboard and Circuit Breaker Schematic

Panelboards are available in several sizes and electrical ratings, depending upon the application that is required:

• Voltage Rating is the design voltage rating of the electrical service that will be connected to the panelboard.

For example: 120/240-volt, single–phase, three-wire (two bus bars and a neutral) is very common in residential applications; 120/208-volt, three-phase, four-wire (three bus bars and a neutral) is very common in commercial applications.

• Current Rating is the capacity of the panelboard bus bars to carry the total electrical current of all the breakers installed on the panelboard.

For example: 100 ampere, 225 ampere and 400 ampere are common current ratings for panelboards. Each bus bar (two or three) has the capacity to carry the current rating of the panelboard.

• Circuits are the number of single pole breakers that can be installed in the panelboard. Two pole breakers take up two circuits, and three pole breakers take up three circuits. The following three examples assume a 120/208-volt, three-phase, four-wire, wye electrical service.

1. Single pole breakers connect to just one of the bus bars in the panelboard for loads such as interior lighting and receptacles that require 120-volt, single-phase power.

2. Two pole breakers connect to two of the bus bars in the panelboard for loads such as submersible pumps and refrigeration condensers that require 208-volt, single-phase power.

3. Three pole breakers connect to all three of the bus bars in the panelboard for loads such as heating and air conditioning units that require 208-volt, three- phase power.

Typical circuit offerings are 12, 20, 30 or 42 circuits. For example, 12-circuit panelboards can handle either 12 single pole breakers, six two pole breakers or four three pole breakers.

No more than 42 circuits are allowed by the NEC in any single lighting and appliance branch circuit panelboard. Typically, if a home or building requires more than 42 circuits, additional panelboards must be used.

Panelboards are classified in two general categories:

• Lighting and appliance branch circuit panelboard has more than 10 percent of its breakers rated at 30 amperes or less for which neutral connections are provided. See Article 384-14(A) of the 1999 NEC. As the name indicates, this type of panelboard is for lighting, appliances, refrigeration, dispensers, submersible pumps and other miscellaneous loads (Photo 1).
• Distribution (power) panelboard typically provides larger ampere-rated breakers that feed power to several other lighting and appliance panelboards and to larger building loads such as heating and air conditioning units (HVAC) as well as for pre-packaged walk-in coolers and freezers. This type of panelboard has 10 percent or fewer of its overcurrent devices protecting lighting and appliance branch circuits. See Article 384-14(B) of the 1999 NEC (Photo 2).

Circuit breaker applications in panelboards

The main circuit breaker is a single breaker that is sized to carry the load of the entire facility and to protect the main service feeder conductors. Turning off this breaker will disconnect all electrical power from the building. The main circuit breaker can either stand alone in its own enclosure or can be installed as part of the distribution panelboard (Photo 3).

Photo 1: Typical lighting and appliance panelboard with branch circuit breakers

Photo 2: Typical distribution panelboard with feeder circuit breakers

Photo 3: Typical main circuit breaker

The feeder circuit breaker is sized to feed power to the individual panelboards and to protect the panelboard feeder conductors. This type of breaker is typically installed in the distribution panelboard (Photo 4).

The branch circuit breaker is sized to feed power to the individual circuits on the panelboard and to protect the branch circuit conductors. This type of breaker is typically installed in the lighting and appliance panelboard (Photo 5).

Photo 4: Typical feeder circuit breaker. Relative size of a feeder circuit breaker is more than twice that of the branch circuit breakers shown in Photo 5
			Photo 5: Typical branch circuit breakers

Diagram 1 shows the relationship between the different categories of panelboards and the associated circuit breaker applications.

Panelboard quality
Panelboards are designed in two general grades of performance:

• Residential grade is designed to be a stand alone “lighting and appliance type” panelboard in a residence or a small commercial application. It typically has a 200-ampere, single-phase electrical service or less.
• Commercial/industrial grade is designed for larger commercial and industrial applications where several “lighting and appliance type” panelboards are required along with one or more “distribution type” panelboards, with typically a 400-ampere electrical service or more.

Panelboards with a “residential grade” typically carry a Short Circuit Current Rating (SCCR) of 10,000 amperes with the installed breakers. The SCCR is a measure of the capacity of the panelboard and breaker combination to withstand the high fault currents if the bus bars are shorted out due to an accident or a major equipment malfunction.

The breakers used are usually a “snap-on” type, where they are held onto the panelboard bus bars using spring pressure from a metal clip. The “residential grade” panelboard structural design is not usually as rugged as the “commercial/industrial grade” panelboards. This is because they are not exposed to the same electrical power environment where much higher short-circuit currents are available from larger utility transformers.

Panelboards with a “commercial/industrial grade” are available with much higher SCCR “series ratings” by placing the larger “distribution type” breakers in series with the smaller “lighting and appliance type” breakers. Typical SCCR “series ratings” are 22,000-amperes, 42,000-amperes and 65,000-amperes. These higher ratings are required in large commercial applications.

In such applications, the utility transformers used to provide the sizable electrical services to the building can provide much greater fault currents under short-circuit conditions. The breakers used are usually a “bolt-on” type, where they are literally bolted in place using a predrilled and tapped hole(s) on the bus bar. This panelboard design usually has a stronger frame, with additional bracing and reinforcement, than does the “residential grade” panelboard in order to withstand the higher short circuit currents.

The electrical utility company provides the available fault current that its transformer can deliver under short-circuit conditions. The electrical engineer for the building must design the electrical system to have an SCCR rating that meets or exceeds the available fault current of the utility transformer—minus a complex derating factor for the length and size of the conductors from the transformer to the electrical service entrance of the building.

Here is an example: The available fault current at the electrical service entrance of the building is 36,000 amperes. The electrical system must be designed to have an SCCR rating of at least 36,000 amperes at the service entrance. Typically, the next size used to accommodate this required rating would be 42,000 amperes.

The electrical equipment manufacturers obtain these “series ratings” through extensive testing of the breakers individually and in combination with the panelboards innsive testing of the breakers individually and in combination with the panelboards in which they are used. Therefore, an SCCR “series rating” only applies to the equipment combina- tions that have been tested by the various manufacturers. This means that, in most cases, the “distribution type” equipment and “lighting and appliance type” equipment should be made by the same manufacturer.

The case for planning
The electrical applications in convenience stores and retail service stations have grown over the years. The needs have skyrocketed, from the threshold of a simple 200-ampere (residential grade) electrical service to the much larger 600, 800 and even 1,200 ampere (commercial/industrial grade) electrical services. In the past when the buildings were relatively simple, the electrical layout was often done on the job site without engineered drawings and using residential grade panelboards. However, the size and complexity of facilities today dictates a properly engineered electrical distribution system as well as the transition to commercial/industrial grade panelboards.

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