Most people are comforted in knowing their homes are protected from electrical hazards by the circuit breakers located behind the door of a home's load center. These residential circuit breakers perform the crucial function of opening circuits if a hazardous condition arises, such as an overload. However, hazard potential is much more pronounced in commercial and industrial buildings, due to the large amount of electricity required for day-to-day operations. For example, big-box retailers often run high levels of current in order to keep the coolers running, lights on, and cash registers humming — not to mention maintain a pleasant temperature via the HVAC system. Similar power demands are less obvious but are also found in hospitals, schools, banks, office buildings, and factories.

Just as in a residential setting, circuit breakers in commercial and industrial buildings help protect personnel and equipment from electrical hazards. However, in these settings, they quietly monitor circuits and open if a hazard becomes present.

This article will focus on the basics of circuit breakers found in commercial and industrial buildings as well as cover common terms associated with these devices and their use.

Core differences

First, let's examine some of the core differences between the electrical demands of a home and those of commercial and industrial facilities. In the latter, voltage is often distributed at a rate of two to three times more than what is typical in a residential application. Although receptacles are commonly 120V in all settings, the power running between electrical panels in commercial and industrial buildings can rise to levels up to 600V. Obviously, this requires more robust circuit breakers to appropriately manage these higher voltages. Voltage ratings are printed on the faceplate labels of circuit breakers — breakers must be rated at or higher than the voltage being distributed.

Another critical element is the interrupting rating, which can be a difficult concept to master at first. Unlike residential settings, available fault current is much higher in commercial and industrial situations. Why? There is much more electricity available for a fault due to the size of the building, its overall power requirements, and the amount of electrical distribution equipment, thus dramatically increasing the hazard potential. Other factors include the size of the utility service feeding the facility, size of transformers, and the distance between the panels and service entrance. In commercial and industrial settings, it's common to see available fault currents ranging from 10,000A to 65,000A or higher. This is in sharp contrast to the 1,000A or lower available fault current in homes.

It's essential for circuit breakers to be appropriately sized for the level of hazard if they are to adequately protect a building, its equipment and machinery, and occupants. Like voltage ratings, interrupting ratings are also located on the faceplate labels of circuit breakers and often vary by voltage. The breaker must have an interrupting rating at or higher than the available fault current in the system, which is typically determined by a facility or specifying engineer. Figure 1 shows a portion of a typical circuit breaker label with the interrupting ratings for that particular device.

On a less dramatic note, protecting the wires and equipment in the electrical distribution system is a key responsibility of circuit breakers. An appropriately sized device is selected by what is commonly referred to as the ampere rating or continuous current rating. When sizing and selecting circuit breakers, careful consideration of the current rating is essential, as the protection settings of the chosen breaker must align with the damage curves for the wire and conductors used in the installation. This is automatically aligned in the trip curves and Underwriters Laboratories (UL) standards for the circuit breaker. Proper sizing places a boundary for the level of current conductors are allowed to carry and ensures the insulation of wires do not overheat or melt. The Photo illustrates typical damage from a cable that has been overheated. The damaged insulation can lead to much more dramatic failures, creating greater risk of arcing faults or shock hazards.

Equally damaging to conductors is an elevated, but temporary, surge of current. This surge is often required of electrical equipment (e.g., motors and transformers) when such equipment is energized. By design, circuit breakers are built to allow as much as eight to 10 times their rated current for a short time (typically less than a few seconds) to advance past the elevated surge of current upon being energized before settling back to operational current. This boundary ensures a quick response for intermediate faults that could occur when equipment is energized, preventing them from escalating and/or causing thermal/mechanical stresses.

Here's another example: Assume a rodent begins chewing on wiring or insulation somewhere within the electrical distribution system. This scenario creates a sharp in-rush of current that would almost instantaneously be cleared by a circuit breaker opening that circuit. The breaker is not going to be able to save the rodent, but it will keep a facility's electrical system from sustaining further damage.

First, let's examine some of the core differences between the electrical demands of a home and those of commercial and industrial facilities. In the latter, voltage is often distributed at a rate of two to three times more than what is typical in a residential application. Although receptacles are commonly 120V in all settings, the power running between electrical panels in commercial and industrial buildings can rise to levels up to 600V. Obviously, this requires more robust circuit breakers to appropriately manage these higher voltages. Voltage ratings are printed on the faceplate labels of circuit breakers — breakers must be rated at or higher than the voltage being distributed.

Time-current curves

A time-current curve (or TCC) is a valuable tool used to illustrate the boundaries a circuit breaker places on electricity. TCCs use a logarithmic scale plotted in an inverse relationship between time and current (referred to as inverse time characteristics). The literal shape of the TCC is closely aligned with the current limitations of conductors to protect them from thermal and mechanical stress. Figure 2 on page C12 is an illustration of the TCC of the circuit breaker and a typical wire damage curve of an electrical conductor.

In some applications, it's important to change the shape and slope of the TCC to better coordinate with other breakers or circuit protection devices. There are three basic zones within a TCC, and many commercial and industrial circuit breakers can be set to adjust these zones as necessary.

Long-time setting (L)

Most smaller circuit breakers (less than 100A) have limited or no adjustment for long-time settings. Because these breakers are typically located near the final loads, there is usually no need to adjust them, unless a specific piece of equipment is being protected. Circuit breakers equipped with electronic trip systems, such as larger molded-case breakers, can provide precise trip thresholds, allowing for better device coordination and equipment protection.

This is the long-time setting for overload faults, which typically occur when too many loads are applied to a specific circuit, similar to having too many appliances using one circuit in a home (Fig. 3). For example, when two times (2x) the continuous current rating is detected, the circuit breaker will trip within approximately 10 and 50 seconds. The greater the overload, the shorter the time it will take the breaker to trip. To determine what level of current is needed for a circuit, the National Electrical Code requires electrical loads be characterized to obtain this value. Because the long-time setting is primarily protecting from thermal damage, there may be special considerations for installations that are in hot or cold environments, which could require a re-rating factor for equipment located outside or on a roof.

Short-time setting (S)

This setting is for when in-rush currents cause low-level short-circuit events or arcing faults (Fig. 4 on page C14). This is common due to the heavy current draw required for electrical motors or transformers. For example, you may notice the lights dim for a split second when starting a saw or heavy power tool due to the current in-rush of the motor. This trip boundary is very quick, typically allowing currents to reach seven to 10 times the rated current for fractions of a second. If that level of current continues for a second or more, permanent damage may be sustained.

Instantaneous setting (I)

The instantaneous setting is the maximum level of current to which the circuit can be exposed. When that threshold is reached, it immediately trips the circuit breaker (Fig. 5). Because the short-time setting previously described has a time-based characteristic, the immediate trip is the crucial element of this setting. In smaller circuit breakers (typically less than 250A), this value is pre-determined and requires no customer adjustment. The instantaneous settings become important when many breakers are located within the same circuit as part of a distribution network. Some installations require each circuit breaker to trip in a precise sequence, starting from the device located closest to the final load, proceeding to larger upstream circuit breakers. This limits a power outage to a much smaller area — a few outlets in an emergency room versus the entire wing of the hospital, for example. The instantaneous setting helps align trip curves to allow downstream breakers enough time to clear a fault for these types of installations.

Beyond these basic functions, circuit breakers have evolved to provide a myriad of other trip characteristics to add safety and reliability to an electrical system. These include devices to monitor and protect from ground faults, voltage irregularities, or current imbalance, and those that measure power quality and harmonics.

Ensuring protection

Like their residential counterparts, commercial and industrial circuit breakers are about protection of the physical plant, machinery/equipment, and occupants. But that's just the beginning. An electrical fire can hinder, if not destroy, a company's ability to be profitable. Properly installed circuit breakers help ensure smooth, orderly operation and contain hazards locally before they escalate.

Given these facts, correct installation of circuit breakers should be viewed as more than just a job to complete before heading to the next destination. It's a responsibility that can prevent a catastrophe and contribute to the economic growth of a city, town, or region.

Mitchell is a senior technical writer for Square D/Schneider Electric in Cedar Rapids, Iowa. Schmidt is product manager of Square D power protection products, also based in Cedar Rapids.