To Serve and Protect

Nov 1, 2007 12:00 PM, By Mark Mitchell and Alan Schmidt, Square D/Schneider Electric

Understanding the basics of commercial and industrial circuit breakers

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.

This photo shows 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. (All graphics courtesy of Square D/Schneider Electric.)

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.

Fig. 1. This graphic represents a portion of a typical circuit breaker label with the interrupting ratings for that particular device.

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.