Over the past half century, advances in molded case circuit breaker technology have led to a product that is a key element in the electrical safety of residences, office buildings, and sophisticated commercial and industrial systems. Circuit breakers have become so commonplace and reliable that their purpose is taken for granted and often misunderstood. In what ways do circuit breakers contribute to electrical safety? Under what conditions do they provide protection?
The National Electrical Code (NEC) defines a circuit breaker as follows:
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.
Simply stated, a circuit breaker must be able to be switched open and closed manually and to protect a circuit from overcurrent by opening automatically. This allows a breaker to be used repeatedly without replacement.
The switching function of a circuit breaker is simple and readily understood. It provides a means of easily isolating a circuit or circuits protected by the circuit breaker from the rest of the electrical system, allowing for the safe modification or maintenance of the circuit, or equipment in the circuit. All circuit breakers are intended to switch loads. In addition, some are specifically rated for switching loads such as the lighting in commercial and in industrial buildings, on a regular basis.
The protection function is not quite so easily understood. What is a molded case circuit breaker intended to protect? What can an overcurrent protective device be expected to do and what is beyond the scope of its design? Sec. 240.1 (FPN) of the NEC has the following to say about overcurrent protection:
Overcurrent protection for conductors and equipment is provided to open the circuit if the current reaches a value that will cause an excessive or dangerous temperature in the conductors or conductor insulation.
So, circuit breakers are intended to protect conductors (insulated wires) by opening automatically before damage is caused by excessive temperatures. This protection applies to the permanently installed wiring of homes, offices, and industrial buildings. In protecting these wires, overcurrent protective devices reduce the risk of fire, personal injury, and damage to equipment and property.
Circuit breakers listed by UL are subjected to a rigorous test program to assure the specifications for overcurrent protection are met. Initial samples are subjected to endurance, overload and short-circuit tests with a length of the wire to be protected in the test circuit. Follow-up tests on samples randomly chosen from production samples are continually checked to be sure ongoing product is properly calibrated.
In summary, circuit breakers are intended to protect the wiring, permanently installed conductors and certain extension and power supply cords, from high temperatures caused by current in excess of the rating of the conductors. While "conductor" protection is the key element in electrical system safety, there are potentially dangerous conditions that do not involve overcurrent. Circuit breakers do not provide protection against these conditions. The following are important examples:
Electric Shock. Standard overcurrent protective devices will not protect personnel from the effects of electric shock. Minute currents, 20mA (0.02A) or less, can have fatal results if they pass through the human body. Obviously, an overcurrent protective device rated 15 or 20A will take no notice of such small currents, however dangerous they may be.
A special form of circuit breaker called a circuit breaker ground fault interrupter (GFCI) can be used to protect personnel against the effects of electric shock. These sophisticated electronic devices will detect currents as small as 5mA and open the circuit so quickly that the shock that does occur will be over before serious injury can occur to a normally healthy person. GFCI protection is required by the NEC for bathrooms, outdoor receptacles, kitchens, and certain other locations where the likelihood of electrical shock hazards is the greatest. Such devices are also available in receptacle form.
Low-level line faults. Suppose the cord supplying power to an electric iron becomes worn and frayed as a result of constant flexing. Strands form opposite sides of the line make contact and an arc is established with current levels at or below the rating of the current levels at or below the rating of the circuit breaker. Once again, there is no overcurrent, the conductors are not experiencing excessive temperature, and the covercurrent protective device is not activated. If the arc persists, it can set fire to flammable materials. It is likely that current levels will eventually increase into the protective range of the circuit breaker, but a fire may have already started. Since the fault described is from line to neutral, ground fault protection is ineffective.
While the example used involved an electric iron power supply cord, all extension cords are susceptible to the "frayed cord" situation. The best protection is proper application, frequent inspection, and maintenance of all flexible cords with immediate replacement or repair when any sign of wear appears.
Low-level ground fault. Ground faults are similar to line faults except the electrical fault is between a line wire and a grounded metal object, such as the frame of a clothes dryer. As with low-level line faults, a standard circuit breaker may not be capable of detecting a low-level ground fault, and such a fault could cause ignition of a flammable material.
Since ground fault circuit interrupters are designed to interrupt the very low levels of current that present a shock hazard, they have the capability to interrupt ground faults at much lower levels than standard circuit breakers. Circuit breaker GFCIs will provide low-level ground fault protection for the entire length of a circuit, further reducing the risk of fire, injury, and property damage.
Voltage surges. During an electrical storm, lightning may strike near the power lines in your neighborhood, causing a voltage surge which follows the power lines into your home. While the voltage may be high enough to damage your TV, VCR, computer or other electronic appliances, the associated current is very low. Since circuit breakers protect against "overcurrent" and not "overvoltage", your electronic equipment is exposed to potential damage.
While circuit breakers are ineffective in this situation, surge protectors are widely available. They are provided in various forms, designed to protect a residence, a particular branch circuit, or a single appliance.
Overheating. If the connection of a branch circuit conductor to an appliance or receptacle is loose, severe overheating may occur at the point of connection. Extreme heating can result, damaging the equipment or device and, in extreme cases, damaging surrounding materials. Despite extensive heat and progressive deterioration, the current level remains normal, there is no "overcurrent", and the circuit breaker is not activated. Consequently, the best protection is careful, professional installation, and periodic inspections of susceptible connection points.
In summary, as defined by the NEC, circuit breakers provide switching and overcurrent protection functions. The overcurrent protection is a key element in electrical safety, protecting conductors from damage due to currents in excess of their ratings and reducing risk of fire, property damage, and personal injury. Special forms of circuit breakers can be applied to protect against damage due to ground fault and personal injury due to electrical shock.
However sophisticated and reliable, circuit breaker technology must be complemented by user and installer care and awareness if electrical safety is to be optimized.
Reprinted with permission from EC&M Magazine - copyright July 1994, Primedia Business Magazines & Media